# Why aren't planetary bodies static?

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Why are planets moving in the first place? Gravity causes them to orbit, but why move at all?

note: this is not a question on why planets orbit each other, I know the reason for that is gravity. What I'm asking is why do planets move at all? For example, if you turned off gravity, why would the planets carry on moving apart rather than be stationary?

It's simply because the sun and planets were formed out of a big pile of dust. Originally the dust was spinning. So once the dust became planets, it kept spinning around. That's all there is to it - that's why the solar system is spinning around.

It's was once a pile of dust that was spinning a bit; it's still spinning.

"For example, if you turned off gravity, why would the planets carry on moving?"

THat's just the same as asking "if I spin something around on a string, and break the string, why does it keep moving?"

For that matter, it's the same as asking "What is momentum?" So, if you push something… why does it keep moving?

At this stage in history, we have utterly no clue, at all, what the heck time, space, matter, and momentum are. "Why does momentum do what it does?" is for now just one of those super-deep questions like "What is time" or "So what caused the big bang" or "What is gravity" or "What's the explanation for this quantum stuff?" or indeed… "What is momentum"?

So, why do the planets keep moving?: answer "momentum". If you want to know "what causes momentum?", that is one of the basic total mysteries.

For now, nobody has a clue. You may as well ask… what is space, what is time, etc.

Regarding momentum, you might like to read up about the so-called "Mach's conjecture". The famous Einstein was, like you, fascinated by the question "WTF is momentum anyway?" One sort of general thinking-point originating with this smart guy called Mach is that momentum could have something to do with "all the other mass in the universe". Nobody has a clue about this, and it's just a vague general idea.

Isaac Newton was a pretty smart guy (if a bit whacky), and in the end he found gravity so mysterious, he just put it down to God. He probably found momentum as mysterious, and he was one of the first guys to think about it clearly.

An interesting point: actually everything astronomers look at (all galaxies, all structures of galaxies) in fact does not (!) behave the way small things (our solar system, as in your question) behaves in terms of gravity and momentum. This is usually explained by invisible unknown matter ("dark matter") or for a few scientists, that gravity works differently than we think presently. So the fact is with issues like gravity/momentum you ask about… not only do we have no clue "why momentum works" but when you look through a telescope, issues like "orbits" work totally differently anyway!

## Hydrostatic equilibrium

In fluid mechanics, hydrostatic equilibrium or hydrostatic balance (also known as hydrostasy) [1] [2] is the condition of a fluid or plastic solid at rest. This occurs when external forces such as gravity [ citation needed ] are balanced by a pressure-gradient force. [3] For instance, the pressure-gradient force prevents gravity from collapsing Earth's atmosphere into a thin, dense shell, whereas gravity prevents the pressure gradient force from diffusing the atmosphere into space.

Hydrostatic equilibrium is the distinguishing criterion between dwarf planets and small Solar System bodies, and has other roles in astrophysics and planetary geology. This qualification means that the object is symmetrically rounded into an ellipsoid shape, where any irregular surface features are due to a relatively thin solid crust. In addition to the Sun, there are a dozen or so equilibrium objects confirmed to exist in the Solar System, with others possible.

## Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science

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When we observe cosmological redshift, where does the energy "go" as the light decreases in frequency?

I've been listening to a lot of John Dobson lately, and his ideas are unconventional, but Iɽ love to get a second opinion on his argument for a static universe, rather than an expanding one. He argues that at border of the observable universe, where matter is receding at the speed of light, the light is redshifted until the energy of the emission, and therefore also the energy of the emitting particles goes to zero. And if that's true, the momentum also goes to zero, as well as the uncertainty in the momentum. Then he brings in Heisenberg to say that if the momentum uncertainty goes to zero, the position uncertainty becomes infinite, and that the particles at the border quantum tunnel right back in to any point in the Universe. Would love to see someone break this down with a bit more mathematics than Dobson uses. Thanks!

When we observe cosmological redshift, where does the energy "go" as the light decreases in frequency?

The energy is lost. Energy is not conserved on cosmological scales.

where does the energy "go" as the light decreases in frequency?

It is lost. There is no global conservation of energy in general relativity.

He argues that at border of the observable universe, where matter is receding at the speed of light

It doesn't. The distance to matter at the edge of the observable universe, 46 billion light years away, increases by

3 times the speed of light. The distance to matter at the edge of the still causally connected part is still increasing by a bit more than the speed of light.

the light is redshifted until the energy of the emission, and therefore also the energy of the emitting particles goes to zero.

And if that's true, the momentum also goes to zero, as well as the uncertainty in the momentum.

That is even more nonsense.

Then he brings in Heisenberg to say that if the momentum uncertainty goes to zero, the position uncertainty becomes infinite, and that the particles at the border quantum tunnel right back in to any point in the Universe.

And here is goes to full-blown crackpot stuff.

Just ignore him unless you want an example how you can't do science.

Part of the issue with that argument is that no object is ever at the border of the observable universe within its own reference frame whatever's happening to the light it emits, it has no effect on what's happening to the matter itself.

But quantum physics is not my strength. What I do know is that multiple lines of evidence point towards a young, expanding universe such as the accumulation of heavy metals, the age distribution of stars, and the properties of distant (and therefore old) galaxies.

Hi all, hope it's ok to post here. I'm not sure if this is an Ask Science or ELI5. Maybe both!
I'm having trouble understanding Atmospheric Emissivity. If daily Atmospheric Emittance values are slightly higher on June 21 than December 21 (say average daily value on June 21 is 300 Wm -2, versus December 21 is 250 Wm -2 on a clear sky day), how would increased cloud cover increase Atmospheric Emittance values?
As I understand, atmospheric emittance depends on the temperature of the atmosphere and atmospheric emissivity atmospheric emissivity is a measure of how effective the atmosphere is at emitting longwave radiant energy.
How can the values be higher on both clear and cloudy days? Is it simply that the atmosphere is warmer in June so emittance is greater? I'm sure I'm probably over thinking this but Iɽ really appreciate if someone could dumb this down for me!
Also, I posted this as a separate thread topic before I saw this ask anything Wednesday is actually a thread for Earth science questions.

How are they able to determine the distance of the solar system body "Farfarout"? As far as I can tell they reported a distance of 140 AU from the sun with only two observations, which seems to rule out any accurate curve-fitting.

It is just a really rough estimate. If the object is in an orbit around Sun and the observations are not too far apart then the object didn't move much relative to the change of Earths' position in the Solar System. You can estimate the distance via the parallax, in a similar way as you would estimate the distance to nearby stars. You know its proper motion must be small if it orbits the Sun.

How exactly to the large particle accelerators discover those tiny bits that make up the parts of atoms? What kinds of sensors do they use, and how do those sensors work, to find elements that are smaller than even the electron? And for that matter, how do they tell that they're discovering what they wanted to discover instead of some unwanted byproduct if the particle explosion.

Particle detectors are like onions. They have layers. Each layer is meant to measure a different thing. Some layers are more necessary and significant than others. We have a calorimeter (made of scintillation crystals which glow when energy is deposited) which measures, with great certainty, all of the electrons and photons that pass through it. We have a drift chamber (think like the old school cloud chambers from the 60s) which measure dE/dx, telling us the direction and velocity of particles passing through. This works because particles in magnetic fields have a radius of curvature which is related to their momentum. We also have heaps of smaller detectors to better refine the origin point of particles, as well as determine which particles are which (muons and electrons look very similar without these dedicated components).

Each collision in a particle accelerator DOES produce heaps of outgoing particles and storing all that info can be problematic. To help with this, we introduce what are called “triggers”. These triggers go off when we have processes that are known to look like background (i.e. uninteresting mess) and they get thrown away without wasting space. We also have some triggers that will label a process as “probably the sort you are looking for” before saving it which makes accessing the data later much easier for physicists.

We know that what we are looking at is what we want to “discover” or measure by using heaps of specially curated selection criteria. To do this, we start by making heaps of fake data (Monte Carlo simulation) and running it through the detector layers (all on a computer). We see what our signal mode (the thing we are trying to find in real data) looks like in the different layers of the detector. We also replicate this process with “background” modes. These are modes that are easily confused for our signal (maybe they have the same types of final state particles or are made of pions pretending to be electrons or whatever).

When we look at the distributions (things like angle the particles hit the detector at or mass or momentum as measured by the detector), we can compare the signal and background modes. From these comparisons, we can find selection criteria to cut out the background (maybe our signal is always at theta = 90 or always has a momentum of at least 1.5 GeV or whatever).

Finally, we will look at data, implement these same cuts, and then see if the stuff left over looks like our fake data we made before.

Another cool thing in this area is the emerging use of neural networks to skip (but not really completely yet) the last few paragraphs of work and help us find “interesting” processes right off the bat.

## Rare Earth Hypothesis

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth (and, subsequently, intelligence) required an improbable combination of astrophysical and geological events and circumstances. The hypothesis argues that complex extraterrestrial life is a very improbable phenomenon and likely to be extremely rare. The term “Rare Earth” originates from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist.

An alternative viewpoint was argued by Carl Sagan and Frank Drake, among others. It holds that Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. Given the principle of mediocrity (also called the Copernican principle), it is probable that the universe teems with complex life. Ward and Brownlee argue to the contrary: that planets, planetary systems, and galactic regions that are as friendly to complex life as are the Earth, the Solar System, and our region of the Milky Way are very rare.

On 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. With the closest found at 16 light-years (Gliese 832 c). Nonetheless, by concluding that complex life is uncommon, the Rare Earth hypothesis is a possible solution to the Fermi paradox: “If extraterrestrial aliens are common, why aren’t they obvious?”

## Are there any models of planetary formation that aren't full of arbitrary fine tuned parameters? [closed]

Want to improve this question? Add details and clarify the problem by editing this post.

I noticed that this was very common in planet formation theories. I would be interested in counterexamples.

Example of finely tuned models:

There are four stages in the supposed evolution of planets, according to the reference:

‘A successful nebular model must account in some detail for four important stages in the solar system’s evolution: the formation of the nebula out of which the planets and sun originate, the formation of the original planetary bodies, the subsequent evolution of the planets, and the dissipation of leftover gas and dust. Modern nebular models (there are more than one!) give tentative explanations for these stages, but many details are lacking. No one model today is entirely satisfactory.’

For the sake of argument, I will just assume that the dust is leftover from a supernova explosion. This is the first stage. Then according to Laplace’s nebular hypothesis, first presented in 1796, the process of planet formation, the second stage, begins with the simple collapse of the dust cloud. There are three theoretical steps in the collapse of the dust cloud and the growth of a planet: 1) gravitational contraction of the dust into small particles, 2) accretion of particles or small aggregates to form large aggregates, and 3) condensation by the accumulation of atoms and molecules on the growing mass.

The most difficult step is the first, gravitational contraction of dust to form small particles. Dust grains must first accrete to form small particles, which must continue to grow until they are at least 10 m in diameter. This size is the point at which gravity is expected to come into its own, accreting and condensing material at a faster and faster rate. Then supposedly, planetesimals would form that are many kilometres across. The planetesimals are finally envisaged to collide to form planets. There are difficult problems with these later steps, but I will focus on the first step: how does the dust collide, stick together and grow before gravity can assert itself? That is the big question. The tiny dust particles must hit each other head on and stick. The process (which is speculative anyhow) is too slow, especially in cold regions of space, according to astronomers. A number of hypotheses are in vogue, but all seem to have fatal flaws.

Steinn Sigurdsson has given up on all the proposed hypotheses because of the extreme unlikelihood that any of them ever occurred. Since planets have obviously formed and they must hold onto their evolutionary belief, he suggests a desperate alternative:

‘ … there could be an extra dimension of space in which gravity alone acts and which until now has gone unnoticed. If this is so, then gravity—which is weak over large distances—gets stronger at the tiny distances encompassed by the extra dimension … .’

In other words, he suggests that gravity would extend into five space dimensions instead of three and would be very strong at very short distances, causing dust and small particles to attract and stick together by gravitational attraction. This would certainly make planet formation much faster and easier. But there is at least one delicate problem with this imaginative hypothesis—the dust grains cannot hit too hard or the incipient particle would break apart:

‘So the turbulence within the disc [flat dust cloud] can’t be too strong, and the acceleration caused by Sigurdsson’s modified gravity can’t be too extreme.’

The idea is actually testable. So far, Newton’s law of gravity still holds down to 218 μm, but experiments are underway to test it at even closer distances. Sigurdsson hopes that his supergravity mechanism will show up when they test gravity at less than 80 μm. It seems to me that if he is correct, there is still the ‘sticky’ problem of how such a small particle can grow larger than 218 μm, above which his hypothetical mechanism would not apply.

Astrophysics makes the absurd ssumption that gravity dominates at small scales.

Reference: Zeilik, M., Astronomy—The Evolving Universe, 8th Ed., John Wiley and Sons, New York, pp. 260–261, 1997

## Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science

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## Dust 'floats' above lunar surface—electrostatic dust transport reshapes surfaces of airless planetary bodies

Credit: NASA

As millions on Earth enjoy a spectacular view of a supermoon on Dec. 14, a NASA-funded research team is reviewing the results of recent laboratory experiments that explain why dust "levitates" on the moon.

The research by a member of NASA's Solar System Exploration Research Virtual Institute (SSERVI), hosted by NASA's Ames Research Center in Silicon Valley, California, explains how dust may be transporting across vast regions above the lunar surface and rings of Saturn, without winds or flowing water.

Learning about these fundamental processes is helping scientists understand how dust and static electricity behave on airless bodies, and how they affect surface mechanical and electrical systems. This and other SSERVI research is helping NASA address key strategic knowledge gaps for airless bodies such as asteroids or the moons of Mars, Phobos and Deimos, which are likely stepping stones along our journey to Mars.

The study builds on observations from the Apollo era to the recent Rosetta comet mission, and brings to closure a long-standing question about electrostatic dust transport seen on the moon and other airless planetary bodies. The research was conducted at the Institute for Modeling Plasma, Atmospheres and Cosmic Dust at the University of Colorado Boulder, and was published recently in the journal of Geophysical Research Letters.

The phenomenon shows up as high-altitude ray-pattern streamers above the lunar surface reported by Apollo astronauts, as well as intermittently appearing radial spokes first seen by the Voyager spacecraft over the rings of Saturn, and the fine dust deposits, or "dust ponds" in craters on Eros. These are all the examples of dust transporting across vast regions without winds or flowing water. Scientists believed electrostatic dust processes could explain these space observations, but until now there were no studies to support these explanations.

Mihaly Horanyi at the University of Colorado in Boulder and his team recorded micron-sized dust particles jumping several centimeters high under ultraviolet (UV) radiation or exposure to plasmas. On Earth's moon, these dust particles would have been lofted more than 4 inches (10 centimeters) above the lunar surface, leading researchers to conclude that the moon's "horizon glow"—seen in images taken by Surveyor 5, 6, and 7 five decades ago—may have been caused in part by sunlight scattering in a cloud of electrostatically lofted dust particles.

"This new 'patched charge model' resolved a fundamental mechanism of dust charging and transport, which has been puzzling scientists for decades," said Xu Wang, the paper's first author.

One of the key science findings is that the emission and re-absorption of photo/secondary electrons at the walls of micro-cavities formed between neighboring dust particles can generate unexpectedly large electrical charges and intense particle-particle repulsive forces. This can cause dust particles to move and lift off the surface, or "levitate." And not just single-sized dust particles—large aggregates can be lofted as well.

"We expect dust particles to mobilize and transport electrostatically over the entire lunar surface, as well as the surface of any other airless planetary body," Wang said. "If so, electrostatic dust activity may be also responsible for the degradation of retroreflectors on the lunar surface."

The laboratory observations also showed dusty surfaces becoming smooth as a consequence of dust mobilization. These electrostatic dust processes could help to explain the formation of the "dust ponds" on asteroid Eros and comet 67P, and the unexpectedly smooth surface on Saturn's icy satellite Atlas.

## We Aren't Martians, Says Astrochemist

Or so says Pascale Ehrenfreund, an astrochemist at George Washington University in Washington, D.C.

Ehrenfreund says that there are two major arguments against “panspermia” (or the notion that microbial life could spread from one planetary body to another).

A mist of hydrocarbons in the Interstellar Medium such hydrocarbons may have played a key role in . [+] the origin of life on earth.

NASA/JPL-Caltech/2MASS/SSI/University of Wisconsin - Spitzer

The first is simply that both ultraviolet radiation from the sun and galactic cosmic rays would likely destroy microbial life in the unprotected vacuum of space. The second is that, even if such life survived a journey from Mars to Earth, among other factors, its survival would also likely depend on entry through a roiling, young planetary atmosphere and adaptation to its new home.

Thus, Ehrenfreund views any “We are Martians” scenario as “highly unlikely.”

So, did early life here get a crucial boost from complex molecules from beyond our forming solar system, or even from our young solar nebula itself?

Ehrenfreund says that although molecules like hydrogen cyanide, formaldehyde and water all form in the interstellar medium, prebiotic chemistry really involves replication and structure.

“I don’t think this happens anywhere in [free-floating] space itself,” said Ehrenfreund.

Even so, to date, some 180 different molecular species have been detected in space.

In their gas phase, Polycyclic Aromatic Hydrocarbons (PAHs) which on Earth encompass everything from naphthalene (the active ingredient in mothballs) to chimney soot, pine tar --- even the “char” residue found on backyard grills, also appear to be ubiquitous throughout the universe.

Some estimates are that PAHs make up some 15 percent of the cosmos’ total carbon supply. But how important are they for our own chemistry on Earth?

Ehrenfreund says PAHs, as well as solid macromolecules, did make it to Earth’s surface, because they were more stable and abundant and may have decayed into smaller subunits incorporated into primitive protocells.

But she says small biomolecules such as amino acids and sugars are fragile and likely were easily destroyed by radiation and high temperatures prevalent on the young Earth.

Even so, Ehrenfreund says complex molecules and gases which form in the interstellar medium will be included in the solar nebula, as such, they are precursors for prebiotic chemistry that may have contributed to life on Earth.

Ehrenfreund says that via meteoritic analysis, there is now ample evidence that processes involving water on asteroids can form new organic compounds such as amino acids.

She points out that an incredible amount of material, including a tiny fraction of amino acids, was delivered through Earth’s atmosphere. But Ehrenfreund says most researchers think that amino acids found in meteorites didn’t form in the Interstellar Medium but rather from water processes in the parent body asteroid.

There is still no undisputed detection of an amino acid in the interstellar medium.

“We have the first undebated indications of life at 3.5 billion years ago,” said Ehrenfreund, who notes that the inner solar system's epoch of asteroidal and cometary Late Heavy Bombardment, ended about 3.9 billion years ago. But even after such an epoch of impactors, she says our young Earth was still plagued by a very violent and hostile surface environment.

Pascale Ehrenfreund Credit: Bernard Foing

The biggest mystery is what basic compounds were available that could also assemble under such inhospitable conditions?

“That’s a key point in the origin of life,” said Ehrenfreund. “That’s a crucial point on which we have a real gap.”

Did these ever-present PAHs make the crucial difference in prebiotic chemistry for life?

Ehrenfreund notes that even though researchers now realize that complex carbon chemistry is universal, they are much less knowledgeable about what happens to such chemistry after an interstellar cloud collapses and forms a planetary system.

That’s because what’s really important for the origin of life on Earth and, by rote, other earthlike planets, is what is actually delivered from extraterrestrial sources.

Although there’s a whole subgroup of microbiologists trying to create artificial life in the lab, says Ehrenfreund, none of these researchers are trying to recreate the assembly of protocells in a simulated early Earth environment.

“It’s intrinsically very difficult because there’s a lot of debate about what the conditions actually were,” said Ehrenfreund.

The so-called Late Heavy Bombardment early in the history of the inner solar system produced a rain . [+] of asteroids and comets (Photo credit: NASAblueshift)

And biotech researchers trying to construct protocells for potential use in contemporary medicine have arguably vastly different goals than astrochemists and astrobiologists.

Partly as a result, Ehrenfreund says that although biotech funding is readily available, it’s very difficult to get funding and grants for research into Earth’s early prebiotic chemistry.

No one has tried to create a protocell under early Earth conditions, she says.

“There are lots of ongoing Mars simulations,” said Ehrenfreund. “But we do much less for simulations of early Earth.”

Answering such fundamental questions about life’s origins on Earth, she says, will require astronomers, geologists and chemists working with simulation chambers that include fluctuations in early Earth changes in temperatures, atmospherics, radiation and hydrothermal conditions.

“We would be able to see which [prebiotic] components could self-assemble and which could not,” said Ehrenfreund. “We could then extrapolate from where those compounds came.”

## Cartesian Vortices

The overlapping circles in Tycho Brahe's geocentric model of the cosmos created a significant problem for the Aristotelian notions of the heavenly spheres. If Brahe was right and the orbits of the planets crossed each other each other then they couldn't be a set of solid.

Rene Descartes offered a solution to this problem in his 1644 Principia Philosophiae. In Descartes system, like Aristotle's, the universe was full of matter, there was no such thing as empty space. To explain motion Descartes introduced the concept of vortices. The system consisted of different kinds of mater or elements rubbing up against each other. His model included three different kinds of elements: luminous, transparent, and opaque. Luminous was the smallest and was what the stars were made of. Earth and the planets were made up of the denser opaque. The space between the planets and the stars was made up of transparent He stated that Lumnious would settle at the center of these vortices and the transparent and opaque elements would keep shifting around each other. This shifting created the movement of objects in the heavens.

## Planets Started Out From Dust Clumping Together. Here’s How

According to the most widely accepted theory of planet formation (the Nebular Hypothesis), the Solar System began roughly 4.6 billion years ago from a massive cloud of dust and gas (aka. a nebula). After the cloud experienced gravitational collapse at the center, forming the Sun, the remaining gas and dust fell into a disk that orbited it. The planets gradually accreted from this disk over time, creating the system we know today.

However, until now, scientists have wondered how dust could come together in microgravity to form everything from stars and planets to asteroids. However, a new study by a team of German researchers (and co-authored by Rutgers University) found that matter in microgravity spontaneously develops strong electrical charges and stick together. These findings could resolve the long mystery of how planets formed.

Put simply, physicists have been in the dark about how nebular material can accumulate to form large bodies in space. Whereas adhesion can cause dust particles to stick together and large particles are drawn together by mutual gravity, the in-between stage has remained elusive. Basically, objects that range from millimeters and centimeters tend to bounce off each other rather than sticking together.

Glass particles in microgravity. Credit: Gerhard Wurm, Tobias Steinpilz, Jens Teiser and Felix Jungmann

For the sake of their study, which recently appeared in the journal Nature, the team conducted an experiment where glass particles were placed in microgravity conditions to see how they behaved. Surprisingly, the team found that the particles developed strong electrical charges. So strong, in fact, that they polarized one another and behaved like magnets.

The team followed up on this by running computer simulations to see if this process could bridge the gap between fine particles clumping together and larger objects aggregating due to mutual gravity. What they found here was that planetary formation models agreed with their experiment data, so long as electrical charging is present.

These results effectively fill a longstanding gap in the most widely accepted model of planetary formation. In addition, they could have numerous industrial applications here on Earth. Said Troy Shinbrot, a professor of biomedical engineering at Rutgers University-New Brunswick and a co-author on the study:

“We may have overcome a fundamental obstacle in understanding how planets form. Mechanisms for generating aggregates in industrial processes have also been identified and that – we hope – may be controlled in future work. Both outcomes hinge on a new understanding that electrical polarization is central to aggregation.”

Artistic rendition of a protoplanet forming within the accretion disk of a protostar Credit: ESO/L. Calçada http://www.eso.org/public/images/eso1310a/

The potential for industrial applications is due to the fact that similar processes are used on Earth in the production of everything from plastics to pharmaceuticals. This consists of gas pressure being used to push particles upwards, during which time they can aggregate due to static electricity. This can cause equipment failures and lead to flaws in the final product.

This study could therefore lead to the introduction of new methods in industrial processing that would be more effective than traditional electrostatic controls. Moreover, it could lead to a refinement of planetary formation theories by providing the missing link between fine particles and larger aggregates.

Another mystery solved, answer piece to puzzle. One step closer to answering the fundamental question, “how did it all begin?”

## What is a planet? Turns out, we aren't sure.

India, Feb. 28 -- The Oxford English dictionary defines a planet as "A celestial body moving in an elliptical orbit around a star." The word is Greek in origin, and the root means "wanderer". The planets were distinguished from the stars, as they had specific motions across the skies, independent of the stars. It is this motion that led to the discovery of planets beyond Saturn. In 1930, a young astronomer by the name of Clyde Tombaugh noticed a moving point of light in between two photographic slides in the direction of the constellation of Gemini. This slight movement indicated the discovery of a new planet, which was named Pluto.

In 1992, over a thousand trans-Neptunian objects (TNOs) were discovered beyond the orbit of Pluto. It looked likely that there would be TNOS larger than Pluto. In 2003, one such object was discovered, with its own moon. It was the discovery of Eris, that led astronomers to first question exactly what a planet is. The media started referring to Eris as the tenth planet in the Solar System, which led to heated discussions within the International Astronomical Union (IAU). This is the body that is responsible for naming celestial objects such as stars, planets and asteroids.

Following an intense debate in the 2006 IAU general assembly, held at Prague, the definition of a planet was changed in a resolution. The resolution reads: (1) 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 (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A "dwarf 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 (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d)is not a satellite. (3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". On 24 August 2006, at the closing ceremony of the general assembly, the resolutions were passed. This was the historic moment when Pluto was "demoted" as a planet, and the Solar System was considered to have 8 planets. According to the new definition of a planet, Pluto was now a "dwarf planet". As of today, the IAU recognises five dwarf planets, Ceres, Pluto, Eris, Makemake and Haumea. Sedna, Orcus, Quaoar are some TNOs, that are not yet recognised as dwarf planets by the organisation.

Almost immediately, scientists began seeing some serious flaws in the IAU definition of the planet. The third criterion requires a planet to have cleared its orbit of other objects. This has a number of implications. None of the eight objects considered to be planets by the IAU has actually cleared their orbits of other objects. The Earth would not be considered a planet because of the Near Earth Objects (NEOs), most of which are asteroids, but some of which are comets. One of these, designated as the asteroid 2016 H03 is a quasi-satellite of the Earth and is a specific example of Earth not having cleared its orbit. The asteroid accompanies the Earth in its annual journey around the Sun but is too distant to be considered a true satellite. It disqualifies the Earth from being considered a planet, according to the definition of the IAU. The same criterion also makes it difficult to designate planets in other star systems. For example, for the 7 identified bodies in the Trappist-1 system to be called exoplanets, it would be necessary to find proof that they have in fact cleared their orbits of all other bodies. Considering how incredibly difficult it is to find exoplanets in the first place, this is a ridiculous exercise.

Mars, Jupiter and Neptune also share their orbits with asteroids, technically disqualifying them from being considered planets according to the definition of the IAU. The third criterionmeans that the farther away from the host star a body is, the more massive it has to be to be qualified as a planet. Even an Earth-sized body as distant from the Sun as Pluto is, would not be able to entirely clear the orbit of all other bodies. Instead of defining a planet by characteristics intrinsic to the body in question, the IAU relies on external factors. The definition also introduces some inconsistencies. Mercury, Venus and Earth are referred to as terrestrial planets. Jupiter and Saturn as the Gas Giants. Neptune and Uranus are the Ice Giants. All of these bodies are considered planets, while only Pluto and the category of bodies called "dwarf planets", are not considered planets. Pluto satisfies the first and second criteria of being a planet, but not the third. It is therefore purely arbitrary that Pluto is not considered a planet. According to planetary scientists, the IAU is saying "Pluto is not a planet because we say it is not a planet", and not providing sufficient and robust scientific reasons, even in the definition that they have adopted.

Planetary scientists have proposed a definition based on the geophysics of the body, one that is simple enough to understand, and one that is in line with how people actually use the word. The proposed definition is: A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters. A simpler definition for school textbooks would be: "Round objects in space that are smaller than Stars".

It was not just the planetary scientists that have a problem with the demotion of Pluto. The new definition of the IAU ended up confusing the general public as well. The New Horizons spacecraft was a probe launched to study objects in the outer reaches in the Solar System, and Pluto was one of the bodies that it studied. One of the most common questions asked to the New Horizons team was why was a probe being sent to study Pluto when it was no longer a planet. Being removed from the roster of planets in orbit around the Sun, somehow made Pluto an object less interesting for study in the minds of the people. The science team members of the New Horizons mission are leading the effort to get Pluto reclassified as a planet.

The proposed definition by the planetary scientists is dependent on the internal characteristics of the body, and not the external environment. If the new definition is adopted, the number of planets in the Solar System would expand significantly. Instead of having 8 planets, the Solar System would have more than 110 planets. This is because apart from the asteroids and stars, every other body is considered to be a planet. This includes the moons of Jupiter and Saturn, as well as our own Moon. Ganymede, a moon of Jupiter, and Titan, a moon of Saturn are both larger than Mercury. This categorisation actually helps give the public a clearer understanding of the nature of these worlds. The definition is more useful than the IAU version to planetary scientists who study geology and other geosciences of these worlds.