# Why isn't Eris considered a planet despite being the body of dominant mass?

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The 2006 definition of a planet states that a planet is a celestial body that

(a) is in orbit around the Sun

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape

(c) has cleared the neighbourhood around its orbit.

Point c is why Pluto allegedly is no longer a planet. However some "proponents" of the definition, namely those who use logic, figured out that according to this vague definition, if we take it literal and set a arbitrary border on "clearing the orbit", then either Pluto is still a planet or we have no planets at all. Therefore these guys actually set up an own definition, thereby replacing point c by the requirement that a planet must be the dominant mass in its orbit. This is not what the 2006 definition is telling, so these anti-Plutoers aren't actually proponents of the 2006 definition. They have their own definition according to which really only Pluto is no planet because Neptune dominates its orbit.

However their definition still doesn't exclude Eris from planethood. Its orbit is far from any of the eight recognized planets. From a vertical point of view, Eris' orbit intersects with that of Pluto. However Eris' orbit is highly tilted to the ecliptic and the two bodies don't really come close enough that their gravities would influence each other strongly. Eris is more massive than any Kuiper belt object (including Pluto). Eris itself is not really a KBO because it only intersects the Kuiper belt while most of its year being outside it. So there is no reason to ban Eris from planethood if point c states (according to some anti-Plutoers) that a planet must be the dominant mass throughout its orbit. So why don't these guys consider Eris a ninth planet?

You are correct that the IAU definition of "clearing the orbit" has the problem of being not explicitly quantified. And a complete clearing was obviously never the intention behind the definition. I like this statement by Steven Soter:

The IAU definition of a planet as a heliocentric body that "has cleared the neighborhood around its orbit” is problematic. Clearance is never complete because the asteroid and comet reservoirs are leaky, and resonant planet-crossing orbits can be stable. A more accurate criterion for planetary status is "dynamical dominance". An object is dynamically dominant if it sweeps up or scatters other objects from its orbital zone in a time much less than the age of the system (theoretical criterion), and/or if its mass is much greater than the total mass of all other objects in its orbital zone (observational criterion), where "orbital zone” can also be simply quantified.

However, there really are substantial difference in between Eris/Pluto and the regular planets concerning their dominance in their respective orbits. The Wikipedia article on "Clearing the neighbourhood" has a neat overview over a few of the proposed ways to make this difference more quantifiable.

Probably the most straight forward definition is the fraction $$mu = M/m$$ between the mass of the object in question $$M$$ and the combined mass of other objects in its orbits zone $$m$$. But there are other options as well:

$$egin{array} { r |r |r |r |r |r |r |r } mathrm{ Name} & mathrm{Margot's ,,} Pi & mathrm{Soter's ,, } mu & mathrm{Stern-Levison, parameter ,, } Lambda & mathrm{Mass} (kg) hline Jupiter & 4.0 imes 10^4 & 6.25 imes 10^5 & 1.30 imes 10^9 & 1.8986 imes 10^{27} Saturn & 6.1 imes 10^3 & 1.9 imes 10^5 & 4.68 imes 10^7 & 5.6846 imes 10^{26} Venus & 9.5 imes 10^2 & 1.3 imes 10^6 & 1.66 imes 10^5 & 4.8685 imes 10^{24} Earth & 8.1 imes 10^2 & 1.7 imes 10^6 & 1.53 imes 10^5 & 5.9736 imes 10^{24} Uranus & 4.2 imes 10^2 & 2.9 imes 10^4 & 3.84 imes 10^5 & 8.6832 imes 10^{25} Neptun & 3.0 imes 10^2 & 2.4 imes 10^4 & 2.73 imes 10^5 & 1.0243 imes 10^{26} Mercury & 1.3 imes 10^2 & 9.1 imes 10^4 & 1.95 imes 10^3 & 3.3022 imes 10^{23} Mars & 5.4 imes 10^1 & 5.1 imes 10^3 & 9.42 imes 10^2 & 6.4185 imes 10^{23} Ceres & 4.0 imes 10^{−2} & 0.33 & 8.32 imes 10^{−4} & 9.43 imes 10^{20} Pluto & 2.8 imes 10^{−2} & 0.08 & 2.95 imes 10^{−3} & 1.29 imes 10^{22} Eris & 2.0 imes 10^{−2} & 0.10 & 2.15 imes 10^{−3} & 1.67 imes 10^{22} Haumea & 7.8 imes 10^{−3} & 0.02 & 2.41 imes 10^{−4} & 4.0 imes 10^{21} Makemake & 7.3 imes 10^{−3} & 0.02 & 2.22 imes 10^{−4} & 4.0 imes 10^{21} end{array}$$

In this paper by Sober he also made some nice plots showing the categorical difference between the two types of objects depending on their M, m and their semi-major axis a:

So, you see, while the IAU definition of "clearing an orbit" really is ambiguous in its interpretation, following its intent, there are quantifiable ways to show the existing difference between planets and dwarf-planets.

Because the "orbital neighbourhood" of Eris (and Ceres and Pluto) is defined as the entire Kuiper belt or in Ceres' case the entire Main belt in the planetary dominance discriminants user SpaceBread is showing. If the IAU defined the orbital dominance region as the body's hill sphere all three bodies would have to be classified as planets. Except if they took a strict approach that no body must even cross their hill sphere, in such case there wouldn't exist any planets at all. And they'd perhaps take the strict approach since, as you say, Eris actually isn't part of the Kuiper belt but just crosses it. Therefore, the IAU took the approach to call the entire belts their "orbital region" and from a 2-dimensional point of view rather than a 3-dimensional one (despite Pluto's and Eris' orbits being highly inclined to the ecliptic) in order to fit their definition of 2006.

According to the argument by some you provide, that another body dominates more than the three bodies, you're right that Neptune dominates Pluto's orbit but one might also conclude that Jupiter dominates Ceres' and farther on that Neptune dominates the entire Kuiper belt and bodies crossing it (like Eris). Still, Eris is the most massive known object beyond Neptune.

## Why Pluto Is a Planet, and Eris Is Too (Op-Ed)

Tim DeBenedictis is the lead developer of the SkySafari line of iOS and Android apps at Simulation Curriculum, the makers of Starry Night, SkySafari and the free Pluto Safari app. DeBenedictis has been writing astronomy software since high school, and graduated from MIT in 1993 with a degree in earth, atmospheric and planetary science. Passionate about space, DeBenedictis is self-taught in mechanical and electrical engineering, and has launched his own private microsatellite into space. He contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

The International Astronomical Union (IAU) got it wrong. Our solar system has 10 planets.

As NASA's New Horizons spacecraft glides its way to the cold outer reaches of our solar system to take the first-ever up-close look at Pluto, the time is right to revise the International Astronomical Union (IAU)'s 2006 definition of a planet, which resulted in Pluto's "demotion" from planet to ambiguous dwarf-planet status.

## Why Is Pluto No Longer Considered a Planet?

Since its discovery in 1930, Pluto has been a bit of a puzzle:

• It's smaller than any other planet -- even smaller than Earth's moon.
• It's dense and rocky, like the terrestrial planets (Mercury, Venus, Earth and Mars). However, its nearest neighbors are the gaseous Jovian planets (Jupiter, Saturn, Uranus and Neptune). For this reason, many scientists believe that Pluto originated elsewhere in space and got caught in the sun's gravity. Some astronomers once theorized that Pluto used to be one of Neptune's moons.
• Pluto's orbit is erratic. The planets in our solar system all orbit the sun in a relatively flat plane. Pluto, however, orbits the sun at a 17-degree angle to this plane. In addition, its orbit is exceptionally elliptical and crosses Neptune's orbit.
• One of its moons, Charon, is about half Pluto's size. Some astronomers have recommended that the two objects be treated as a binary system rather than a planet and satellite.

These facts contributed to the long-running debate over whether to consider Pluto a planet. On Aug. 24, 2006, the International Astronomical Union (IAU), an organization of professional astronomers, passed two resolutions that collectively revoked Pluto's planetary status. The first of these resolutions was Resolution 5A, which defines the word "planet." Although many people take the definition of "planet" for granted, the field of astronomy had never clearly defined what is and is not a planet.

Here's how Resolution 5A defines a planet:

Pluto is relatively round and orbits the sun, but it does not meet the criteria because its orbit crosses Neptune's orbit. Critics of the resolution argue that other planets in the solar system, including Earth, have not cleared the neighborhood around their orbits. Earth, for example, regularly encounters asteroids in and near its orbit.

Resolution 5A also established two new categories of objects in orbit around the sun: dwarf planets and small solar-system bodies. According to the resolution, a dwarf planet is:

Small solar-system bodies are objects that orbit the sun but are neither planets nor dwarf planets. Another resolution, Resolution 6A, also specifically addresses Pluto, naming it as a dwarf planet.

Not all astronomers supported Resolutions 5A and 6A. Critics have pointed out that using the term "dwarf planet" to describe objects that are by definition not planets is confusing and even misleading. Some astronomers have also questioned the resolutions' validity, since relatively few professional astronomers had the ability or opportunity to vote (less than 4 percent of the world's astronomers and planetary scientists voted.)

Here's how the two resolutions classified the objects in orbit around our sun:

• Planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
• Dwarf planets: Pluto, Ceres (an object in the asteroid belt between Mars and Jupiter), 2003 UB313 (an object farther from the sun than Pluto)
• Small solar-system bodies: Everything else, including asteroids and comets

But this may not be the last word on Pluto. In 2014, after a debate among scientists sponsored by the Harvard-Smithsonian Center for Astrophysics, the majority of the nonexpert audience voted for a simpler definition of planet — basically, that it had to be spherical and orbit around a star or the remnants of one — that included Pluto, according to an article on the center's website.

A 2019 paper, authored by a number of distinguished planetary scientists, concluded that the argument made back in 2006, that Kuiper Belt Objects should be classified as non-planets was "arbitrary," based on their assessment of 200 years' worth of studies. The paper argued that none of the studies (expect for one paper) talked about the non-sharing of an orbit as a criterion for distinguishing planets from asteroids. You can read more about the case to reinstate Pluto at our article "Pluto: Is It a Planet After All?"

## Contents

The article's use of the IAU definition of "planet" has too much weight the entire article is structured around it. As has been said at this link, the IAU's definition is not the "final word". The geophysical definition of "planet" (in which moons such as Ganymede are considered planets) should be given more weight. Possibly a major rewrite of the article should be done by someone to address this problem. For example, the first paragraphs of the article are very IAU-centric and should include a non-IAU perpsective. LumaP15 (talk) 01:04, 3 September 2018 (UTC)

Again, the IUGS would have to approve it before it could be added here. Serendi pod ous 07:33, 3 September 2018 (UTC) New to this discussion here. Why would the IUGS be the final authority? Grinspoon and Stern https://www.washingtonpost.com/news/speaking-of-science/wp/2018/05/07/yes-pluto-is-a-planet/?utm_term=.1bde2f497aa0 and other scientists https://phys.org/news/2017-02-geophysical-planet-definition.html argue that different definitions should be acceptable depending on context. The topic is described as controversial in the community (example in Phys.org) and books are written that emphasize that dwarf planets are also planets https://www.scholastic.com/teachers/books/13-planets-by-david-a-aguilar/. Highlighting the proposed "geophysical definition" https://www.hou.usra.edu/meetings/lpsc2017/pdf/1448.pdf would probably be going too far, but I think it would be good to have a small section on the controversy behind the definition. It's not universally accepted! WorldsWanderer (talk) 01:41, 4 September 2018 (UTC) Yeah. I'm sure they do. But they're two people. One of whom has a VERY personal interest in this particular issue. Only designated authorities have the final say on issues of nomenclature. For astronomy, that's the IAU for geology, i's the IUGS. Believe me, if the IUGS voted to accept the definition, it would go in here topswitch. But it hasn't. Serendi pod ous 09:07, 4 September 2018 (UTC) I respectfully disagree with your view of the relation between IUGS and science. If your look at their mission they state that they are involved in ("fostering international agreement on nomenclature and classification. "). Fostering is not the same as legislating. Scientists are under no obligation to follow their decisions. The same goes for the IAU although I admit I didn't find an equivalent statement after a quick search. As for the motivation, please don't impart motivations to one of the authors (I assume you mean Stern). Yes, he's been an advocate for Pluto to be recognized at the same level as other planets. You may argue that it's because he's the PI of the New Horizon mission. However, New Horizon is en route to Ultima Thule. He's not advocating for that to be planet. Others on the team have strongly supported the IAU definition (e.g. Rick Binzel). One of the most vocal supporters of the IAU definition is (Mike Brown), whose Tweeter handle is "Plutokiller" and wrote a book "How I killed Pluto". Could he have a vested interest in the topic? Also, the issue is not just about Pluto: all dwarf planets would be included in any of the other definitions. There are other opponents of the IAU definition (see the previous references I listed, and thanks for pointing the better way to include a reference in the Talk page). I don't want to take down the IAU definition, but I think we do a disservice to the Wikipedia users if we don't recognize that this definition is not universally accepted by scientists. A significant fraction of the community does not agree with the perception that there are two "levels" of planets and has gathered several times to discuss the issue. WorldsWanderer (talk) 20:49, 6 September 2018 (UTC) I think the current article makes clear the changing definitions of "planet" (which once included the sun and moon). The IAU definition is supported by major dictionaries. Dbfirs 21:18, 6 September 2018 (UTC) I Agree with WorldsWanderer the non-IAU definition is worth at least a mention on the article even if it doesn't have the same weight as the IAU definition. People reading the article should at least be aware that the issue of planetary definition isn't a settled one. Just because the IAU is an authority doesn't necessarily mean they're right. LumaP15 (talk) 00:08, 7 September 2018 (UTC) Just because the IAU is an authority doesn't necessarily mean they're right.

It does for Wikipedia's purposes. We seek verifiability, not truth. – Juliancolton | Talk 00:16, 7 September 2018 (UTC)

Why isn't Eris among the objects formerly considered planets in that section? Eris was considered a planet by many including NASA and still is by people rejecting the IAU definition. 212.186.0.174 (talk) 06:08, 9 August 2019 (UTC)

Not officially and for a very short time only. Ruslik_ Zero 06:53, 9 August 2019 (UTC) Eris is still officially recognized by NASA as a dwarf planet. So technically is still a planet just smaller and belongs on this page. [1] Liebecon (talk) 03:13, 9 May 2021 (UTC)

15760 Albion (image on the right) and (181708) 1993 FW are the two bodies in question.

"two newly discovered planets are called Smiley and Karla."ISBN 978-1-84239-912-5

I searched those two bodies up and it turns out they refer to the bodies listed at the very top, discovered by David C. Jewitt and Jane X. Luu. These bodies do orbit the Sun and do appear to be round despite their small dimensions. They may not have cleared their Kuiper Belt neighbourhood (this is being debated on, see clearing the neighbourhood#Disagreement) but unlike Pluto their orbits appear satisfyingly circular and not as eccentric.

I know I and my source are probably the only ones in the world shouting about these two bodies being planets, but unlike Pluto I classify them both as legitimate planets in a ten-planet Solar System. Should we debate over these being planets? Honestly, even though I see Pluto as a dwarf planet or even a binary comet or asteroid I don't think we should always follow the IAU.

They're not planets they're not even dwarf planets. Whoever called them planets was being either careless or facecious. And in future, don't put refs in talk page comments they stay at the bottom no matter how long the page gets. Serendi pod ous 09:22, 9 October 2019 (UTC)

An editor has asked for a discussion to address the redirect Escaping the Earth. Please participate in the redirect discussion if you wish to do so. Steel1943 (talk) 19:16, 23 January 2020 (UTC)

An editor has asked for a discussion to address the redirect Planet-sam. Please participate in the redirect discussion if you wish to do so. Steel1943 (talk) 19:18, 23 January 2020 (UTC)

1. Add the template to the section "Objects formerly considered planets" linking to the main article: List of former planets 2. Change "was ruled by Aphrodite, the goddess of love" in section "Mythology and naming" to "was ruled by Aphrodite, the goddess of love, and Hera, the queen of the gods" with source [2] thanks 2407:7000:A2AB:D00:51C0:E732:65B0:DA84 (talk) 06:23, 17 May 2020 (UTC)

1. ^ NASA. "Eris". NASA.gov . Retrieved 05/08/2021 . Check date values in: |access-date= (help)
2. ^https://www.theoi.com/Summary/Hera.html

A discussion is taking place to address the redirect Space Planet. The discussion will occur at Wikipedia:Redirects for discussion/Log/2020 May 18#Space Planet until a consensus is reached, and readers of this page are welcome to contribute to the discussion. TheAwesome Hwyh 13:24, 18 May 2020 (UTC)

Not done: it's not clear what changes you want to be made. Please mention the specific changes in a "change X to Y" format and provide a reliable source if appropriate. JTP (talk • contribs) 07:11, 3 July 2020 (UTC)

I think this might be a good general ref for either this article, definition of 'planet', or our article on the IAU definition of 'planet'. I don't see it in any of them, but IMO it might make a useful counterweight to the claims that the IAU didn't adequately consider astronomer's views.

Ron Ekers (2018) 'The Prague IAU General Assembly, Pluto and the IAU processes'. In C. Sterken, J. Hearnshaw & D. Valls-Gabaud, eds., Under One Sky: The IAU Centenary Symposium. Proceedings, IAU Symposium No. 349.[1]

It does note that there were 3 conceptions of 'planet' under consideration, something that seems to have gotten lost in our articles -- the traditional 9 plus Ceres and a couple more TNOs, the Classical 8, and the Sterns concept including moons. — kwami (talk) 05:40, 24 October 2020 (UTC)

did they really use the term "classical 8"? if so, that was kinda bogus of them..in classic times, there were only 5. (the sun and moon were NOT originally considered planets, but they sort of came to be considered planets in late/post classical times, but even then not fully). Uranus, Neptune, and Pluto were all discovered in modern times. so to call Uranus and Neptune classical seems a bit arbitrary Firejuggler86 (talk) 07:58, 4 January 2021 (UTC)

Thought I should give some of my reasons here. We often equate the non-IAU def with Alan Stern, because he's vocal and is the head of New Horizons. But he badly contradicts himself, criticizes the IAU for adopting his own proposals (e.g. for claiming that Neptune had cleared its neighborhood, claiming that if it had, Pluto wouldn't be there -- despite his lambda criterion being in complete accord with the IAU), counts hundreds of known TNOs as dwarf planets when his colleagues/collaborators have shown that most are not even solid bodies, etc. So e.g. in the table of former planets, we need to be careful when stating that a body fits "the" geophysical definition, because there isn't just one, and even restricting ourselves to a single researcher, the def changes from year to year. — kwami (talk) 03:23, 13 November 2020 (UTC)

Well that raises the question of whether we should include the geophysical definition at all. If, as you say, it changes all the time, then the logical thing to do would be to appeal to a higher authority, like we do re: the definition of the Kuiper belt. But in this case there is no higher authority. Serendi pod ous 06:13, 13 November 2020 (UTC) Agree with Serendipodous. Double sharp (talk) 08:04, 3 June 2021 (UTC)

Could we get a citation needed (or better yet, a citation) for that? The Moon is clearly rounded under its own gravity, unless there's some very specific reason to consider it otherwise it is in equillibrium at least as much as the Earth is. — Preceding unsigned comment added by 185.206.227.142 (talk) 23:10, 20 February 2021 (UTC)

Done corrected and cited. Geogene (talk) 00:38, 21 February 2021 (UTC) That citation, on the other hand, includes "

110" geophysical planets and shows the ones smaller than Earth and Venus in a picture. Including Huya, which according to current thinking is probably not a solid body and hence not in equilibrium. And including Iapetus, which is not actually in HE now (it's too oblate for its current rotation period). And including Pallas, Vesta, and Hygiea. So I'm unconvinced. Stern certainly considers the Moon a planet, but whether his definition as stated actually agrees with his intent is in question. Double sharp (talk) 07:32, 3 June 2021 (UTC)

The citations for the Moon not being in HE are at List of possible dwarf planets. That said, one of those papers (doi:10.1007/BF00055525) says that Mercury and Venus are not in HE either. On the other hand, HE is part of the IAU definition, so if that's correct, then they are not planets. I don't think anyone actually believes that. After all: dynamically speaking, Mercury and Venus are clearly gravitationally dominant objects within their spheres of influence. And besides: as Soter remarks: "In a population of small bodies spanning a continuum of sizes and shapes,does gravity dominate the shape of a body if the cross-section deviates from hydrostatic equilibrium by 10%, or by 1%? Nature provides no unoccupied gap between spheroidal and non-spheroidal shapes, so any boundary would be an arbitrary choice."

I kind of suspect that each side already knows what kind of things they want to be planets and just has a problem finding a formal definition. It's just that the geophysical definition has bigger problems with edge cases, so far as I can see. Double sharp (talk) 08:03, 3 June 2021 (UTC)

## Why Pluto Lost Its Status as a Planet

We all went to school and learned that there were nine planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, and it is very rare before scientific facts get changed.

However, this fact did change in 2006, when the International Astronomical Union (IAU) voted that Pluto be reclassified as a dwarf planet instead of a real one. This implies that only the gas giants, which are Jupiter, Saturn, Uranus, and Neptune, and terrestrial planets like Mercury, Venus, Earth, and Mars will be considered as planets from now on.

Members of the public were surprised and enraged at the same time. Some authors had to rewrite their textbooks to avoid spreading the wrong information.

But the question lingering in the minds of people is this: Why isn’t Pluto a planet anymore?

The word “planet” is from the Greek word “planetes,” which implies “wandering star.” Mercury, Venus, Mars, Jupiter, and Saturn can be seen with the naked eye and are constantly shifting in bizarre pathways across the sky, just the way more distant background stars do. When the telescopes came into existence, scientists were able to discover Uranus and Neptune, which are too faint to be visible to the human eye.

When, then, was Pluto established as a planet?

Pluto was discovered and classified as a planet in 1930 when Clyde Tombaugh (from the Lowell Observatory) made some comparisons of different photographic plates of the sky on different occasions and noticed that there was a tiny dot that moved to and fro against the star backdrop. At that point, the solar system had a possible new candidate that was odd. It could actually get closer to the sun than Neptune for up to 20 years of its 248 years trip.

In 1992, some scientists discovered 1992 QB1, a very tiny object orbiting around Pluto’s area, and it was the first Kuiper Belt object. Many more objects were discovered with time. Pluto, however, remained the lord of the area until July 2005, when scientists discovered Eris, a distant body that was thought to be way larger than Pluto at first sight.

That’s when scientists began to question their findings. Things they considered included:

• If Pluto is a planet, does it mean that Eris is one too?
• What exactly is the basis for classifying a planet as one? A word that sounded so straightforward and simple no longer made sense.
• What would we call all those other objects in the Kuiper Belt?

Different controversies and suggestions arose in the Prague Conference, especially when a new definition for the word “planet” was being discovered. One of the controversies suggested that the total number of planets should be up to 12, and this includes Pluto’s moon, Charon, and the largest asteroid, Ceres.

At the end of the day, the astronomers created three new categories for the solar system objects, and this made Mercury to Neptune the only planets that existed. Pluto and its buddies – the objects that shared their orbits with other bodies, were classified as dwarf planets. The other objects that orbit around the sun will now be referred to as solar system bodies.

## Ganymede Moon Facts

1. Ganymede is the largest moon in our solar system. It has a diameter of 3,273 mi (5,268 km).
2. Ganymede is actually larger than the planet Mercury, being around 10% bigger than the planet.
3. However, Ganymede cannot be considered as a planet or even a dwarf planet because it orbits Jupiter.
4. Like our solar system, Ganymede has likely been here since the beginning, forming around 4.5 billion years ago.
5. Ganymede takes it’s name from the Trojan prince from Greek mythology.
6. Ganymede orbits fully around Jupiter in around 7 days.
7. Ganymede makes up one of the four Galilean moons along with Io, Europa and Callisto. They are the four largest moons of Jupiter, discovered by Galileo Galilei.
8. Ganymede orbits Jupiter from a very far distance, being over 665,000 miles (1.070 million kilometers) away.
9. The surface of Ganymede is often talked about. It is made up of a dark region (approximately 40%) filled with craters, and a lighter region (approximately 60%) filled with grooves and patterns.
10. Ganymede is the only moon that has it’s own magnetic field like a planet does.
11. Many different spacecrafts and probes have flown past Ganymede, from the Pioneer 10 in the 1970s, to Juno as recently as December 2019.
12. Ganymede has an iron core, and is also made up of silicate rocks and water in the inner layer, in fairly equal amounts.
13. Underneath the surface of Ganymede lies a saltwater ocean, which is just beneath the crust.

This moon is well known for having two types of terrain on its surface. They’re easily distinguishable from one another, and they give the planet a unique appearance that it is pretty well known for.

Around one third of the planet is made up of dark regions across its surface. These areas of Ganymede have larger craters, which we can actually date back to occurring approximately 4 billion years ago. Across the rest of the surface of Ganymede, the terrain is significantly lighter and smoother, which is likely what the entire surface was originally like.

## Neil deGrasse Tyson and the Museum Exhibit

Scientists knew as the 1990s rolled along that the parameters of Pluto&aposs planet status were becoming fuzzy. The Voyager probes showed moons with surface features and activities like what we see here at home. The Galileo probe found that asteroid Ida has a moon named Dactyl. To top this off, the grouping of the planets was always sketchy, with the terrestrial planets, the gas giant planets, and then. Pluto, all by itself. Heck, even Kuiper himself felt Pluto should have been demoted, all the way back in a February 20, 1956 article in Time (Tyson "The" 50-1, 59).

It was with these things in mind that everyone&aposs favorite public-relations astrophysicist Niel deGrasse Tyson decided to see if the then-new \$230 million Rose Center for Earth and Space could address. He needed to have a museum built that could be easily updated with the latest science and yet not gloss over too many details. After mulling over this and reading "When is a Planet Not a Planet?" by David H. Friedman in the February 1998 Atlantic Monthly, Niel wrote an article entitled, "Plutos Honor" for the February 1999 issue of National History. He went in depth about the similarities that seem to be arising between Pluto and Ceres (more on this later) and so recommends that Pluto be reclassified as a KBO. He did not plan to include this in the exhibit, however, because of a lack of scientific consensus on the matter and for that matter because Neil&aposs field of expertise was stars, not planets (61, 64-5).

With such a lack of agreement, Tyson arranged for a panel to debate the situation. With over 800 people gathered at the American Museum of National History, the 90 minute debate occurred on May 24, 1999 and was entitled, "Pluto&aposs Last Stand: A Panel of Experts Discuss and Debate Classification of the Solar System&aposs Smallest Planet." Amongst the scientists who were present as debaters were Michal Hearn (the then-President of the International Astronomical Society &aposs Planetary Sciences Division), David H. Levy (a Clyde Tombaugh expert), Jane Luu, Brian Marsden (a comet expert), and Alan Stern (a scientist at Southwest Research and Principal Scientist for New Horizons). Many big selling points were pointed out during the meeting in the for-Pluto camp including the roundness criteria along with examining the origins of the object as opposed to using an arbitrary label for the object. Also pointed out in defense of the counter group was the low density value versus the arbitrary use of labeling for sentimental reasons. Some even suggested using dual labeling for Pluto and her sisters since she has planet-like and KBO-like features. At the end of the debate, Tyson had audience indicate their viewpoint, with a majority in favor of retaining planethood for Pluto (69-75).

This debate gave Tyson an idea for the layout of his new Rose Center. Rather than worry about labeling, just state the facts: Pluto is in the Kuiper Belt and resides there. No mention of it being a planet or not removes being overly judgmental, in his eyes. Besides, the planetarium has other non traditional groupings such as magnetic objects so why not try something new? To top it off, the Center has a Scale of the Universe which shows planets but. no Pluto. That is because it isn&apost in that region but in the Kuiper Belt, Tyson argued. Upon the Center&aposs opening on February 19, 2000, no one questioned the move. It wouldn&apost be until a reporter at the Center overhears a child inquiring about the location of Pluto in the Scales that a major article was written about it (76-78, 80).

That story would see light as, "Pluto&aposs Not a Planet?" on January 22, 2001 in The New York Times. Written by Kenneth Chang, it interviews people and gets some emotional reasoning for Pluto being a planet. The author points out during the article that the Center doesn&apost say either way if Pluto is a planet or not, but that didn&apost stop Richard Binzel (MIT) from saying, "They (The Rose Center) went too far in demoting Pluto, way beyond what the mainstream astronomers think." Alan Stern added to this, with, "They (The Rose Center) are a minority viewpoint. its absurd. The astronomical community has settled this issue. There is no issue." Cheng then follows up with some of the odd characteristics of Pluto and finally ends with Tyson&aposs viewpoint about the museum not actually having a stance on the subject (81-3).

## Once Upon a Time, the Sun Was a Planet

But what, exactly, is a planet? The definition has changed as new observations accrued, says Harvard astronomer and historian Owen Gingerich, who chaired the IAU committee charged with defining the word. For some, the definition of a planet is kind of like the Supreme Court definition of pornography: You know it when you see it.

"'Planet' is a culturally defined word that has changed its meaning over and over again," Gingerich said during the Harvard debate. "My feeling is that in retrospect, the IAU should not have attempted to define the word 'planet.'"

Millennia ago, when the Greeks were staring at the stars and charting the heavens, there were seven planets: Mercury, Venus, Mars, Jupiter, Saturn, the sun, and the moon. Centuries later, after Copernicus redrew the solar system and placed the sun at its center, Earth became a planet, and the sun and moon lost their planethood.

By 1850, when Ceres and its contingent of rocky worlds in the realm between Mars and Jupiter were emerging from the darkness, they, too, were called planets. At the time, astronomy textbooks listed as many as 18 planets, and the tally threatened to grow as more were discovered.

"People said, 'This can't go on. We can't have this many planets. We've got to call them something else,'" Gingerich said. So Ceres and its friends became known as asteroids (meaning "star-like"). More terms followed: Among them, minor planets, plutinos, gas giants, ice giants, Jovian planets, terrestrial planets, ice dwarfs, trans-Neptunian objects, centaurs, and Kuiper Belt objects joined an overflowing list of classes.

And now, there are dwarf planets.

Even though he chaired the IAU committee that redefined planets, Gingerich is not pleased with the outcome. "I thought it was really dumb that the IAU took as a category 'dwarf planet' and then said, 'But they're not planets,'" he said. "I was disappointed that it happened that way."

## Should Pluto Be A Planet Again? 13 Years After Being Demoted NASA Boss Wants ‘Dwarf Planet’ Back

A near-true-color image taken by New Horizons after its flyby. Numerous layers of blue haze float in . [+] Pluto's atmosphere. Along and near the limb, mountains and their shadows are visible.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Could Pluto soon be a planet again?

It’s been over a decade since Pluto was relegated from being the ninth planet in the solar system and deemed to be a mere “dwarf planet” by astronomers, but NASA administrator Jim Bridenstine says he wants it re-instated.

“I am here to tell you, as the NASA administrator, I believe Pluto is a planet,” said Bridenstine during a keynote on the final day of the International Astronautical Congress in Washington, D.C. on October 25.

Pluto was uncovered in detail by NASA’s New Horizons mission in July 2015, the best image from which has just been published.

This composite of enhanced color images of Pluto (lower right) and its moon Charon (upper left), was . [+] taken by NASA’s New Horizons spacecraft as it passed through the Pluto system on July 14, 2015.

When did Pluto lose its planet status?

Bridenstine has said as much before about Pluto, which was demoted in 2006 when the International Astronomical Union (IAU) made a controversial call to downgrade it from planet to a new designation of “dwarf planet,” with its official name changed to “minor planet 134340 Pluto.”

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To a generation brought up on nine planets, it was difficult news to accept, and an outpouring of grief spawned music videos and songs. There’s even a Facebook group called the Society of Unapologetic Pluto Huggers.

Why did Pluto lose its planet status?

It was actually the discovery of 2003 UB313—now called Eris—in 2005 that led to Pluto being reclassified. Eris has a lot more mass than Pluto, so at its 2006 meeting, the IAU set a new definition of a planet to avoid having a tenth planet (and in practice, many more). The IAU says a planet is round if:

• It is in orbit around the sun
• It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)
• It has cleared the neighborhood around its orbit.

Pluto fails at the last hurdle because it orbits the sun along with many other small Kuiper Belt Objects. The IAU also created the new term “dwarf planet,” which is defined as a celestial body that meets the first two criteria. There are five known dwarf planets in the solar system Ceres in the asteroid belt between Mars and Jupiter, and Pluto, Haumea, Makemake and Eris in the outer solar system.

So it’s easy to see why the IAU demoted Pluto instead of having to sell the idea of having 13 planets.

Ironically, New Horizons lifted off from Cape Canaveral on January 19, 2006, just a few months before Pluto’s change of status.

Why does NASA want Pluto back as a planet?

“Some people have argued that in order to be a planet you have to clear your orbit around the sun,” said Bridenstine. “If that’s the definition we’re going to use then you could undercut all the planets—they’re all dwarf planets—because there isn’t a planet that clears its entire orbit around the sun.” He’s talking about asteroids, which come close to all of the planets in the solar system.

“I think it’s a sloppy definition,” said Bridenstine. “I think the way you should define a planet is based on its intrinsic value, not values that constantly change like orbital dynamics.” Others argue that a planet is simply an object large enough to be spherical under its own gravity.

There are scientists on Bridenstine’s side. A paper published earlier this year called “The reclassification of asteroids from planets to non-planets” explained that “asteroid” was broadly recognized by scientists as a subset of “planet” for 150 years. The paper argues that:

• scientists considered asteroids to be planets until the 1950s
• the fact that thousands of asteroids share orbits is not relevant to planet status
• asteroids were reclassified as non-planets on the basis of geophysical characteristics
• terms like “planet” are determined by the scientific process, not voting (as with the IAU decision in 2006 over Pluto)

Although the paper doesn’t go as far as to say that Pluto should be reclassified as a planet, it does say: “We suggest attempts to build consensus around planetary taxonomy not rely on the non-scientific process of voting, but rather through precedent set in scientific literature and discourse, by which perspectives evolve with additional observations and information, just as they did in the case of asteroids.” It seems a fair point.

This enhanced color view from NASA’s New Horizons spacecraft zooms in on the southeastern portion of . [+] Pluto’s great ice plains, where at lower right the plains border rugged, dark highlands informally named Krun Macula.

### What do we know about Pluto?

This is really at the core of Bridenstine’s argument. Since the New Horizons mission that photographed it up close for the first time on July 14, 2015, astronomers know a lot about Pluto. “In 2006 Pluto got downgraded from a planet to a dwarf planet, but that happened before the images and science we got in 2015,” he said.

“From New Horizons we know that Pluto has an ocean under its surface, Pluto has complex organic compounds on its surface, Pluto has a multi-layered atmosphere and Pluto has its own moon.” It’s hardly some boring minor object that’s not worth further investigation. In fact, planetary scientist and principal investigator of the New Horizons mission to Pluto, Dr. Alan Stern, is proposing a Pluto Orbiter Mission.

### When was Pluto discovered?

On February 18, 1930 astronomer Clyde Tombaugh found Pluto on a photographic plate he’d taken using a 13-inch astrograph at Lowell Observatory in Flagstaff, Arizona. He actually discovered it after lunch, making it the only “planet” discovered in daylight. An ounce of Tombaugh's ashes are on New Horizons, now in the Kuiper Belt after its unveiling of Ultima Thule/2014 MU69—the most distant object ever explored up close—on New Year’s Day 2019.

## Why Pluto Is a Planet, and Eris Is Too

Jeez, why are we still having discusssions about this. I get it, the definition is rather ambigous, but it has been established that we have 8 planets and everything else is a dwarf planet, asteroid a moon etc. I think the definition given by IAU is just a formality to shut up people.

Take a look at continents. There is no single definition. It's mostly: large landmass seperated by oceans. Which is obviously stupid, because that creates 4 continents (America, Afro-euro-asia, Australia and Antarctica). So instead you will see: large landmass seperated by ocean (Eg: list of continents here).

So I guess it's the same with planets. There are 8 planets. Get over it. Nothing changed about Pluto when it stopped being a planet. Planet is just a bracket, nothing else. No need for discussion about it.

It's the linguistic fallacy that bothers me the most about the new definition. Dwarf planets aren't planets, yet dwarf stars are still stars and a dwarf nebula is still nebula. Why can't we just say that we have 8 primary planets. 5 or so dwarf planets. Many more KBO's, comets, asteroids, ect.

Ceres, Pallas, Juno, and Vesta were considered planets for 50 years before they got reassigned to "asteroids".

The Stern-Levison parameter is interesting since it's applicable even to extrasolar planets.

First of all, can we please shut up about Pluto and it being a planet?

Second, I don't buy the 1000 km definition. It's far too arbitrary. Resolution 5A's wording is poor, but it's trying to say that objects that orbit with lots of other objects (IE: Ceres has a similar orbit to Vesta, Juno, Pallas, etc) shouldn't be considered planets. This seems to be a very sensible definition.

Third, don't complain about "Dwarf Planet". Planetoid would be a much better word, but they picked Dwarf Planet for one reason everyone is forgetting: Resolution 5B (or, as /u/DrMikeBrown put it, the Pluto Escape Clause). Resolution 5B would've made objects that followed all three rules "Major Planets" and those that only followed the first two "Dwarf Planets", turning "planets" into an umbrella term (which I think it should, but that's besides the point). So stop complaining about the evil IAU killing Pluto, because they created Resolution 5B to try and save it.

TLDR: Stop Complaining, there's a good reason Pluto isn't a planet, and the IAU did try to save it.

As much as I hate agreeing with imbeciles, the IAU is correct: Pluto doesn’t make much sense being a planet unless the rest of the KBIs are significantly smaller than it is (are they? I don’t remember).

Eris, on the other hand, absolutely is a planet. It even meets the IAU’s mentally defective “definition” that they created to try to keep it out (kicking out other real planets instead).

How does Eris fit the IAU's definition?

What it comes down to is the dynamicists managed to hijack the process to come up with a really bad definition.

As the article points out, if you were to move the Earth to Pluto's orbit, by the current definition it would now be a ɽwarf planet' as well. While Pluto in Mercury's orbit would now be magically upgraded to full 'planet'.

If you were to move the Earth to Pluto's orbit it would still be considered a planet, not a dwarf planet. The Earth would have to be moved out to more than 2000 AU (50 times farther out than Pluto) for it to borderline not clear its orbit (see last column of this table).

As the article points out, if you were to move the Earth to Pluto's orbit, by the current definition it would now be a ɽwarf planet' as well. While Pluto in Mercury's orbit would now be magically upgraded to full 'planet'.

This is kind of missing the point though. That might be the case if these objects were to suddenly teleport into these new orbits but the point is that if an Earth-like object had formed in a Pluto-like orbit, the Kuiper Belt would look quite different to how we see it today, if it was to even exist at all. These KBOs would have spent billions of years being perturbed by a much larger object than they were previously. The "cleared it's orbit" criteria is more about how the object has influenced the space around it over time it's not merely a too-many-objects-near-it proximity criteria. Certainly, Jupiter's trojan asteroids don't make it less of a planet, and you could absolutely imagine the chaos it would have if it were suddenly teleported into a Pluto-like orbit - the Kuiper Belt wouldn't exist as we know it for long.

The definition isn't great. Iɽ like to see a lot more involvement from the planetary geologists. But regardless, it's unlikely that we're going to see a wholey "good" definition any time soon. Especially because we know we're only currently scratching the surface of understanding exoplanet populations (our detection methods still feature massive biases). We're trying to make these distinctions by examining maybe a few dozen objects in our own solar system, despite knowing that there's hundreds of billions (more likely trillions) in the Milky Way alone. Despite knowing that there are solar systems that look essentially nothing like our own. This is partly why the 2006 IAU definition strictly refers only to our Solar System. It's basically arbitrary by design.