Why are the stars, planets and moons round, when comets and asteroids aren't?

Toowoomba (Australia), July 16

Why are planets, stars, and moons all around us when other large and small objects like asteroids and meteorites have irregular shapes? — Lionel Young, Launceston, Tasmania, age 74.

Lionel, you've asked a terrific question and made an excellent observation! When we look out at the Solar System, we see everything from small dust grains to enormous planets and the Sun. The huge ones are (more or less) circular, whereas the small ones are uneven, which is a recurrent motif among those objects. But why is that?

Gravity is the key to turning large objects around

The influence of gravity was found to be the reason why the larger items are circular. The gravitational force of an item will always point to its mass center. The more massive something is, the stronger its gravitational attraction becomes.
The strength of the thing itself opposes such force in solid objects. The downward force you feel as a result of Earth's gravity, for example, does not drag you into the planet's center. Because the ground is too strong to allow you to sink through it, it pushes back at you.
Earth's might, on the other hand, has its bounds. Consider how a massive mountain, such as Mount Everest grows in size as the planet's plates collide. Everest's weight increases as it grows bigger, to the point where it begins to sink. The added weight will drive the mountain further into the Earth's mantle, limiting its height.
Mount Everest would sink all the way to the Earth's core if the Earth were totally constituted of water (displacing any water it passed through). Any regions where the water level was unusually high would sink as the Earth's gravity drew them down. Water pushed from elsewhere would fill in areas where the water level was shallow, causing this hypothetical ocean Earth to become perfectly spherical.

But the truth is that gravity is feeble. Before an object can exert a strong enough gravitational attraction to overcome the material's strength, it must be huge. Smaller solid objects (a few meters or kilometers in diameter) have fewer gravitational pulls, which prevent them from forming a spherical shape.
This is also why you won't collapse into a spherical shape with your own gravitational attraction – your body is far too robust for the minuscule gravitational pull it exerts to do so.

achieving hydrostatic balance

When an object is large enough that gravity triumphs over the object's material strength, it will tend to draw all of the thing's materials into a spherical shape, too-high parts of the object will be dragged down, displacing material beneath them, causing regions that are too low to push outward.
When the object achieves that spherical shape, we call it "hydrostatic equilibrium." But, to reach hydrostatic equilibrium, how huge must an item be? That depends on the material. It would be quite easy for an object composed entirely of liquid water to handle it, as it would have no strength - water's molecules move about very quickly.
Meanwhile, an object constructed entirely of pure iron would need to be significantly larger for gravity to overcome the iron's intrinsic strength. The diameter required for an icy object to become spherical in the Solar System is at least 400 kilometers. The threshold is even higher for objects formed mostly of stronger material.

Mimas, Saturn'smoon that resembles the Death Star, is spherical and has a diameter of 396 kilometers. It's currently the tiniest object we're aware of that might fit the bill.

Always on the move

Things grow much more difficult when you consider that all objects tend to spin or tumble across space. When an object spins, places near the equator (the point halfway between the two poles) have a somewhat lower gravitational pull than those near the pole.
As a result, the completely spherical shape that you'd anticipate in hydrostatic equilibrium is changed to an "oblate spheroid," where the object's equator is wider than its poles. Our spinning Earth has an equatorial diameter of 12,756 kilometers and a pole-to-pole diameter of 12,712 kilometers.
The more quickly a space object spins, the more dramatic this effect becomes. Saturn, which has a lower density than water, rotates on its axis every ten and a half hours (as opposed to Earth's 24-hour cycle). As a result, it is not quite as spherical as Earth.
The equatorial diameter of Saturn is slightly over 120,500 kilometers, while the polar diameter is just over 108,600 kilometers. That's a distance of about 12,000 kilometers. Some celebrities are even more outlandish. One such oddity is the bright star Altair, which may be seen in the northern sky from Australia during the winter months. It rotates every nine hours or so. That's so quick that its equatorial diameter is 25% bigger than the distance between its poles

The quick response is

To put it another way, the reason that large celestial objects are spherical (or nearly spherical) is that their gravitational force is strong enough to overcome the material's strength. (Source: The Conversation)