I've always wondered: Why are the stars, planets and moons round, when comets and asteroids aren’t?

This article was first published by The Conversation. Space.com's Expert voices: Op-Ed and Insights was contributed by the publication.
Jonti Horner is Professor of Astrophysics, University of Southern Queensland

"I am puzzled by why the planets and stars, moons, are all round (when other large and smaller objects such as asteroids or meteorites are in irregular shapes). Lionel Young, age 74, Launceston, Tasm

Lionel, this is a great question and a very good observation.

We see many objects in the solar system. They range from tiny grains of dirt to huge planets and the sun. The common thread among these objects is that the larger ones are more or less round while the smaller ones are irregular. Why?

Here are some of the small bodies in the solar system, scaled. While larger objects may appear rounder, the smaller ones are much more compact. (Image credit: Wikipedia/Antonio Ciccolella)

Gravity is the key to making great things happen

Gravitation is the reason why larger objects appear rounder. The center of an object's mass will be its gravitational pull. The gravitational pull of an object's mass will always point towards its center, regardless how massive it is.

Solid objects are subject to the force of gravity, which is countered by the object's strength. The downward force that Earth's gravity exerts on you doesn't pull you towards the center of the Earth. The ground pushes back at you, it is too strong to allow you to sink through it.

But Earth's strength is not unlimited. As the planet's plates press together, a mountain like Mount Everest grows larger. Everest's height increases, and its weight begins to drop. This extra weight will cause Everest to sink into the Earth's mantle and limit its height.

Mount Everest would sink all the way to Earth's centre if it were entirely made of ocean (displacing any water that it passed through). Earth's gravity would pull down any areas with unusually high water levels. The water in areas that were unusually high would be filled with water from another source, resulting in an imaginary ocean Earth that is perfectly spherical.

Gravitation is actually quite weak. A large object cannot exert enough gravitational force to overcome the strength and weight of the material it is made from. Gravitational pulls for smaller solid objects (meters and kilometers in diameter) are not strong enough to make them spherical.

This is, in fact, why you don’t need to worry about your body collapsing under the small gravitational pull that it exerts.

Continue reading: Curious Kids: How and when did Mount Everest become Mount Everest? Will it stay so?

Hydrostatic equilibrium

If an object is large enough that gravity prevails over the strength of the material it is made from, it will tend to shape all its material into a sphere. The object's higher parts will be pulled down. This will displace material below them. This will make it difficult to push upwards.

We call an object in "hydrostatic equilibrium" when it reaches that spherical form. How massive is an object needed to reach hydrostatic equilibrium? It all depends on the material it is made from. A liquid object would be able to handle it easily as the water molecules would move very easily.

An object made from pure iron, however, would have to be larger to allow its gravity to overcome the iron's inherent strength. The minimum diameter for an icy object in the solar system to be spherical, is 400 km. For objects made primarily from stronger materials, this threshold is higher.

Saturn's moon Mimas looks like the Death Star. It is spherical with a diameter 396 km. It is currently the smallest known object that might meet this criterion.

The Cassini spacecraft captured Saturn's moon Mimas as it appears in Cassini images. It is just large enough that gravity can pull it into a spherical form. Mimas looks almost like the Death Star because of the huge crater Herschel. Image credit: NASA/JPL-Caltech/Space Science Institute

Always in motion

Things get even more complicated when you consider the fact that all objects spin and tumble through space. If an object spins, it will feel a slight reduction in gravitational pull than locations closer to the pole.

Continue reading: Even planets have limits in size

This results in a perfectly spherical shape that you would expect from hydrostatic equilibrium. The object becomes an "oblate-spheroid", where it is larger at its equator and smaller at its poles. This holds true for our spinning Earth which has an equatorial radius of 12,756 km, and a pole to pole diameter of 12,712km.

The more dramatic an object's spin rate in space, the greater its effect. Saturn, which is denser than water, spins around its axis approximately every 10 and a quarter hours, compared to Earth's 24-hour cycle. It is therefore less spherical that Earth.

Saturn's equatorial diameter measures just above 120,500 km, while its polar diameter measures just over 108.600 km. This is a difference of nearly 12,000 km

Cassini's September 2017 Cassini spacecraft's final widefield mosaic of Saturn, its moons and Saturn gives us a good idea of how large the planet is. (Image credit: NASA/JPL-Caltech/Space Science Institute)

Some stars are even more extreme. Altair, a bright star visible in the northern skies from Australia during winter months, is an example of such an oddity. It spins about once every nine hours. It spins at a speed of 25 mph, which is faster than the distance between its poles.

The short answer

You learn more when you get closer to a question like this. To put it another way, the reason large astronomical objects appear spherical or nearly so is that they are massive enough to overcome the strength and gravitational pull of the material from which they were made.

This article is part of I've Always Wondered. It's a series in which readers submit questions for experts to answer. Send your question to alwayswondered@theconversation.edu.au

This article was republished by The Conversation under Creative Commons. You can read the original article.

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