Why are planets spherical?

Seth Jarvis

Anthony Garcia wrote in to ask, “Why are planets perfect spheres, or at least appear to be perfect?”

Nature loves spheres. It can’t get enough of them.

Soap bubbles are spherical because that shape most efficiently balances the outward pressure of the air within the bubble against the surface tension of the soap film.

When water splashes and for a brief instant a droplet of water is neither rising nor falling and is momentarily weightless, what shape does the droplet’s surface tension force the water to take?  A sphere.

bubble-droplet_450

Nature loves spheres. In the case of bubbles and droplets of liquid, surface tension creates a sphere to minimize surface area.

Stars are perfect examples of natural spheres.  The mass of a star is mind-bogglingly large and creates an equally mind-bogglingly large amount of gravity. What shape does Mother Nature give to so much mass to minimize its enormous volume?  A sphere.

Stars are huge, dynamic, energy-making monsters shaped by their enormous gravity into spheres.

Stars like our Sun are huge, dynamic, energy-producing concentrations of Hydrogen and Helium, compacted by their enormous gravity into spheres.

The reason planets appear spherical is because gravity compresses the planet into a shape that most evenly distributes the gravitational force among the planet’s mass.

Whether it is shaping water droplets, stars, soap bubbles or planets, nature seeks to minimize the surface area needed to contain a given volume, and the shape that keeps volume at the absolute minimum a sphere.

Any object in weightless space larger than a couple of hundred miles in diameter has enough mass for its gravity to overcome large-scale irregularities and force it into a spherical shape.  This gravitational compression also generates significant amounts of heat at the center of the planet. This heat melts, or at least softens, any solid materials within the planet, facilitating the planet’s collapse into a spherical shape.

Objects in space smaller than about 100 miles in diameter, such as most asteroids, comet nuclei and small moons, lack the mass to create a gravitational field strong enough to compress themselves into spheres.  These little worlds often take on what I call the “sick potato” look.

gaspra_91_galileo_450

The 12.5 mile-long, 7.5 mile wide asteroid Gaspra, imaged in October 1991 from a distance of 1,600 miles by the Galileo spacecraft en route to Jupiter.

A really large asteroid, such as Ceres (diameter = 600 miles), has enough mass for its gravity to compress it into a sphere.

ceres_450

The 600 mile-wide asteroid Ceres as seen by the Hubble Space Telescope.

However, “perfect” spheres are hard to find in space.

Pretty much everything is space rotates, and rotating a non-rigid sphere causes it to “bulge” at its equator from the centrifugal forces acting on it.

This spinning distorts large planets into a slightly squashed shape known as an “oblate spheroid.” This means that a planet’s diameter measured through its poles is smaller than the diameter measured through its equator.

Whereas the difference between the polar diameter and the equatorial diameter of Earth is a barely noticeable 0.3%, the oblateness of Saturn, a large, gaseous and rapidly spinning planet,  is greater than 10%.  You can easily see Saturn’s polar flattening through a telescope.

oblate-saturn

Saturn's polar diameter is 33,700 miles, but its equatorial diameter is 37,360 miles.

There may not be such a thing as a “perfect” sphere in nature, but there is no doubt that spheres, nature’s favorite shape, are perfectly lovely.

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52 thoughts on “Why are planets spherical?

  1. thnx i need more info on dis for my science project. by any chance you could put more details so i could pass my science class.

    -Ms.Harwood’s student

  2. Thanks for the information. It seems odd that we have so many spherical things in our universe yet very few of them (or none of them) are actually spheres?

  3. My brother would like to know how the gaseous planets stay spherical… Does this apply to them too? And if not, could you explain why the gaseous planets stay spherical??

  4. Amber – love the question!

    Yes, gravity will cause any object in space with significant mass to collapse itself into a spherical shape – including gasses. That’s why stars, including our Sun, are spherical, too. Even if all you’re talking about is a lot of hydrogen and helium, if there’s enough of it to exert a significant gravitational pull on itself then you’ll end up with a spherical collection of matter as the mass redistributes itself to respond to gravitational forces equally from the gravitational center of its mass.

    Notice that I’m saying “spherical” rather than “perfect spheres.” If the only force acting on a collection of matter was gravity, then you would end up with a perfect sphere. However, there are generally a lot of other forces at work on stars and planets, rotation being chief among those forces.

    Centrifugal forces cause a rotating sphere to bulge at the equator, causing the equatorial diameter of the sphere to increase and the polar diameter (the diameter measured between the sphere’s north and south pole) to decrease. The faster the rate of rotation, and the lower the density of the sphere (star or planet) the greater the equatorial bulge.

    And since pretty much everthing in the Universe is spinning, pretty much all spheres are thought to actually be “oblate spheroids.”

    One type of massive collection of matter that is almost certainly spherical _and_ almost certainly rotating very rapidly is a Neutron Star. A Neutron Star’s surface gravity is fantastically powerfull, and they often spin at a rate of several rotations PER SECOND. Are Neutron Stars oblate, too? Or is their surface gravity so powerful that in spite of their rotation they are somewhat oblate?

    Maybe someday someone will get close enough to one to make such a measurement. That’s the stuff of sci-fi stories.

  5. I also need more info on why are they actually spherical, but anyhow I cant complain as I enjoyed your piece of knowledge.

  6. Bence, spheres are nature’s way of evenly distributing gravitational attraction. Every part of a sphere at any given distance from the sphere’s center experiences exactly the same gravitational attraction as any other part of the sphere at that same distance. Think of any shape other than a sphere and you’ll quickly see lots of places on the objects surface are signifcantly different in their distance from the object’s center of mass. That’s fine for small objects like asteroids, comets and spaceships, but for anything that’s more than a couple of hundred miles in diameter the gravitational forces will just naturally pull the mass into a spherical shape in order to evenly distribute the mass around the center of gravity. I hope that helps – Seth

  7. Planets move at high speed (Earth’s speed is nearly 67,000 miles per hour) in a direction that is nearly at a right angle to the direction from the planet to the Sun. (See graphic at: http://www.gcsescience.com/Gravity-Earth-Orbit-Circle.gif ). In space, there is no friction or air-resistance to slow a planet. In the absence of gravity, planets would move in straight lines. However, the Sun’s gravity pulls them away from a straight line path into an elliptical path. If a planet were to stop its sideways motion, it would fall into the Sun.

  8. Hello!

    Magnetism is almost infinitely stronger than gravity.

    Could a pyramid shaped Neodymium magnet maintain its non spherical shape if it were 50 meters tall * 50 Meters wide * 50 meters long?

    The reason I ask is because 2 tiny magnets pull on each other so forcefully. Do magnets pull within themselves too?

    Question 2:

    One of Jupiter’s moons is in a very elliptical orbit, so it experiences great internal squeezing (and heat generation) when it hits the pointy parts of its orbit. Does this make the orbit decay or is it free energy?

  9. Isn’t your explainations, while not incorrect, a little misleading? You state “Soap bubbles are spherical because that shape most efficiently balances the outward pressure of the air within the bubble against the surface tension of the soap film”. Outward pressure vs surface tension is not the point.

    Isn’t the most basic governing law that everything seeks balance in the most energy efficient manner possible. The spherical shape does this because it is the most symmetrical form in existance. Planets are spherical because the creation process is the same process that governs all physical free forming construction?

  10. Richard, I think we’re both trying to say the same thing here.

    In the weightless vacuum of space gravity will always try to squeeze matter in on itself, and a sphere is the shape that permits matter to occupy the smallest possible volume.

    In the case of a gas in otherwise empty space, gravity will collapse the gas in on itself and evenly distribute the mass into the smallest possible volume. Absent any external forces to change this arrangement, the gasses will be most compressed at the center of the spherical volume and the least compressed at the surface of the spherical volume.

    For rocky objects smaller than a hundred miles or so, such as many asteroids, gravity lacks the power to compress the mass of the object into a sphere and so we’re left with lots of lumpy, potato and peanut-shaped asteroids.

    When a lump of rock in space is larger than a couple of hundred miles in diameter, such as the minor planet Ceres, gravity is strong enough to produce a spherical shape.

    Soap bubbles don’t experience much shaping due to gravity, but they do experience an analog of gravity, which is the surface tension of the soap film. Again, it’s all about natually distributing matter to achieve the smallest possible volume for the air surrounded by the soap film – which is a sphere.

  11. Leviathan,

    Atoms in a solid object are held together by electric and magnetic forces. Electric forces are much stronger than magnetic forces. Inside a magnet (or any solid), it is mostly electric forces between positive and negative charges that bind atoms together. Unlike gravity, electric forces both attract and repel. In normal matter, with equal numbers of positive and negative charges, the attractive and repulsive forces cancel each other at any appreciable distance away. So, electric forces from an individual atom act only on nearby atoms, not on the entire object, unlike gravity where every atom attracts every other atom.
    That means that the electrical forces between atoms are about the same for small and large objects

    Magnetic forces also attract and repel, so within a magnet, any magnetic attraction that exists between atoms does not significantly change as the magnet is made larger. (Magnetic forces within a magnet mostly act to align the magnet moments of the atoms). So, a large pyramid shaped Neodymium magnet could easily maintain its shape.

    Regarding the tidal forces on the moons of Jupiter – yes, there is an exchange of energy. The tidal stresses and deformations experienced by these moons (and indeed by our Moon from Earthly tidal forces) represents a transfer of angular momentum. It’s a complex exchange of energy between planet and moon, but the bottom line is that the tidal energy that deforms a moon in orbit around a planet comes at the expense of the rotational energy of the planet. Tidally deforming an orbiting moon makes the planet spin a tiny, and I mean REALLY TINY, bit slower. It’s not free energy.

  12. So if a planet is gaseous is there a certain size it must be in order to be spherical? The gaseous planets in our solar system are very large. Is it possible to have an Earth-sized planet that is gaseous?

  13. Katie,

    That’s an EXCELLENT question!

    First, we should clarify that “gas” planets would more accurately be described “fluid” planets, because beneath the topmost layers of clouds these planets have enough gravity to compress their atmospheres into a density where the differences between “gas” and “liquid” are not meaningful.

    The smallest of the “Gas Giant” planets in our solar system is Uranus, which has a diameter about 4 times larger than Earth and a mass that’s not quite 15 times the mass of Earth.

    Neptune is a little bit smaller than Uranus, but has slightly more mass, about 17 times that of Earth.

    What the Gas Giant planets in our solar system all have in common is that they’re principally made of Hydrogen, and they’re many times farther from the Sun than Earth.

    That distance is important, because if they were significantly closer to the Sun the extra warmth would cause their atmospheres to slowly escape into space and over time much of the planet’s mass would be lost.

    If you had a planet that was far from its parent star, made of gasses that would remain fluid even at intense pressures and wouldn’t freeze into a solid even at very cold temperatures (Hydrogen with significant amounts of Helium and Carbon Monoxide would make a nice mixture), you could, in theory, get a spherical, stable “gas” planet with not much more than a few times the mass of Earth.

    Even then, however, this “little” gas planet would have to be considerably larger than Earth. Earth is a “rocky” planet with a density to match – roughly 5.5 grams per cubic centimeter. That’s a lot higher than the density of a typical gas planet, which is around 0.7 to 1.6 grams per cubic centimeter.

    To contain more mass than Earth, but at a lower density than Earth, you need a considerably bigger volume than Earth. That pretty much rules out any Earth-sized gas planets.

  14. Sir,my question is why does planets prefer to take only spherical shape ? gravity is the answer but there must be some planets whose gravity bounds the molecules in triangular , ellipse or some other shape . And how does it decide that how big the planet should be (the size)? Is it the electronegativity of molecules that create gravitational force?
    What do you thing sir what is behind universe , will the universe contract henceforth , just like it is expanding ????????

  15. sir i dont if my ques reached u . But if not i will put myques in front of you again . My ques was why planets prefer to take only spherical shape there must be some planets that shows triangular ellipse shape ??????? Can electonegativity of atom could be the reason for existence of gravity ???? What do you think is behind universe and will the universe contract henceforth just like it has expanded ???
    If we fall under free fall how deep we will go????????

  16. Himanshu,

    You ask several questions. I’ll do my best to give you answers, but you’ll likely also need to do further research on your own.

    For small solid objects, those with sizes of less than a few kilometers, all kinds of non-spherical shapes are possible. You would first, however, have to identify a set of natural processes and environments that could support the formation of kilometers-wide, isolated triangular shapes in space. (I’ll leave that up to you!)

    Once a solid object is bigger than a few hundreds of kilometers in diameter, however, then the single, overwhelming, large-scale physical force acting on the object is gravity, and gravity will always work to compress such an object into a sphere.

    While it is not known exactly what “causes” the force of gravity, we do know that it is not electrical. Gravity is a sub-atomic property associated with all matter, regardless of atomic or molecular configuration.

    Will the universe ever contract? Probably not. All available evidence describes a universe that is going to keep expanding forever, or at least a length of time so great that the term “forever” may accurately be used. How long is that, you ask? That would be when the universe is a trillion-trillion-trillion-trillion-trillion-trillion-trillion-trillion times older than it is now.

    For a fun topic of casual internet research, start looking for things like “heat death of the universe” or “fate of the universe.”

    Finally, you asked, “If we fall under free fall how deep we will go?”
    Here on Earth, if you free-fall you go until you reach something solid.
    However, there are other forms of “free falling.” The International Space Station is itself in a perpetual free fall towards Earth, but due to the ISS’s velocity, the ground keeps curving away beneath the ISS as fast as the ISS is falling towards it.

    Our Moon is also in a state of perpetual free fall around Earth, and Earth and all the other planets, asteroids, and comets in our Solar System are in perpetual free fall around the Sun.

    Our Sun is merely one of a few hundred-billion stars in perpetual free fall (and taking us along for the ride) around the gravitational center of our Milky Way Galaxy, and our galaxy is itself in perpetual free fall with several hundred-billion other galaxies adrift within an expanding universe.

    How’s that for “deep”?

  17. Amazingly correct. But still is there any evidence that supports this fact that universe is expanding ?????? Big bang is only an assumption.

  18. The most easily understandable scientific evidence for the expansion of the universe can be found by observing the “red shift” of distant galaxies and the Cosmic Microwave Background Radiation.

    A great over view of many forms of evidence for the “Big Bang” and the expanding universe can be found here.

  19. Hi!
    Thanks so much for this article. I was reading up on 2012 and my curious mind ventured into the ‘anatomy of planets’ topic. This article was very easy for me to understand and laid down the basics, and I can’t wait to raise my hand in science class when we are discussing this topic. I presume I’ll need it next year, in 8th grade. Kudos for encouraging learning, haha!
    Best regards,
    Kris

  20. The earth from space certainly , like most other large heavenly bodies appear spherical . However , on earth we have both land and water . With the water we certainly take on a look of spherical or oblate spherical form from a distance but the rock and sediment portion of our planet would reveal vast differences in height and depth if the water were not here or somehow removed ( hypothetically ). The deepest part of the ocean ..the Marianas Trench is some 35000ft. deep while Mt.Everest is around 27000 ft. These are vast differences of highs and lows on surface area . If we were to make an exact duplicate of a waterless earth the size of let’s say ..a basketball ….would it roll like a sphere ? I don’t think so . So my question is …Was the earth ever smooth and uniform like a basketball ? If so why didn’t it stay that way ?

  21. Not sure if this is a silly question or not, but it’s been bugging me, so here goes. If an object has to reach a certain size in order to mold itself into a sphere, and this is why smaller objects like asteroids are all misshapen and gnarly looking, why are there only gas giants, and no gas “minis?” Why aren’t there gaseous bodies in weird looking shapes like there are for asteroids and such?

    Thanks!

  22. Tim asked, “why are there only gas giants, and no gas ‘minis?’ Why aren’t there gaseous bodies in weird looking shapes like there are for asteroids and such?”

    Tim, the answer has to do with the compressability of gasses and the minimum mass needed to create a gas planet.

    Gasses in the presence of a significant gravitational field (such as on a decent-sized planet) behave like liquids and will always seek the lowest point in the gravitational field, thus ensuring that the distribution of gasses around a spherical gravitational field is smooth and round.

    The least massive gas planet in our solar system is Uranus, with a mass about 15 times greater than Earth’s. Uranus itself is not 100% gas; it has a small core of various ices and rock surrounded by a thick, cold envelope of hydrogen, helium, methane and ammonia.

    How small you could make a gas planet depends on what kind of core it has, what gasses are present and how close it is to a star.

    Some astronomers speculate that a planet with a small rocky core about the mass of Earth that’s located far from its star could have the bulk of its mass represented by Hydrogen and Helium (making it a “gas” planet”) and still have a total mass of only about five times that of Earth.

  23. Matthew,
    Unfortunately, gravity is so weak that there is no way to show this in a classroom or a laboratory. The smallest known object in the solar system that is spherical in shape due to its own gravity is Saturn’s moon Mimas. It has a diameter of about 250 miles. So, in order to demonstrate this concept, a quantity of matter about the mass of Mimas would be needed and the demonstration would need to be done in space, far from Earth.

  24. My nephew has a question about magnetic strength . If you had let’s say a magnetic bullet or an iron bullet in a gun and you fired this bullet across a very large and very very strong magnetic field ( let’s assume a mile long natural magnet and the gun and the bullet are parrallel to the magnet ) , would the bullet be instantaneously snatched by the magnet ? My answer to him was that it depended on the mass and velocity of the bullet in relation to the relative strength of the magnet . Am I correct . In other words the bullet could be snatched the moment it left the gun if the magnetic field was strong enough…….Cy Davis

  25. I guess my nephew’s above Question is off topic. Sorry about that. Is there a page here it might be answered for him?

  26. Hi Cy,
    Your nephew’s question is off-topic, but I’m going to go ahead and answer it here because it introduces the concept of the “thought experiment.”

    The thought experiment is a time-honored tool employed by physicists to use what we already know about the physical world to help answer interesting new questions about the physical world.

    Your initial answer to your nephew’s question is correct; the behavior of the bullet in flight depends on the forces acting on the bullet. The gun is simply a way of introducing into a thought experiment the idea of an object being accelerated to a certain velocity and then released.

    I recommend you and your nephew explore behavior of a magnetic bullet acting within a large magnetic field by examining a few questions using what you already know about objects in motion:

    How would this giant magnetic environment be any different than one in which a normal bullet is fired from a normal in gun under the influence of the force of gravity?

    If the bullet is fired parallel to the ground, will it hit the ground at the same time as another bullet that’s been simply dropped from the gun at the same instant?

    What forces are acting on the bullets as they are released from the gun, and what force(s) determines when the bullet hits the ground?

    What would happen to the bullet if Earth’s gravity were 10x more powerful?

    What would happen to the bullet if the magnet/gravity instantly switched off shortly after the bullet was fired?

    Is the magnetic field you talk about in your original question working on the bullet before the bullet leaves the gun?

    What do you mean by “instantaneously snatched”? If you mean the bullet instantaneously stops its forward movement the instant it leaves the gun and it begins travelling in a straight line towards the magnet (or gravitational field), why would this occur?

    What force, and acting in which direction, would be necessary to completely cancel the bullet’s forward motion?

    If the gravitational attraction of Earth, or the mile-long magnetic field described in your question pull at right angles to the motion of the bullet, could either gravity or the magnetic field provide that velocity-canceling force?

    If these questions continue to intrigue your nephew, then I recommend a trip to the library to check out some introductory physics books and enrolling him in some math and physics courses in school.

    Good luck!

  27. Thank you again for taking the time to answer . I’ll show this to my nephew . That’s what I love about science . One question always leads to so many more .The good thing is that it keeps our thought processes flowing . I have to say that the manner in which you answer questions is both intriguing and impressive . Regards ..CY

  28. The Moon definitely has gravity. On the surface of the Moon you feel about 1/6th the gravity that you do on Earth.

    The Moon’s gravity is also responsible for most of the tidal forces on Earth.

    Tides go in… tides go out…

    It’s the Moon’s gravity.

  29. If you found yourself transported to an unfamiliar planet, what methods could you apply to verify that it is spherical and how will you find its radius?

  30. Amazing information. So much is spherical. It helps me appreciate “nature” and it’s maker even more.

    I had not realized the many large stars had gravity. It is something that I would like to look into more. Thanks

  31. Hello! I am curious as to how gravity even appears in gaseous planets, allowing them to remain spherical. How does gravity even exist when there really is nothing to physically cling to?… Actually, I think this question applies to all planets, not just the gaseous ones.

    How does gravity work in space, bring together rocks and gases to form planets and stars?

  32. I think I understand why stars and planets with sufficient mass are spherical, but what is different about protoplanetary discs that they form into a disc shape (and galaxies and accretion discs)?
    Thanks.

  33. If hypothetically we made an object containing a space larger than a couple of hundred miles in diameter that was not spherical in shape and sent it into weightless space would it then turn spherical? And if so would this be instant or would it take some time and if so how long? Also if it was to be formed into a spherical mass shape if it was mounted with an engine to thrust it would this counteract the force and if so how much force or speed would it have to go at to stop this? Many Thanks Sean.

  34. Sean,

    Imagine a box measuring a mile long, a mile wide and a mile high. Now fill that box with rocks – everything from sand to boulders. That’s one cubic mile of rock.

    Now imagine one million of these rock-filled boxes.

    Now empty your collection of one million cubic mile-sized boxes of rock in roughly the same place in space. After some number of years (any number between a few, to many thousands, depending on the original distribution of the rocks) you would end up with a roughly spherical ball of rocks about 120 miles in diameter.

    That amount of rock has just barely enough mass to pull itself together into a sphere, but not enough mass for the gravity-induced pressure at the center of sphere the heat the interior to its melting point to create a differentiated interior like Earth has (core, mantel, lithosphere, etc.)

    Gravity on this little rock-ball would be extremely weak – about one-half of one percent the gravity we feel here on Earth. A person who weighs 160 pounds on Earth would only feel like they only weighed about 13 ounces on your little rock-ball world. With so little gravity it would take a very long time for the pile of rock to assemble itself into a sphere. It would definitely not happen instantaneously.

    And yet your little 120 mile-wide ball of rock would still have a mass of more than nine million-billion tons.

    When you figure out a rocket engine that can move that kind of mass around, I’ll post a discussion of what would happen to your spherical rock-ball when it starts to move.

  35. Answers to multiple questions from above:

    Joseph: You might try testing the shape and size of the planet you found yourself on by observing whether or not the angle of the stars above you change as you change your location on the planet. This was an experiment originally performed by Eratosthenes (Google him!) in the 3rd Century B.C.E.

    Eratosthenes measured changes in the apparent location of the Sun at midday on the Summer Solstice from two locations hundreds of miles apart, and he then used that information to not only conclude that the Earth was round, he also calculated the circumference of Earth with remarkable precision.

    Xavi: Everything has gravity, including wisps of gas drifting in space. It just takes a lot of gas and a lot of time for them to fall in on themselves through mutual gravitational attraction.

    Amanda: A large enough amount of mass in space will begin to fall in on itself through mutual gravitational attraction.

    There is virtually no chance that the movement of any one in-falling bit of matter is always exactly matched and cancelled out by an equal and opposite movement by another in-falling bit of matter. This means that by the time all the in-falling matter gets close to itself there is some small dominant direction motion.

    Look at the problem this way…

    Think of dumping a million pennies from a high-altitude balloon – there’s virtually no chance you’ll get exactly 500,000 “heads” and exactly 500,000 “tails.” Instead, you’re almost always going to get a few more heads than tails, or vice-versa. It may be a tiny imbalance favoring one side over the other, but you’re pretty well guaranteed to always end up with more of one than the other.

    The same thing holds true to tiny bits of dust and gas experiencing mutual gravitational attraction in the emptiness of space. There will always be some slight imbalance of motion as gravity pulls matter together, which means that eventually the whole in-falling collection of matter begins to spin. The spin is very slow at first, but then gets faster as gravity pulls the matter in closer and closer together. It is exactly the same thing you see when a spinning figure skater spins faster as she pulls her arms in close to her body. (The phenomenon is called Conservation of Angular Momentum, if you want to look it up.)

    As the collection of matter begins to spin faster and faster, centrifugal forces begin to make the collection bulge outward in the direction perpendicular to the spin axis, and flatten along the plane of the spin. The same thing happens to spinning pizza dough when it’s being tossed to make a pizza crust. Spinning flattens the pizza dough into a disk. (Yummy!)

    That’s why matter in any solar system, especially when you get close to the central star, tends to be largely flattened into a disk. It’s also why a planet’s moons tend to orbit on a plane that’s very close to the planet’s equatorial plane, and why spiral galaxies are disc-shaped.

  36. With the many articles about the immanent close approach to Earth of the asteroid 2005 YU55, it’s repeatedly stated that the object is spherical. The radar image made by Arecibo in 2010 appears to confirm this. It shows not only a well rounded hemisphere, but a curved terminator across the middle. Since YU55 is only about 400 meters across, I was wondering how it could have sufficient gravity to form itself into a sphere.

  37. Thaks for that last answer, also. I was also wondering about galaxies and planetary nebulae. Wikipedia articles also talk about ( in the case of planet formation) ” sweeping the field” , where dust, rocks, planetoids and moons get sucked into the gravitational field, or knocked out of it, clearing away remnants of the disc. And then there’s Saturn.
    Another article I read on spherical planets mentioned the idea that all the gathered particles that make up a planet, on such a large scale act in a way analogous to liquids, sorta’ like creek bed sand does when you pour it, but acted on by forces of even greater magnitude. Is this one of the reasons I sometimes see formulas related to liquid mechanics ( and way beyond my comprehension) in articles on planetary mechanics?

  38. Jabbadah, I think we’d need to look at the “liquid mechanics” formulae provided in relation to planetary mechanics before attempting to answer your question.

  39. Quick question. I recall hearing that for an object in space to become spherical the radius must be at least 500 miles. But you said that Ceres is 600 miles in diameter making the radius 300 miles. So my question is how large does something need to be for gravity to form it into a sphere?

    P.S I noticed you first posted this in 2009 and continued to answer questions even up to this year. Thank you for just trying to share the knowledge. =)

    -Criss

  40. Criss,

    I don’t know of any rule about “at least 500 miles” for the minimum radius of an object in space to become spherical. Just off the top of my head that seems quite a bit high – even for a diameter, let alone a radius. But it is, more-or-less, in the correct order of magnitude. An object in space with a diameter of more than 1,000 miles is pretty much guaranteed to be spherical. Objects smaller than about 100 miles in diameter tend to look like regular asteroids, i.e., very lumpy – sort of like sick potatoes. Examples of this would be Mars’ two moons, Deimos and Phobos, and any number of asteroids that have been imaged with sufficient resolution to determine their shape.

    The limiting factor of course is the object’s ability to flow in response to gravity and establish what’s known as “hydrostatic equilibrium.” The more elastic the material, the more easily it can change its shape in response to gravity. That’s why the ice-rich satellites of the planets of the outer solar system are highly spherical in spite of their small size. Internal heating from tidal stresses induced by the planet around which they orbit keeps them warm enough for their own week gravity to mold them into spheres.

    Mimas, a moon of Saturn, is only 250 miles in diameter, but because it is mostly ice and subjected to tidal forces from Saturn it’s reached hydrostatic equilibrium and become spherical.

    Conversely, the asteroid 2 Pallas, with a diameter of 338 miles, receives virtually no tidal stresses from nearby planets and is basically a large rock, and appears distinctly lumpy. It’s gravity is not quite strong enough to force itself into a sphere.

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