What wavelengths are used to search for distant solar system objects like KBOs and Oort cloud members?

What wavelengths are used to search for distant solar system objects like KBOs and Oort cloud members?

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What wavelengths of light are used/most suited to search for distant solar system objects like KBOs and Oort cloud members? I suppose they are brightest in reflected sunlight, so they are best searched for in visible and near infrared. Is that correct?

Virtually all KBO searches use mosaic CCD cameras operating in the visible part of the spectrum, usually with a red filter like SDSS-r' (centered at approx. 622nm SDSS filters) on big (~4m telescopes). This is what we did with the OSSOS survey I was (lightly) involved with ( Although cold KBOs are brightest in the infrared, the background from the atmosphere is much higher which means you are less sensitive, unless you go to space (very expensive).

Also IR detectors are smaller and less sensitive than optical CCDs and much harder to make into mosaic cameras and keep cold (to increase sensitivity). This means you can't cover as much sky area per night which makes finding rare and faint KBOs much slower. A CCD and red filter is a good balance between brightness of the objects you are looking for, the sensitivity of your detector (CCDs get less sensitive as you move more to the red or blue of the green-red central part of the optical spectrum) and a low background from the atmosphere.

What wavelengths are used to search for distant solar system objects like KBOs and Oort cloud members? - Astronomy

Are the objects in the Kuiper Belt asteroids?

This is a question that's currently being discussed by astronomers. So far, Kuiper Belt Objects (also called KBOs) are treated like they're a separate class of objects, partly to avoid having to call them asteroids or comets. What we know so far is that KBOs have different compositions than most asteroids, and different orbits than the objects traditionally called comets.

Are they comets? Some short-period comets come from the Kuiper Belt, so in that respect KBOs can be considered comets. Also, "typical" asteroids are mostly composed of rock, while "typical" comets are a mix of ice and rock. We think most KBOs are about half ice and half rock, so they may be more like comets in this way too. We think some comets "change" into asteroids as they repeatedly pass close to the sun and lose their ice, so the difference between the two classes is a little fuzzy.

Are they asteroids? You're right that some people consider KBOs to be more like asteroids than comets. Comets that come close to the Sun (including all the ones that we see in the sky with beautiful tails) have very elliptical orbits. But the KBOs have fairly circular orbits around the Sun, and most of them don't come close to the Sun at all. So if an object needs to have an icy composition and a highly elliptical orbit to be considered a comet, then KBOs are more like an "icy asteroid belt" then a group of comets.

I think what it comes down to is that our classification for comets and asteroids was based on what we knew 60 or so years ago, and it's now clear that the the Solar System is much more complicated. As we learn more about the Kuiper Belt, we'll probably come up with a different classification that works better, but for now the trend is to just refer to smaller classes of objects to avoid confusion. For example Main Belt Asteroids, Kuiper Belt Objects, Near-Earth Asteroids, Long-Period Comets, etc., as opposed to just "asteroids" or "comets".

What is your opinion on whether an object that is in the Kuiper Belt should be called a planet if it is bigger than Pluto?

Hmmmm. I'm not sure what I would think about this one! So far, our classification for planets has relied on size: Things larger than Pluto are planets. So for the classification to make sense, I guess it would have to be considered a planet as well. If such an object was discovered, it would increase the chances that Pluto would be dropped from the list of planets, although I personally don't think there's anything wrong with calling Pluto and anything larger a planet, as long as we're consistent.

2006 Update by Karen Masters: In fact the recent discovery of Eris (previously Xena) - an object larger than Pluto and in a similar orbit did raise this question. At the IAU meeting in August 2006 Astronomers debated the formal definition of a planet and decided to exclude Pluto from the list of "classical planets" but create a new category of "dwarf planets" of which Pluto and Eris are both members (along with Sedna, and the largest member of the asteroid belt, Ceres).

If so, should Quaoar be called a planet?

Well, Quaoar is about half the size of Pluto, so I think it should be called a KBO, not a planet. I guess my personal opinion is that we should leave Pluto as a planet for two reasons: 1. Historically Pluto was considered a planet and I don't think there's a serious scientific reason to change it (changing the designation won't change what we know about it), and 2. Pluto is currently the largest KBO and is bigger than any asteroids that we know of, so (at least for now) it's a special object. Quaoar is considerably smaller than Pluto, and is about the size of large asteroids, like Ceres.

2006 Update by Karen Masters: so obviously this didn't happen - Pluto was reclassified as a dwarf planet in August 2006. There is still some debate about the size of Quaoar. At present it is not formally a member of the new "dwarf planet" class, although it may join that class when better observations confirm its size.

Another thing to keep in mind is that, in the end, this is all just terminology. Classifications are useful because they highlight similarities and differences between large numbers of objects, and when you need to talk about a large group (in this case "small objects orbiting the Sun"), it's easier to talk about general properties when they're arranged as asteroids, comets, KBOs, etc.

But even if we decided to call the KBOs asteroids, they would still be different from the Asteroid Belt asteroids (different compositions, different orbits, and different histories). So it wouldn't change how we studied the Kuiper Belt, it would just alter how our classifications were decided. The same with planets. It doesn't matter so much that Pluto is or isn't a planet: if it is considered a planet, everyone knows it's much different from the other planets. And if Pluto is just a Kuiper Belt Object, then people will still know that it's the largest KBO and most likely different from other small Kuiper Belt objects. So I think most astronomers aren't too excited about how the KBOs are classified. They'd rather learn more about KBO compositions, find out how many KBOs there are and figure out where all the Kuiper Belt Objects came from!

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For most of history, humanity did not recognize or understand the concept of the Solar System. Most people up to the Late Middle Ages–Renaissance believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system. [11] [12]

In the 17th century, Galileo discovered that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it. [13] Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. [14] Edmond Halley realised in 1705 that repeated sightings of a comet were recording the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets orbited the Sun. [15] Around this time (1704), the term "Solar System" first appeared in English. [16] In 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. [17] Improvements in observational astronomy and the use of uncrewed spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun.

The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally. [18] The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System's total mass. [g]

Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic, whereas comets and Kuiper belt objects are frequently at significantly greater angles to it. [22] [23] As a result of the formation of the Solar System, planets (and most other objects) orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above Earth's north pole). [24] There are exceptions, such as Halley's Comet. Most of the larger moons orbit their planets in this prograde direction (with Triton being the largest retrograde exception) and most larger objects rotate themselves in the same direction (with Venus being a notable retrograde exception).

The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt of mostly icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. [25] Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. [26]

Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which, Titan and Ganymede, are larger than the planet Mercury). The four giant planets have planetary rings, thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent. [27]

Kepler's laws of planetary motion describe the orbits of objects about the Sun. Following Kepler's laws, each object travels along an ellipse with the Sun at one focus. Objects closer to the Sun (with smaller semi-major axes) travel more quickly because they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its perihelion, whereas its most distant point from the Sun is called its aphelion. The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. The positions of the bodies in the Solar System can be predicted using numerical models.

Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum. [28] [29] The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets. [28]

The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium. [30] Jupiter and Saturn, which comprise nearly all the remaining matter, are also primarily composed of hydrogen and helium. [31] [32] A composition gradient exists in the Solar System, created by heat and light pressure from the Sun those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points. [33] The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, and it lies at roughly 5 AU from the Sun. [4]

The objects of the inner Solar System are composed mostly of rock, [34] the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula. [35] Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapour pressure, such as hydrogen, helium, and neon, which were always in the gaseous phase in the nebula. [35] Ices, like water, methane, ammonia, hydrogen sulfide, and carbon dioxide, [34] have melting points up to a few hundred kelvins. [35] They can be found as ices, liquids, or gases in various places in the Solar System, whereas in the nebula they were either in the solid or gaseous phase. [35] Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit. [34] [36] Together, gases and ices are referred to as volatiles. [37]

Distances and scales

The distance from Earth to the Sun is 1 astronomical unit [AU] (150,000,000 km 93,000,000 mi). For comparison, the radius of the Sun is 0.0047 AU (700,000 km). Thus, the Sun occupies 0.00001% (10 −5 %) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10 −6 ) that of the Sun. Jupiter, the largest planet, is 5.2 astronomical units (780,000,000 km) from the Sun and has a radius of 71,000 km (0.00047 AU), whereas the most distant planet, Neptune, is 30 AU (4.5 × 10 9 km) from the Sun.

With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearer object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the Titius–Bode law), [38] but no such theory has been accepted.

Some Solar System models attempt to convey the relative scales involved in the Solar System on human terms. Some are small in scale (and may be mechanical—called orreries)—whereas others extend across cities or regional areas. [39] The largest such scale model, the Sweden Solar System, uses the 110-metre (361 ft) Ericsson Globe in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at Stockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object, Sedna, is a 10 cm (4 in) sphere in Luleå, 912 km (567 mi) away. [40] [41]

If the Sun–Neptune distance is scaled to 100 metres, then the Sun would be about 3 cm in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm, and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm) at this scale. [42]

Distances of selected bodies of the Solar System from the Sun. The left and right edges of each bar correspond to the perihelion and aphelion of the body, respectively, hence long bars denote high orbital eccentricity. The radius of the Sun is 0.7 million km, and the radius of Jupiter (the largest planet) is 0.07 million km, both too small to resolve on this image.

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud. [h] This initial cloud was likely several light-years across and probably birthed several stars. [44] As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System, known as the pre-solar nebula, [45] collapsed, conservation of angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. [44] As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU [44] and a hot, dense protostar at the centre. [46] [47] The planets formed by accretion from this disc, [48] in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed, leaving the planets, dwarf planets, and leftover minor bodies. [49]

Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun, and these would eventually form the rocky planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large. The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud. [49] The Nice model is an explanation for the creation of these regions and how the outer planets could have formed in different positions and migrated to their current orbits through various gravitational interactions. [51]

Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion. [52] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved: the thermal pressure equalled the force of gravity. At this point, the Sun became a main-sequence star. [53] The main-sequence phase, from beginning to end, will last about 10 billion years for the Sun compared to around two billion years for all other phases of the Sun's pre-remnant life combined. [54] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process. The Sun is growing brighter early in its main-sequence life its brightness was 70% that of what it is today. [55]

The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now. This will mark the end of the Sun's main-sequence life. At this time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be much greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler (2,600 K at its coolest) than it is on the main sequence. [54] The expanding Sun is expected to vaporize Mercury and render Earth uninhabitable. Eventually, the core will be hot enough for helium fusion the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will move away into space, leaving a white dwarf, an extraordinarily dense object, half the original mass of the Sun but only the size of Earth. [56] The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium.

The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses), [57] which comprises 99.86% of all the mass in the Solar System, [58] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium, making it a main-sequence star. [59] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light. [60]

The Sun is a G2-type main-sequence star. Hotter main-sequence stars are more luminous. The Sun's temperature is intermediate between that of the hottest stars and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as red dwarfs, make up 85% of the stars in the Milky Way. [61] [62]

The Sun is a population I star it has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars. [63] Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This high metallicity is thought to have been crucial to the Sun's development of a planetary system because the planets form from the accretion of "metals". [64]

The vast majority of the Solar System consists of a near-vacuum known as the interplanetary medium. Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour, [65] creating a tenuous atmosphere that permeates the interplanetary medium out to at least 100 AU (see § Heliosphere). [66] Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms. [67] The largest structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium. [68] [69]

Earth's magnetic field stops its atmosphere from being stripped away by the solar wind. [70] Venus and Mars do not have magnetic fields, and as a result the solar wind is causing their atmospheres to gradually bleed away into space. [71] Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions create aurorae seen near the magnetic poles.

The heliosphere and planetary magnetic fields (for those planets that have them) partially shield the Solar System from high-energy interstellar particles called cosmic rays. The density of cosmic rays in the interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown. [72]

The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes the zodiacal light. It was likely formed by collisions within the asteroid belt brought on by gravitational interactions with the planets. [73] The second dust cloud extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt. [74] [75]

The inner Solar System is the region comprising the terrestrial planets and the asteroid belt. [76] Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to the Sun the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is also within the frost line, which is a little less than 5 AU (about 700 million km) from the Sun. [77]

Inner planets

The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals, such as the silicates—which form their crusts and mantles—and metals, such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather all have impact craters and tectonic surface features, such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).


Mercury ( 0.4 AU from the Sun) is the closest planet to the Sun and on average, all seven other planets. [78] [79] The smallest planet in the Solar System (0.055 M ), Mercury has no natural satellites. Besides impact craters, its only known geological features are lobed ridges or rupes that were probably produced by a period of contraction early in its history. [80] Mercury's very tenuous atmosphere consists of atoms blasted off its surface by the solar wind. [81] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, or that it was prevented from fully accreting by the young Sun's energy. [82] [83]


Venus (0.7 AU from the Sun) is close in size to Earth (0.815 M ) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C (752 °F), most likely due to the amount of greenhouse gases in the atmosphere. [84] No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions. [85]


Earth (1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only place where life is known to exist. [86] Its liquid hydrosphere is unique among the terrestrial planets, and it is the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen. [87] It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.

Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 M ). It has an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6% of that of Earth). [88] Its surface, peppered with vast volcanoes, such as Olympus Mons, and rift valleys, such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago. [89] Its red colour comes from iron oxide (rust) in its soil. [90] Mars has two tiny natural satellites (Deimos and Phobos) thought to be either captured asteroids, [91] or ejected debris from a massive impact early in Mars's history. [92]

Asteroid belt

  • Sun
  • Jupiter trojans
  • Planetary orbit
  • Asteroid belt
  • Hilda asteroids
  • NEOs(selection)

Asteroids except for the largest, Ceres, are classified as small Solar System bodies [f] and are composed mainly of refractory rocky and metallic minerals, with some ice. [93] [94] They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), depending on different, somewhat arbitrary definitions.

The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter. [95] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. [96] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth. [21] The asteroid belt is very sparsely populated spacecraft routinely pass through without incident. [97]


Ceres (2.77 AU) is the largest asteroid, a protoplanet, and a dwarf planet. [f] It has a diameter of slightly under 1000 km , and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in 1801 and was reclassified to asteroid in the 1850s as further observations revealed additional asteroids. [98] It was classified as a dwarf planet in 2006 when the definition of a planet was created.

Asteroid groups

Asteroids in the asteroid belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets, which may have been the source of Earth's water. [99]

Jupiter trojans are located in either of Jupiter's L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit) the term trojan is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter that is, they go around the Sun three times for every two Jupiter orbits. [100]

The inner Solar System also contains near-Earth asteroids, many of which cross the orbits of the inner planets. [101] Some of them are potentially hazardous objects.

The outer region of the Solar System is home to the giant planets and their large moons. The centaurs and many short-period comets also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid. [49]

Outer planets

The four outer planets, or giant planets (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. [g] Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of the gases hydrogen and helium, hence their designation as gas giants. [102] Uranus and Neptune are far less massive—less than 20 Earth masses ( M ) each—and are composed primarily of ices. For these reasons, some astronomers suggest they belong in their own category, ice giants. [103] All four giant planets have rings, although only Saturn's ring system is easily observed from Earth. The term superior planet designates planets outside Earth's orbit and thus includes both the outer planets and Mars.


Jupiter (5.2 AU), at 318 M , is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has 79 known satellites. The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating. [104] Ganymede, the largest satellite in the Solar System, is larger than Mercury.


Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 M . Saturn is the only planet of the Solar System that is less dense than water. [105] The rings of Saturn are made up of small ice and rock particles. Saturn has 82 confirmed satellites composed largely of ice. Two of these, Titan and Enceladus, show signs of geological activity. [106] Titan, the second-largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.


Uranus (19.2 AU), at 14 M , is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other giant planets and radiates very little heat into space. [107] Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel, and Miranda. [108]


Neptune ( 30.1 AU ), though slightly smaller than Uranus, is more massive (17 M ) and hence more dense. It radiates more internal heat, but not as much as Jupiter or Saturn. [109] Neptune has 14 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen. [110] Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by several minor planets, termed Neptune trojans, that are in 1:1 resonance with it.


The centaurs are icy comet-like bodies whose orbits have semi-major axes greater than Jupiter's (5.5 AU) and less than Neptune's (30 AU). The largest known centaur, 10199 Chariklo, has a diameter of about 250 km. [111] The first centaur discovered, 2060 Chiron, has also been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun. [112]

Comets are small Solar System bodies, [f] typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such as Hale–Bopp, are thought to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent. [113] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult. [114] Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids. [115]

Beyond the orbit of Neptune lies the area of the "trans-Neptunian region", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt. The entire region is still largely unexplored. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System. [116]

Kuiper belt

  • Sun
  • Jupiter trojans
  • Giant planets
  • Kuiper belt
  • Scattered disc
  • Neptune trojans

The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice. [117] It extends between 30 and 50 AU from the Sun. Though it is estimated to contain anything from dozens to thousands of dwarf planets, it is composed mainly of small Solar System bodies. Many of the larger Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may prove to be dwarf planets with further data. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth. [20] Many Kuiper belt objects have multiple satellites, [118] and most have orbits that take them outside the plane of the ecliptic. [119]

The Kuiper belt can be roughly divided into the "classical" belt and the resonances. [117] Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU. [120] Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, 15760 Albion (which previously had the provisional designation 1992 QB1), and are still in near primordial, low-eccentricity orbits. [121]

Pluto and Charon

The dwarf planet Pluto (with an average orbit of 39 AU) is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet this changed in 2006 with the adoption of a formal definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. Pluto has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos. [122]

Charon, the largest of Pluto's moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycentre of gravity above their surfaces (i.e. they appear to "orbit each other"). Beyond Charon, four much smaller moons, Styx, Nix, Kerberos, and Hydra, orbit within the system.

Makemake and Haumea

Makemake (45.79 AU average), although smaller than Pluto, is the largest known object in the classical Kuiper belt (that is, a Kuiper belt object not in a confirmed resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. It was assigned a naming committee under the expectation that it would prove to be a dwarf planet in 2008. [6] Its orbit is far more inclined than Pluto's, at 29°. [123]

Haumea (43.13 AU average) is in an orbit similar to Makemake, except that it is in a temporary 7:12 orbital resonance with Neptune. [124] It was named under the same expectation that it would prove to be a dwarf planet, though subsequent observations have indicated that it may not be a dwarf planet after all. [125]

Scattered disc

The scattered disc, which overlaps the Kuiper belt but extends out to about 200 AU, is thought to be the source of short-period comets. Scattered-disc objects are thought to have been ejected into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs' orbits are also highly inclined to the ecliptic plane and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered disc objects as "scattered Kuiper belt objects". [126] Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc. [127]

Eris (with an average orbit of 68 AU) is the largest known scattered disc object, and caused a debate about what constitutes a planet, because it is 25% more massive than Pluto [128] and about the same diameter. It is the most massive of the known dwarf planets. It has one known moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.

The point at which the Solar System ends and interstellar space begins is not precisely defined because its outer boundaries are shaped by two forces, the solar wind and the Sun's gravity. The limit of the solar wind's influence is roughly four times Pluto's distance from the Sun this heliopause, the outer boundary of the heliosphere, is considered the beginning of the interstellar medium. [66] The Sun's Hill sphere, the effective range of its gravitational dominance, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud. [129]


The heliosphere is a stellar-wind bubble, a region of space dominated by the Sun, in which it radiates its solar wind at approximately 400 km/s, a stream of charged particles, until it collides with the wind of the interstellar medium.

The collision occurs at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind. [130] Here the wind slows dramatically, condenses and becomes more turbulent, [130] forming a great oval structure known as the heliosheath. This structure is thought to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field. [131]

The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space. [66] Voyager 1 and Voyager 2 are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively. [132] [133] Voyager 1 is reported to have crossed the heliopause in August 2012. [134]

The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere. [130] Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way. [135]

  • inner Solar System and Jupiter
  • outer Solar System and Pluto
  • orbit of Sedna (detached object)
  • inner part of the Oort Cloud

Due to a lack of data, conditions in local interstellar space are not known for certain. It is expected that NASA's Voyager spacecraft, as they pass the heliopause, will transmit valuable data on radiation levels and solar wind to Earth. [136] How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere. [137] [138]

Detached objects

90377 Sedna (with an average orbit of 520 AU) is a large, reddish object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 940 AU at aphelion and takes 11,400 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt because its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, sometimes termed "distant detached objects" (DDOs), which also may include the object 2000 CR105 , which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years. [139] Brown terms this population the "inner Oort cloud" because it may have formed through a similar process, although it is far closer to the Sun. [140] Sedna is very likely a dwarf planet, though its shape has yet to be determined. The second unequivocally detached object, with a perihelion farther than Sedna's at roughly 81 AU, is 2012 VP 113 , discovered in 2012. Its aphelion is only half that of Sedna's, at 400–500 AU. [141] [142]

Oort cloud

The Oort cloud is a hypothetical spherical cloud of up to a trillion icy objects that is thought to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year (ly)), and possibly to as far as 100,000 AU (1.87 ly). It is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way. [143] [144]


Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU. [145] Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun. [146] Objects may yet be discovered in the Solar System's uncharted regions.

Currently, the furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun, but as the Oort cloud becomes better known, this may change.

The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing more than 100 billion stars. [147] The Sun resides in one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur. [148] The Sun lies about 26,660 light-years from the Galactic Centre, [149] and its speed around the center of the Milky Way is about 247 km/s, so that it completes one revolution every 210 million years. This revolution is known as the Solar System's galactic year. [150] The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega. [151] The plane of the ecliptic lies at an angle of about 60° to the galactic plane. [i]

The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Its orbit is close to circular, and orbits near the Sun are at roughly the same speed as that of the spiral arms. [153] [154] Therefore, the Sun passes through arms only rarely. Because spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, this has given Earth long periods of stability for life to evolve. [153] However, the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth, according to the Shiva hypothesis or related theories. The Solar System lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life. [153] Even at the Solar System's current location, some scientists have speculated that recent supernovae may have adversely affected life in the last 35,000 years, by flinging pieces of expelled stellar core towards the Sun, as radioactive dust grains and larger, comet-like bodies. [155]


The Solar System is in the Local Interstellar Cloud or Local Fluff. It is thought to be near the neighbouring G-Cloud but it is not known if the Solar System is embedded in the Local Interstellar Cloud, or if it is in the region where the Local Interstellar Cloud and G-Cloud are interacting. [156] [157] The Local Interstellar Cloud is an area of denser cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light-years (ly) across. The bubble is suffused with high-temperature plasma, that suggests it is the product of several recent supernovae. [158]

There are relatively few stars within ten light-years of the Sun. The closest is the triple star system Alpha Centauri, which is about 4.4 light-years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the small red dwarf, Proxima Centauri, orbits the pair at a distance of 0.2 light-year. In 2016, a potentially habitable exoplanet was confirmed to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun. [159] The stars next closest to the Sun are the red dwarfs Barnard's Star (at 5.9 ly), Wolf 359 (7.8 ly), and Lalande 21185 (8.3 ly).

The largest nearby star is Sirius, a bright main-sequence star roughly 8.6 light-years away and roughly twice the Sun's mass and that is orbited by a white dwarf, Sirius B. The nearest brown dwarfs are the binary Luhman 16 system at 6.6 light-years. Other systems within ten light-years are the binary red-dwarf system Luyten 726-8 (8.7 ly) and the solitary red dwarf Ross 154 (9.7 ly). [160] The closest solitary Sun-like star to the Solar System is Tau Ceti at 11.9 light-years. It has roughly 80% of the Sun's mass but only 60% of its luminosity. [161] The closest known free-floating planetary-mass object to the Sun is WISE 0855−0714, [162] an object with a mass less than 10 Jupiter masses roughly 7 light-years away.

Comparison with extrasolar systems

Compared to many other planetary systems, the Solar System stands out in lacking planets interior to the orbit of Mercury. [163] [164] The known Solar System also lacks super-Earths (Planet Nine could be a super-Earth beyond the known Solar System). [163] Uncommonly, it has only small rocky planets and large gas giants elsewhere planets of intermediate size are typical—both rocky and gas—so there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). Also, these super-Earths have closer orbits than Mercury. [163] This led to the hypothesis that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection. [165] [166]

The orbits of Solar System planets are nearly circular. Compared to other systems, they have smaller orbital eccentricity. [163] Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined. [163] [167]

This section is a sampling of Solar System bodies, selected for size and quality of imagery, and sorted by volume. Some large objects are omitted here (notably Eris, Haumea, Makemake, and Nereid) because they have not been imaged in high quality.

  1. ^ ab As of August 27, 2019.
  2. ^Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar System" and "solar system" structures in their naming guidelines document. The name is commonly rendered in lower case ("solar system"), as, for example, in the Oxford English Dictionary and Merriam-Webster's 11th Collegiate Dictionary.
  3. ^ The natural satellites (moons) orbiting the Solar System's planets are an example of the latter.
  4. ^ Historically, several other bodies were once considered planets, including, from its discovery in 1930 until 2006, Pluto. See Former planets.
  5. ^ The two moons larger than Mercury are Ganymede, which orbits Jupiter, and Titan, which orbits Saturn. Although bigger than Mercury, both moons have less than half its mass. In addition, the radius of Jupiter's moon Callisto is over 98% that of Mercury.
  6. ^ abcde According to IAU definitions, objects orbiting the Sun are classified dynamically and physically into three categories: planets, dwarf planets, and small Solar System bodies.
    • A planet is any body orbiting the Sun whose mass is sufficient for gravity to have pulled it into a (near-)spherical shape and that has cleared its immediate neighbourhood of all smaller objects. By this definition, the Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Because it has not cleared its neighbourhood of other Kuiper belt objects, Pluto does not fit this definition. [5]
    • A dwarf planet is a body orbiting the Sun that is massive enough to be made near-spherical by its own gravity but that has not cleared planetesimals from its neighbourhood and is also not a satellite. [5] Pluto is a dwarf planet and the IAU has recognized or named four other bodies in the Solar System under the expectation that they will turn out to be dwarf planets: Ceres, Haumea, Makemake, and Eris. [6] Other objects commonly expected to be dwarf planets include Gonggong, Sedna, Orcus, and Quaoar. [7] In a reference to Pluto, other dwarf planets orbiting in the trans-Neptunian region are sometimes called "plutoids", [8] though this term is seldom used.
    • The remaining objects orbiting the Sun are known as small Solar System bodies. [5]
  7. ^ ab The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses), [19] the Kuiper belt (estimated at roughly 0.1 Earth mass) [20] and the asteroid belt (estimated to be 0.0005 Earth mass) [21] for a total, rounded upwards, of

37 Earth masses, or 8.1% of the mass in orbit around the Sun. With the combined masses of Uranus and Neptune (

New Dwarf Planet Candidate Discovered at Solar System's Edge: Could a Hidden Planet Be Orbiting Farther Out?

The discovery images of 2012 VP113. Three images of the night sky, each taken about two hours apart, were combined into one. The first image was artificially colored red, the second green, and the third blue. 2012 VP113 moved between each image as seen by the red, green, and blue dots. The background stars and galaxies did not move and thus their red, green, and blue images combine to show up as white sources. Image Credit/Caption: Scott Sheppard/Carnegie Institution for Science

A team of astronomers has announced the discovery of a new dwarf planet candidate called 2012 VP113 at the outermost edges of the Solar System, possibly confirming the existence of the Oort Cloud, the hypothetical vast reservoir of trillions of icy minor planetary bodies where comets are thought to originate. More interestingly, this discovery leaves the door open for the possibility of a bigger, more massive planet orbiting beyond the known reaches of the Solar System.

With all the small, icy Solar System bodies being discovered during the last 20 years beyond the orbit of Neptune, you could be forgiven for not knowing exactly where the Solar System ends. For decades, it seemed like it was a neatly organized place, consisting of two distinct planetary families: the terrestrial planets (Mercury, Venus, Earth, and Mars) closer to the Sun, the gas giants (Jupiter, Saturn, Uranus, and Neptune) farther out, and an asteroid belt in between the orbits of Mars and Jupiter separating the two groups. And then there was Pluto, an oddball celestial body whose highly inclined orbit beyond Neptune and very small size made it look more like a rogue moon than a full-blown planet. Nevertheless, and despite this apparent observed tidiness, astronomers had theorised for years that the left-over material of the Solar System’s formation would have formed a vast belt of icy planetoids extending beyond the orbit of Neptune, in essence constituting a new, third class of Solar System objects which they called the “ Kuiper Belt .”

An orbit diagram for the outer Solar System, showing the orbits of 2012 VP113 and Sedna, relative to those of the outer gas giant planets. Image Credit: Scott Sheppard, Department of Terrestrial Magnetism

The discovery of 1992 QB1 in 1992, a small object with a diameter of a few hundred kilometers, confirmed the existence of the Kuiper Belt and gave more credence to previously held notions among the scientific community that Pluto itself should not be considered a planet, rather a large member of a new class of objects orbiting in these distant reaches of the Solar System, called Kuiper-Belt Objects, or KBOs. The systematic discovery of thousands more of similar objects helped astronomers to determine that the boundaries of the Kuiper Belt extended from beyond the orbit of Neptune, 30 Astronomical Units from the Sun, to 50 AU away (1 AU is the average Earth-Sun distance, approximately 150 million km). Most importantly, the discovery of Eris in 2005, an object with a diameter the same as Pluto’s, forced the International Astronomical Union the following year to reclassify the then-ninth planet from the Sun as one of the largest members of the newly adopted “dwarf planet” category, alongside Eris and a handful of KBOs a bit smaller than Pluto that had been discovered in previous years.

One of these dwarf planets, named Sedna , caused quite a stir when it was discovered in 2003. Whereas the orbits of other dwarf planets like Pluto, Eris, Haumea, and Makemake were positioned inside the Kuiper Belt, Sedna’s orbit around the Sun was much more elongated, with an aphelion of 937 AU and a perihelion of 76 AU. This meant, according to many astronomers, that Sedna’s place of origin was the Oort Cloud, the hypothetical spherical cloud of trillions of comets and other icy planetesimals that envelops the Solar System and whose existence had been theorized by Dutch astronomer Jan Oort in 1950.

The recently discovered 2012 VP113 (affectionately called “Biden,” after the incumbent U.S. Vice President) is the second minor Solar System body besides Sedna to have such an elongated orbit. In fact, even though 2012 VP113’s orbit never brings it past 500 AU from the Sun, it nevertheless has a perihelion of 80 AU, even larger than Sedna’s.

These images show the discovery of 2012 VP113 taken about 2 hours apart on Nov. 5, 2012. The motion of 2012 VP113 stands out compared to the steady state background of stars and galaxies. Image Credit: Scott Sheppard/Carnegie Institution for Science

The discovery and overall research work that was supported and partially funded by NASA was made by Chadwick A. Trujillo, assistant astronomer of the Gemini Observatory in Hawaii, and Scott S. Sheppard, an astronomer at the Carnegie Institution for Science’s Department of Terrestrial Magnetism, in Washington, D.C. Both researchers are no strangers to new Solar System discoveries, with Sheppard being the co-discoverer of many dozens of small moons around Jupiter, Saturn, Uranus, and Neptune, while Trujillo is most famous for being a co-discoverer of Sedna and more than a dozen Kuiper Belt Objects, such as Eris, Haumea, Makemake, and Quaoar. Trujillo and Sheppard first observed 2012 VP113 during November 2012, with the Dark Energy Camera , or DECam, mounted on NOAO’s 4-meter Víctor M. Blanco Telescope in Chile. They later used the 6.5-m Magellan telescope at Carnegie’s Las Campanas Observatory to track its orbit over a series of months and gathered more details about its surface features. “The detection of 2012 VP113 confirms that Sedna is not an isolated object instead, both bodies may be members of the inner Oort cloud, whose objects could outnumber all other dynamically stable populations in the Solar System,” the researchers write in their study , titled “A Sedna-like body with a perihelion of 80 astronomical units” and published in the March 27 edition of Nature.

From their observations Trujillo and Sheppard calculated the object’s diameter to be no more than 450 km (approximately half that of Sedna) and consisting almost entirely of frozen volatiles, such as water, methane, and carbon dioxide. Although relatively small, it is possible that 2012 VP113 could have a round shape, thus qualifying as a dwarf planet candidate, according to Mike Brown , professor of planetary astronomy at Caltech and co-discoverer of Sedna and many Kuiper Belt objects: “Ice is not as hard as rock, so it less easily withstands the force of gravity, and it takes less force to make an ice ball round. The best estimate for how big an icy body needs to be to become round comes from looking at icy satellites of the giant planets. The smallest body that is generally round is Saturn’s satellite Mimas, which has a diameter of about 400 km. So somewhere between 200 and 400 km an icy body becomes round.”

An illustration showing the relative distances in Astronomical Units from the Sun of 2012 VP113 and Sedna relative to the rest Solar System planets. Image Credit: Nature

Yet the most fascinating aspect of the study is its implications for the possibility of more massive, distant objects orbiting the Sun inside the unknown expanses of the Oort Cloud. The reason for such speculation is the greatly elongated orbits of Sedna and 2012 VP113 themselves, which means something must be altering them. There are currently three hypotheses that explain how objects within the inner Oort Cloud could be perturbed: by the gravitational effects of a star passing close to the Sun, by a rogue extrasolar planet captured by the Sun’s gravity, or by a massive Solar System planet, orbiting hundreds of Astronomical Units away from the Sun, inside the inner Oort Cloud. The latter, according to theoretical models, seems the most probable hypothesis. Studies have shown that the close passage of stars or rogue extrasolar planets near the Sun would not have any noticeable gravitational effects on the region where Sedna and 2012 VP113 reside. These effects would be much greater on the outer part of the cloud, which is thought to extend from roughly 1,500 to 50,000 AU away from the Sun. The elongated orbits of Sedna and 2012 VP113 could then best be explained by the presence of hundreds of more massive planetary bodies, according to the study’s authors. “Some of these inner Oort cloud objects could rival the size of Mars or even Earth,” says Sheppard. “This is because many of the inner Oort cloud objects are so distant that even very large ones would be too faint to detect with current technology.”

The discovery of 2012 VP113 reveals the presence of a new realm within the Solar System waiting to be explored. Image Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

Searches for these distant objects have been ongoing in recent years. As detailed in a recent AmericaSpace article , NASA’s Wide-field Infrared Survey Explorer, or WISE, conducted two all-sky surveys at infrared wavelengths, with a sensitivity strong enough to detect a Jupiter-sized planet out to 26,000 AU and a Saturn-sized planet out to 10,000 A.U. Yet WISE found no traces of any such planets. But if the hypothesized objects in Trujillo’s and Sheppard’s study were smaller than Saturn, they would have been unobservable by WISE. Ultimately, the authors state that more objects similar to 2012 VP113 need to be discovered so that astronomers can better understand which model of the Oort Cloud’s evolution best fits the observations: “Each theory of inner Oort cloud object formation predicts different orbital configurations for the population. Therefore, as more inner Oort cloud objects are discovered, their orbits will provide strong constraints on the inner Oort cloud object formation models and thus our Solar System evolution.”

Despite being highly speculative, the notion of an undetected massive planet lurking somewhere inside the inner Oort Cloud makes the vast expanses lying beyond the orbit of Neptune much more interesting than previously thought. “The discovery of 2012 VP113 shows us that the outer reaches of our Solar System are not an empty wasteland as once was thought,” says Trujillo. “Instead, this is just the tip of the iceberg telling us that there are many inner Oort Cloud bodies awaiting discovery. It also illustrates how little we know about the most distant parts of our Solar System and how much there is left to explore.”

Currently, NASA’s New Horizons spacecraft is on its way to become the first ever man-made object to study Pluto and its moon, Charon, in July 2015. It is hoped that following the Pluto encounter, the spacecraft will continue on its journey while studying more objects within the Kuiper Belt, revolutionising our understanding of this largely unexplored region of the Solar System along the way. The exciting discovery of 2012 VP113 opens up a whole new realm of the Sun’s family for further, more detailed study.

What else is out there to explore? NASA, the ball is in your court now.

Want to keep up-to-date with all things space? Be sure to “Like” AmericaSpace on Facebook and follow us on Twitter: @AmericaSpace

Mass and size distribution

Despite its vast extent, the collective mass of the Kuiper belt is relatively low. The total mass is estimated to range between 1/25 and 1/10 the mass of the Earth. Conversely, models of the Solar System’s formation predict a collective mass for the Kuiper belt of 30 Earth masses. This missing >99% of the mass can hardly be dismissed, because it is required for the accretion of any KBOs larger than 100 km (62 mi) in diameter. If the Kuiper belt had always had its current low density, these large objects simply could not have formed by the collision and mergers of smaller planetesimals.[3] Moreover, the eccentricity and inclination of current orbits makes the encounters quite “violent” resulting in destruction rather than accretion. It appears that either the current residents of the Kuiper belt have been created closer to the Sun, or some mechanism dispersed the original mass. Neptune’s current influence is too weak to explain such a massive “vacuuming”, though the Nice model proposes that it could have been the cause of mass removal in the past. Although the question remains open, the conjectures vary from a passing star scenario to grinding of smaller objects, via collisions, into dust small enough to be affected by solar radiation. The extent of mass loss by collisional grinding is limited by the presence of loosely bound binaries in the cold disk, which are likely to be disrupted in collisions.

Bright objects are rare compared with the dominant dim population, as expected from accretion models of origin, given that only some objects of a given size would have grown further. This relationship between N(D) (the number of objects of diameter greater than D) and D, referred to as brightness slope, has been confirmed by observations. The slope is inversely proportional to some power of the diameter D:

d N d D ∝ D − q

>propto D^<-q>>
>propto D^ <-q>where the current measures[63] give q = 4 ±0.5.

This implies (assuming q is not 1) that

(The constant may be non-zero only if the power law doesn’t apply at high values of D.)

Less formally, if q is 4, for example, there are 8 (=23) times more objects in the 100–200 km range than in the 200–400 km range, and for every object with a diameter between 1000 and 1010 km there should be around 1000 (=103) objects with diameter of 100 to 101 km.

If q was 1 or less, the law would imply an infinite number and mass of large objects in the Kuiper belt. If 1<q≤4 there will be a finite number of objects greater than a given size, but the expected value of their combined mass would be infinite. If q is 4 or more, the law would imply an infinite mass of small objects. More accurate models find that the “slope” parameter q is in effect greater at large diameters and lesser at small diameters. It seems that Pluto is somewhat unexpectedly large, having several percent of the total mass of the Kuiper belt. It is not expected that anything larger than Pluto exists in the Kuiper belt, and in fact most of the brightest (largest) objects at inclinations less than 5° have probably been found.

For most TNOs, only the absolute magnitude is actually known, the size is inferred assuming a given albedo (not a safe assumption for larger objects).

Recent research has revealed that the size distributions of the hot classical and cold classical objects have differing slopes. The slope for the hot objects is q = 5.3 at large diameters and q = 2.0 at small diameters with the change in slope at 110 km. The slope for the cold objects is q = 8.2 at large diameters and q = 2.9 at small diameters with a change in slope at 140 km. The size distributions of the scattering objects, the plutinos, and the Neptune trojans have slopes similar to the other dynamically hot populations, but may instead have a divot, a sharp decrease in the number of objects below a specific size. This divot is hypothesized to be due to either the collisional evolution of the population, or to be due to the population having formed with no objects below this size, with the smaller objects being fragments of the original objects.

As of December 2009, the smallest Kuiper belt object detected is 980 m across. It is too dim (magnitude 35) to be seen by Hubble directly, but it was detected by Hubble’s star tracking system when it occulted a star.

The Kuiper Belt and a Mysterious World Hidden Beyond Pluto

Far from our Sun’s welcoming heat and brilliant light, there is a dark and distant domain inhabited by frigid, frozen bodies that are well-hidden from the prying eyes of curious observers. Indeed, astronomers are just beginning to explore this weird region of everlasting twilight that has, up until now, been so far away that it has not yielded its many enticing and bewitching secrets easily. This distant domain, in our Solar System’s outer limits, is called the Kuiper Belt, and the ice dwarf planet Pluto and its quintet of moons have–so far–been the only denizens of this region to have been visited by spacecraft. In June 2017, planetary scientists from the University of Arizona in Tucson, announced their new finding that the plane of our Solar System is warped in the outer fringes of the Kuiper Belt, revealing the possible presence of a Mars-to-Earth-mass world far beyond Pluto. This unknown, unseen “planetary mass object” may lurk far, far away, according to this new research on the orbits of minor planets to be published in the Astronomical Journal. This hidden, mysterious, and icy world would be different from, and much closer than, the so-called Planet Nine, whose existence yet awaits confirmation.

In the new research paper, Dr. Kat Volk and Dr. Renu Malhotra of the University of Arizona’s Lunar and Planetary Laboratory (LPL), present strong evidence of a yet-to-be-discovered world sporting a mass somewhere between that of our Earth and Mars. The authors show that this mysterious mass has given away its existence–for now–only by controlling the orbital planes of a population of icy space rocks known as Kuiper Belt Objects (KBOs), that do their mysterious, distant dance in the dark and frozen outskirts of our Solar System.

Our Solar System was born about 4.56 billion years ago, and the frozen denizens of the distant Kuiper Belt are the lingering leftovers of that ancient era. While most KBOs circle our Star with orbital inclinations (tilts) that average out to what planetary scientists term the invariable plane of the Solar System, the most distant KBOs do not. Indeed, Dr. Volk and Dr. Malhotra discovered that the most remote of these frozen objects show an average plane that is tilted away from the invariable plane by approximately eight degrees. This means that there is something unknown–something mysterious–haunting this frigid distant shadow-region that is warping the average orbital plane of the outer limits of our Solar System.

“The most likely explanation for our results is that there is some unseen mass. According to our calculations, something as massive as Mars would be needed to cause the warp that we measured,” explained Dr. Volk in a June 20, 2017 University of Arizona Press Release. Dr. Volk is a postdoctoral fellow at LPL and the lead author of the study.

The Kuiper Belt is situated beyond the orbit of Neptune, and it extends out to a few hundred Astronomical Units (AU). One AU is the average distance between Earth and Sun, which is about 93,000,000 miles. In a way that is analogous to the inner Solar System’s Main Asteroid Belt, situated between Mars and Jupiter, the Kuiper Belt plays host to a myriad of relatively small objects.

There are also a few frozen dwarf planets hiding in the secretive shadows of the Kuiper Belt. In this remote realm of twilight, our Sun shines with only a feeble stellar fire that can barely reach this frigid, remote shadow-region. Indeed, in this distant realm of darkness, our Sun appears to be only a particularly large star, swimming within a sparkling sea, swarming with countless other stars.

Our Solar System’s Deep Freeze

Although the Kuiper Belt bears a family resemblance to the Main Asteroid Belt, it is considerably more vast–it is about 20 times as wide and 20 to 200 times as massive as the Asteroid Belt. However, like the Astroid Belt, the Kuiper Belt is the home of a host of relatively small objects, that are tattle-tale primordial remnants of our Solar System’s birth. While many asteroids are made up of rock and metal, the majority of KBOs are composed mostly of frozen volatiles (“ices”), such as water, ammonia, and methane. The Kuiper Belt is also home to a trio of officially confirmed dwarf planets: Pluto, Haumea, and Makemake. Some of our Solar System’s most mysterious moons–such as Triton of Neptune and Saturn’s Phoebe–are frequently considered to be bodies that originated in the Kuiper Belt, but escaped from their birthplace long ago. Triton circles Neptune backwards, a clear clue that it is really a captured moon that was snared by its beautiful, blue, banded planet’s powerful gravity, only to become an adopted member of its family. Triton circles its adopted parent-planet in the wrong direction, and it is probably doomed to plunge, one day, into the waiting thick blanket of clouds that veil the blue planet that it has circled since it wandered away from its frozen birthplace in the Kuiper Belt.

The Kuiper Belt was named after the Dutch-American astronomer Gerard Kuiper, even though he did not predict its existence. In 1992, 1992 QB1 was detected, and it became the first KBO to be discovered since Pluto was spotted back in 1930 by the American astronomer Clyde Tombaugh. Since the discovery of 1992 QB1, the number of known KBO’s has skyrocketed to over a thousand, and more than 100,000 KBOs over 62 miles in diameter are believed to exist. Originally, astronomers thought that the Kuiper Belt was the main home for periodic comets, which are those with orbits lasting less that 200 years (short-period comets). However, more recent studies, dating from the mid-1990s, have shown that the Kuiper Belt is really dynamically stable, and that short-period comets’ true birthplace is in the scattered disc. The scattered disc is a dynamically active region created by the outward migration of Neptune 4.5 billion years ago. Scattered disc objects, such as Eris, sport extremely eccentric orbits that carry them as far as 100 AU from our Sun.

Another distant home for Solar System comets is the still hypothetical Oort Cloud. The Oort Cloud is thought to be approximately a thousand times more remote than the Kuiper Belt and is mostly spherical. The distant denizens of the Kuiper Belt, along with the frozen inhabitants of the scattered disc, are together termed trans-Neptunian objects (TNOs). The ice dwarf planet Pluto is the largest known and most massive constituent of the Kuiper Belt. It is also the largest and second-most-massive known TNO–after Eris–to dwell in the scattered disc. Pluto was originally classified as a major planet, but it was unceremoniously booted out of the pantheon of major Solar System planets–only to be reclassified as a dwarf planet in 2006. In composition, Pluto is similar to many other KBOs, and its orbital period is characteristic of a particular class of KBOs called “plutinos”. Plutinos are objects that share the same 2:3 resonance with Neptune.

After Pluto’s discovery in 1930, many astronomers suspected that it might not really be a solitary world in our Solar System’s dark and cold outer limits. The first astronomer to propose the existence of a trans-Neptunian population was Dr. Frederick C. Leonard. Shortly after Clyde Tombaugh spotted Pluto, Dr. Leonard considered whether it was “not likely that in Pluto there has come to light the first of a series of trans-Neptunian bodies, the remaining members of which still await discovery but which are destined eventually to be detected”. That same year, the astronomer Dr. Armin O. Leuschner proposed that Pluto “may be one of many long-period planetary objects yet to be discovered.”

In 1943, Dr. Kenneth Edgeworth proposed that in the outskirts of our Solar System, beyond Neptune, the material of the primeval solar nebula was too thin and dispersed to coagulate into planets. Instead, this frigid place evolved into the home of comparatively small bodies that sometimes were disturbed by the antics of other objects. These small, icy, dirty bodies, as a result, would occasionally wander far from their home, to become occasional invaders screeching into the inner Solar System–becoming brilliant comets with flashing, thrashing tails cutting brilliant capers in the sky.

In 1951, Gerard Kuiper published a paper in the journal Astrophysics: A Topical Symposium, where he speculated that a disc had developed early in our Solar System’s ancient formation. However, he did not think that such a belt could still exist. Kuiper was devising his theory on the erroneous assumption, common in his time, that Pluto was the same size as Earth and had, as a result, scattered these small frozen bodies out toward the Oort Cloud–or even out of our Solar System altogether. If Kuiper’s theory had been correct, there would not be a Kuiper Belt today.

In 1977, Charles Kowal discovered 2060 Chiron, a frozen planetoid with an orbit situated between the giant planets, Saturn and Uranus. In 1992, another icy object, 5145 Pholus, was detected in a similar orbit. Currently, a sizeable population of comet-like objects, named centaurs, do their distant dance in the region between Jupiter and Neptune. However, the orbits of the centaurs are not stable and, as a result, have dynamical lifetimes of only a few million years. From the time of Chiron’s discovery, astronomers have contemplated on the possibility that centaurs must be frequently replenished by some objects inhabiting an outer reservoir.

Additional evidence for the existence of the Kuiper Belt ultimately emerged from the study of comets. Comets are delicate, fragile, ephemeral objects with finite lifespans. As they wander towards the glaring light and melting heat of our Star, its stellar fires cause their volatile surfaces to sublimate into interplanetary space–slowly scattering them. In order for comets to continue to be visible over the 4.56 billion-year-existence of our Sun and its family, they must be replenished frequently. One such reservoir of icy replacements is the Oort Cloud, composed of a spherical swarm of frozen comets that reach out beyond 50,000 AU from our Sun. The existence of the Oort Cloud was first proposed by the Dutch astronomer Jan Oort in 1950. The still-hypothetical Oort Cloud is generally thought to be the origin of long-period comets. Long-period comets travel on orbits that can last thousands of years.

In 1987, astronomer Dr. David Jewitt, who was then at the Massachusetts Institute of Technoogy (MIT) in Cambridge, started to wonder about the strange emptiness of the outer Solar System. Because he was fascinated by this intriguing mystery, he encouraged his then-graduate student Dr. Jane Luu to help him hunt for the location of another object beyond Pluto’s orbit. Finally, after five years of searching, Dr. Jewitt and Dr. Luu announced on August 30, 1992 that they had discovered a candidate KBO (15760) 1992 QB 1. Six months later they found a second object in this distant, and previously unexplored, region of our Solar System.

A Mysterious World Is Hidden Beyond Pluto

For their recent paper, Dr. Volk and Dr. Malhotra analyzed the tilt angles of the orbital planes of over 600 remote, icy objects inhabiting the Kuiper Belt. The two planetary scientists did this in order to determine the common direction about which these frozen worldlets all precess. Precession is the term used to show change or “wobble” in the orientation of a rotating body.

“Imagine you have lots and lots of fast-spinning tops, and you give each one a slight nudge. If you then take a snapshot of them, you will find that their spin axes will be at different orientations, but on average, they will be pointing to the local gravitational field of Earth,” Dr. Malhotra explained in the June 20, 2017 University of Arizona Press Release. Dr. Malhotra is a Louise Foucar Marshall Science Research Professor and Regent’s Professor of Planetary Sciences at LPL.

“We expect each of the KBOs orbital tilt angle to be at a different orientation, but on average, they will be pointing perpendicular to the plane determined by the Sun and the big planets,” she added.

Imagine the average orbital plane of objects in the outer Solar System as a sheet. It should be very flat past 50 AU, according to Dr. Volk.

“But going further out from 50 to 80 AU, we found that the average plane actually warps away from the invariable plane. There is a range of uncertainties for the measured warp, but there is not more than a 1 or 2 percent chance that this warp is merely a statistical fluke of the limited observational sample of KBOs,” Dr. Volk explained in the June 20, 2017 University of Arizona Press Release.

This means that the effect is probably a real signal and not merely a statistical fluke. According to the new calculations, an object with the same mass as Mars orbiting approximately 60 AU from our Sun on an orbit tilted approximately eight degrees–to the average plane of the known planets–can exert a sufficiently powerful gravitational influence to warp the orbital plane of the remote KBOs within about 10 AU to either side.

“The observed distant KBOs are concentrated in a ring about 30 AU wide and would feel the gravity of such a planetary mass object over time. So hypothesizing one planetary mass to cause the observed warp is not unreasonable across that distance,” Dr. Volk continued to note.

This observation rules out the possibility that the proposed object could be the hypothetical Planet Nine. The possible existence of a Planet Nine is based on other observations. That planet is predicted to be about 10 times more massive than Earth, and much farther out at 500 to 700 AU.

“That is too far away to influence these KBOs. It certainly has to be much closer than 100 AU to substantially affect the KBOs in that range,” Dr. Volk added.

According to the International Astronomical Union’s (IAU’s) definition of a planet, in order for an object to be classified as a planet, it must have swept its orbit clean of minor planets–such as icy little KBOs. This is the reason why the authors refer to a “hypothetical planetary mass object” in their paper. In addition, the data also fail to rule out the possibility that the warp could really be the result of the presence of more than merely one planetary mass object.

So why haven’t they seen this elusive object yet? The most likely answer, according to Drs. Malhotra and Volk, is because they haven’t yet searched the entire sky for remote Solar System objects. The most probable place a planetary mass object could be lurking in secret would be in the galactic plane. The galactic plane is a region so densely filled with a myriad of stars that Solar System surveys have come to avoid it.

“The chance that we have not found such an object of the right brightness and distance simply because of the limitations of the surveys is estimated to be about 30 percent,” Dr. Volk commented in the June 20, 2017 University of Arizona Press Release.

One potential alternative to an unseen object, that could have rudely ruffled the plane of outer KBOs, would be a star that wandered too close to our Solar System in recent history–at least “recent” by astronomical standards.

“A passing star would draw all the ‘spinning tops’ in one direction. Once the star is gone, all the KBOs will go back tp precessing around their previous plane. That would have required an extremely close passage of about 100 AU, and the warp would be erased within 10 million years, so we don’t consider this a likely scenario,” Dr. Malhotra noted in the University of Arizona Press Release.

But humanity’s chance to see this elusive and mysterious object might come soon. Once the construction of the Large Synoptic Survey Telescope (LSST) is at last finished, it might provide astronomers with a precious peek into this shadowy, frigid, and faraway region of our Solar System’s outer limits. The LSST is run by a consortium, and is scheduled for first light in 2020. At that time the telescope will take unprecedented, real-time surveys of the sky–night after night after night.

According to Dr. Malhotra: “We expect LSST to bring the number of observed KBOs from currently about 2000 to 40,000. There are just a lot more KBOs out there–we just have not seen them yet. Some of them are too far and dim even for LSST to spot, but because the telescope will cover the sky much more comprehensively than current surveys, it should be able to detect this object, if it’s out there.”

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, and magazines. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.


In this paper, we analyse the attributes of some well-known objects in the solar system using clustering methods. Objects such as planets (with/without an estimate of Planet Nine), dwarf planets and some candidates for dwarf planets are considered. Different distances such as the Canberra, Jaccard, Lorentzian and Manhattan are adopted to calculate the relations between all objects based on their characteristics. The high dimensional information can thus be represented in a smaller number of dimensions via the maps generated by the hierarchical clustering and multidimensional scaling. The numerical measures and the computational tools allow the visualization of the solar system objects in a locus reflecting their characteristics.

Special Issue: Cosmic Dust IX

Frédéric Galliano , in Planetary and Space Science , 2017

3 Theoretical and methodological challenges

3.1 An emerging grain framework

Jones et al. (2017 , THEMIS) have accounted for the Planck results on the Milky Way to design a new model reproducing the interstellar extinction, non-polarized emission and elemental depletions. This grain model is entirely based on optical properties measured in the laboratory. One of its originalities is the use of amorphous carbons, potentially hydrogenated (noted a-C(:H)), carrying both aromatic and aliphatic features, in place of PAHs (see Li and Draine, 2012 Yang et al., 2013 , for constraints on the aliphatic-to-aromatic ratio in carbon grains). We refer to Jones (2014) for a presentation of a-C(:H). This dust model also contains a mixture of large silicates coated with aromatic mantles. This coating provides the necessary increased emissivity to account for the Planck flux constraints.

The left panel of Fig. 3 shows the dust mixture, heated by the solar neighborhood radiation field ( Mathis et al., 1983 intensity U = 1). The far-IR bump is emitted by the large grains (silicates and a-C(:H)), at thermal equilibrium with the radiation field. The mid-IR continuum is carried by out-of-equilibrium small a-C(:H) (radius ≲ 20 nm). The aromatic features are carried by the smallest a-C(:H) (radius ≲ 1.5 nm).

Fig. 3 . Dust SED modelling. Left panel: dust model of Jones et al. (2017) uniformly illuminated by the solar neighborhood radiation field (U = 1). Right panel: an example of SED fit. The observations are the open circles with error bars (simulated data, for demonstration). The filled circles are the synthetic photometry of the model. The total model is the sum of uniformly illuminated SEDs with radiation field intensity ranging between U min and U max (rainbow curves).

3.2 Accounting for the mixing of physical conditions

Such a dust model can not be used, as is, to fit the SED of galaxies, since there can be significant mixing of physical conditions in the beam or along the line of sight. Ideally, we should model the radiative transfer inside the object, but we usually lack the knowledge of its actual 3D structure. An alternative is to empirically account for the mixing, focussing only on quantities that are weakly dependent on radiative transfer processes. The prescription of Dale et al. (2000) , assuming that the radiation field intensity, U, within the studied region follows a power-law distribution, is commonly used:

It is demonstrated on the right panel of Fig. 3 . The three parameters of the distribution ( α , U min , U max ) are constrained by the position and width of the far-IR peak. The dust mass Mdust is a scaling factor. More complex distributions are also found in the literature (e.g. Draine and Li, 2007 ).

The far-IR peak is mainly emitted by large grains at thermal equilibrium. Thus, its spectrum does not depend on the spectral shape of the incident radiation field, as it depends only on the total absorbed power. On the contrary, the spectral shape is important for small stochastically heated grains, as their temperature distribution depends on the mean photon energy. Fortunately, these grains do not account for a large fraction of the mass. Notice that the fraction of small grains can not be unambiguously constrained with the approach of Eq. (1) , as it is degenerate with the fraction of hot equilibrium grains (i.e. U > r s i m 100 ). However, it is not the case for the carriers of the aromatic features, since they are effectively heated by a narrower wavelength range of photons ( λ > r s i m 0.0912 μ m e.g. Draine, 2011 ).

In summary, assuming a given dust mixture composition (e.g. Jones et al., 2017 ), this type of model provides constraints on the dust mass, average radiation field intensity and fraction of aromatic feature carrying grains, but can not provide any information on the topology of the ISM.

3.3 Fitting biases

SED models being highly non-linear, several degeneracies and biases are encountered with a classical χ 2 minimization fit. This is weel-known for modified black body (MBB) models, where the monochromatic luminosity is parameterized by the dust mass, Mdust, the equilibrium temperature, Tdust, and the emissivity index, β:

where κ0 is the opacity at wavelength λ0 = 160 μm, and Bν, the Planck function. Shetty et al. (2009) have demonstrated that single modified black body least-squares fits of an ensemble of pixels lead to biases and false correlations between Tdust and β. This is also true for more complex models like the one presented in Sect. 3.2 ( Galliano, 2017 ). In the case of a modified black body, Kelly et al. (2012) have demonstrated that hierarchical Bayesian (HB) inference could remove these biases and false correlations. In simple terms, the HB method consists in sampling the multidimensional probability distribution of the physical parameters of each pixel, and of some “hyperparameters”. These hyperparameters are the mean and covariance matrix of the parameter distribution. In other words, this inference combines physical modelling (SED fit) with statistical modelling (inferring the distribution of parameters). Several other studies have discussed such an approach ( Veneziani et al., 2013 Juvela et al., 2013 Shetty et al., 2016 ). We have developed a new code (HerBIE Galliano, 2017 ) implementing the HB method for the more realistic dust models of Sect. 3.2. Galliano (2017) has performed extensive tests of this model, applying it to simulated data, in order to assess its performances. We have shown that such a code can always recover the true parameters better than a least-squares method, for different signal-to-noise ratios, sample sizes, SED shapes and the distributions of physical parameters. We have not found any bias in the model's results.

There are also biases induced by our ignorance of the origin of certain physical processes. In particular, the “submm excess” is an emission excess particularly strong beyond 500 μm in low-metallicity environments ( Galliano et al., 2003, 2005 Galametz et al., 2009 ), but it has also been detected in the Milky Way ( Reach et al., 1995 Paradis et al., 2012 ). It can not be accounted for by regular dust models, free-free, synchrotron and molecular line emission. Its origin is still debated: (i) very cold dust, although unlikely ( Galliano et al., 2011 ) (ii) magnetic grains ( Draine and Hensley, 2012 ) (iii) temperature dependent grain emissivity ( Meny et al., 2007 ) (iv) intrinsic grain optical properties ( Coupeaud et al., 2011 ). The only way to avoid this excess is to not use constraints beyond 500 μm. It is important to note that there is a degeneracy in the submm range between the radiation field distribution (Eq.), the intrinsic emissivity slope of the grain mixture and this possible excess.

3.4 Comparison with the gas tracers

Despite these uncertainties, careful modelling can lead to consistency with the gas tracers. For example, Chevance et al. (2016) modelled the PDR in the massive star forming region 30 Doradus of the LMC. One of the derived parameters of this study is the V -band extinction in each pixel A V P D R . This A V P D R was found in very good pixel-to-pixel agreement with the A V d u s t derived from SED fitting in the same region. Similarly, Wu et al. (2015) modelled the spatially resolved spectral line energy distribution of CO within the central region of M 83. The derived gas pressure was found to correlate well with the average radiation field derived from SED fitting in the same pixels. These studies demonstrate that we can find good agreements between quantities derived from different sets of data, with different physical models.

For that reason, dust is often used as a mean to study the “dark gas” ( Wolfire et al., 2010 ). This gas component can indeed be mainly traced by γ-rays ( Grenier et al., 2005 ) or dust emission ( Planck Collaboration et al., 2011 ). It is probably a combination of optically thick H i gas and CO-dissociated H2 gas ( Bolatto et al., 2013 ), and can account for a significant fraction of the gas budget.

Scattered disc is out by the Kuiper belt.

The Scattered disc is a distant region of our solar system. The scattered disc is thinly populated by icy minor planets known as scattered disc objects (SDOs), a subset of the broader family of trans-Neptunian objects (TNOs). The innermost portion of the scattered disc overlaps with the Kuiper belt. The scattered disc's outer limits extend much farther away from the Sun and above and below the ecliptic than the belt proper.

Formation of the scattered disc.

The scattered disc is still poorly understood, although prevailing astronomical opinion suggests it was formed when Kuiper belt objects (KBOs) were "scattered" by gravitational interactions with the outer planets, principally Neptune, into highly eccentric and -inclined orbits. While the Kuiper belt is a relatively "round" and "flat" doughnut of space extending from about 30 AU to 44 AU with its member-objects locked in autonomously circular orbits (cubewanos) or mildly-elliptical resonant orbits (Plutinos and Twotinos), the scattered disc is by comparison a much more erratic milieu. SDOs can often, as in the case of Eris, travel almost as great a "vertical" distance as they do relative to what has come to be defined as "horizontal". Orbital simulations show SDO orbits may well be erratic and unstable and that the ultimate fate of these objects is to be permanently ejected from the core of the solar system into the Oort cloud or beyond.

There is an emerging sense that Centaurs may simply be objects just like SDOs that were knocked inwards from the Kuiper belt rather than outwards, making them simply "cis-Neptunian" SDOs. Indeed, some objects like (29981) 1999 TD10 blur the distinction, and the Minor Planet Center (MPC) now lists centaurs and SDOs together. In recognition of this blurring of categorization, some scientists use "scattered kuiper belt object" (or SKBO) as an umbrella term for both centaurs and member bodies of the scattered disc.

Although the TNO 90377 Sedna is officially considered an SDO by the MPC, its discoverer Michael E. Brown has suggested that because its perihelion distance of 76 AU is too distant to be affected by the gravitational attraction of the outer planets it should be considered an inner Oort cloud object rather than a member of the scattered disc . This line of thinking suggests that a lack of gravitational interaction with the outer planets disqualifies a TNO from scattered disc membership, which would create an outer edge somewhere between Sedna and more conventional SDOs like Eris. If Sedna is beyond the scattered disc, it may not be unique 2000 CR105, which was discovered before Sedna, may also be an inner Oort cloud object or (more likely) a transitional object between the scattered disc and the inner Oort cloud.

Such objects referred to as Detached, have orbits which cannot be created by Neptune scattering. Instead, a number of explanations have been put forward including a passing star (Morbidelli 2004) or a distant, planet-sized object (Gomes 2006) See Sedna.

Orbits of the scattered disc.

The first SDO to be recognized was (15874) 1996 TL66, first identified in 1996 by astronomers based at Mauna Kea. The first object presently classified as an SDO to be discovered was (48639) 1995 TL8, found by Spacewatch.

The diagram on the right illustrates the orbits of all known scattered disc objects up to 100AU together with Kuiper belt objects (in grey) and resonant objects (in green). The eccentricity of the orbits is represented by segments (extending from the perihelion to the aphelion) with the inclination represented on Y axis.

Typically, the scattered objects are characterised by orbits with medium and high eccentricities but their perihelia bring them no closer than 35AU, clear from direct influence of Neptune (red segments). Plutinos (grey segments for Pluto and Orcus) as well as resonant object at 2:5 (in green) can approach Neptune closer as their orbits are protected by resonances. This perihelion > 35 AU condition is actually one of the defining characteristics of scattered objects.

Scattered disc: Extremes.

The scattered disc is the place where extreme eccentricity and high inclination appears to be the norm and circular orbits are exceptional. Some exceptional orbits are plotted in yellow

    1999 TD10 has an orbit with extreme eccentricity (

scattered disc: Some order in the chaos?

Resonant objects (shown in green), are not considered to be members of the scattered disc. Minor resonances are also populated and some computer simulations show that many objects could be actually on weak, higher order resonances (6:11,4:9,3:7,5:12,3:8,2:7,1:4). Quoting one of the researchers : the scattered disc might not be so scattered after all.

Scattered objects versus classical objects

The inserts in the diagram on the right compare the eccentricity and inclination of the scattered disc population to the cubewanos. Each small coloured square represents a given range for both the eccentricity e and the inclination i 1 . The relative number of objects within the square is represented with cartographic colours 2 (from small numbers plotted as green valleys to brown peaks).

The two populations are very different: more than 30% of all cubewanos are on low inclination, near circular orbits (the low bottom corner 'peak') and their eccentricity peaks at 0.25. Scattered objects on the other hand are, well, scattered. The majority of the known population have medium eccentricity in 0.25-0.55. Two local peaks correspond to e in the 0.25--0.35 range, inclination 15-20 o and e=0.5--0.55, low i<10 o respectively. The extreme orbits show up as outliers in grey. Characteristically, there are no known SDO objects with eccentricity lower than 0.3 (with the exception of 2004 XR190).

It is the eccentricity, more than the orbit's inclination, that is the distinctive attribute of the family of scattered objects.

1 As near-circular orbits occupy the first column (e<0.05) and the orbits with the lowest inclination (i<5 degrees) occupy the lowest row, the square in the bottom left corner represents the number of near circular, very lowly inclined orbits.

2 A grey square represents a single object (an outlier) in this range.

Orbit plots of the scattered disc.

More traditional, the graph on the left represents polar and ecliptic views of the (aligned) orbits of the scattered disc objects (in black) on the background of cubewanos (in blue) and resonant (2:5) objects (in green). As yet unclassified objects in 50-100AU region are plotted in grey 1 .

The solid blue ring is not an artist's representation but a real plot of hundreds of overlapping orbits of the classical objects, fully deserving the name of the main (classical or cubewanos) belt. The minimum perihelion mentioned above is illustrated by the red circle. Unlike SDOs, the resonant objects approach Neptune&rsquos orbit (in gold) .

On the ecliptic view, the arcs represent the same minimum perihelion 2 of 35AU (red) and Neptune&rsquos orbit (at

30AU, in yellow). As this view illustrates, the inclinations alone do not really distinguish SDO from the classical objects. Instead, the eccentricity is the distinctive attribute (long aphelion segments).

1 For roughly a half of known TNO the orbits are not yet known with the precision sufficient for the classification (a particularly delicate task for resonant objects).

2 The precise value is not too important the value of 35 AU is quoted for coherence with Jewitt. Other authors prefer to use 30AU instead while the data used here appear to fit 34AU.

Detached objects, or an extended scattered disc?

The recently discovered objects 2000 CR105 with a perihelion too far away from Neptune to be influenced by it, led to a discussion among astronomers about a new minor planet set, called the Extended scattered disc (E-SDO, Gladman). More recently, these objects are referred to as detached objects (Jewitt,Delsanti) or Distant Detached Objects (DDO, Gomes et al..

The classification suggested by Deep Ecliptic Survey team, introduces a formal distinction between Scattered-Near objects (which could be scattered by Neptune) from Scattered-Extended objects (e.g. 90377 Sedna) using Tisserand's parameter value of 3.

The diagram illustrates all known scattered and detached objects together with the largest Kuiper belt objects for reference. The very large eccentricities of Sedna and (87269) 2000 OO67 are partly shown with the red segments, extending from the perihelion to the aphelion, well outside the diagram (>900AU and >1020AU respectively).

1 Note that the positions on the diagram represent semi-major axis (mean distance to the Sun) and not the current positions of the objects. Sedna is currently actually closer than Eris.