User: Why are all the planets and moons round in shape? What makes them round? Does there exist another shape?
Assistant: Not all planets and moons are round. There is some variety in their shapes depending on their masses, and curiously looking worlds might even exist, unlike anything we know from our system.

Everything within a planet or a moon is under the influence of gravity in such a way that bits of it would move to the center if they could. Under ideal conditions, it would acquire a spherical shape. For objects made of something other than liquids, it varies at what point they become round. It is generally assumed that they mostly become spheres around 400 kilometers/248 miles in diameter when made from icy material commonly found in our Solar System. Still, if something is made from something more rigid, it needs to be larger in diameter than that - around 750 km/466 miles. At a sufficient mass, even rock becomes fluid-like. The moon of Mars Deimos is only 12.6 km/7.8 miles across and is not spherical, and there are many other small moons of other planets that don’t have a round shape.

This, however, is not all. Astronomical objects spin, giving them the flattened shape of a sphere the faster their rotation if they are massive enough. On top of it, they can have moons. This causes a bulge to form at the position of their orbiting body around them. In the case of Earth, our Moon causes tides via this process. Also, our Sun gives our planet tides. Elsewhere in the Universe, there could be planets with massive moons or two similarly massive planets, called double planets, orbiting their common center of mass, the barycenter, at a close distance, which can cause even stronger bulging. Their shape would be affected this way more visibly.

Finally, a bizarre, toroidal planet, shown above, can be stable from what we know from physics, at least for a while. As I mentioned earlier, even rigid rock behaves like a fluid at large masses. An adequate rotation speed can counterbalance the strength of gravity with centrifugal acceleration, and a torus-shaped planet could exist. A possible scenario for such a world to come to be would be if two sufficiently fast rotating planets collided and formed a doughnut of vaporized rock, a so-called synestia.

We currently think that the events that would lead to the formation of a world of this shape are so unlikely that there might not be a single toroidal planet in the Visible Universe, but we were quite a few times surprised by what we discovered during our exploration of Cosmos. Perhaps we will even find a doughnut shape world one day.