# Why are planets round? [duplicate]

I was studying our solar system and all celestial objects are depicted as round objects, including black holes.. worm holes as hollow cylinders as a pathway across two universes.. even the galaxy is show circular.. Why does this happen?

• Just a comment about wormholes: Do not believe the cylindrical shape you've seen in pictures - this is just a common method to attempt to illustrate them on two-dimensional paper. They may not even exist by the way, so do not believe everything you read about them, especially on the internet.
– Andy
Mar 23, 2016 at 15:37
– Dean
Mar 23, 2016 at 16:12
• @SirCumference, perhaps you should post that as an answer so that this subject doesn't show up in an unanswered search. Mar 23, 2016 at 18:08
• Can least surface area be also a point? Mar 24, 2016 at 15:39
• @zephyr - There is no stackexchange rule that says that the newer of two questions that ask the same thing has to be closed as a duplicate of the older question. An older question can be closed as a duplicate of a newer question. Which is marked as a duplicate and which is left open is a judgement decision that depends on the quality of the two questions and on the quality of the answers to those two questions. Jan 26, 2018 at 15:16

Gravity makes objects compress to their centers of mass, since gravity extends in all directions.

For planets, gravity continues compressing the object until the rocks cannot be compressed anymore, since the pressure will fight against gravity. At this point (called hydrostatic equilibrium), the object has become a spheroid. One of the requirements for an object to be called a "planet" is that it has reached hydrostatic equilibrium.

For black holes, the shape of their event horizons really depend on whether the black hole is spinning or not. For non-spinning black holes, gravity extends in all directions, so the event horizon will become spherical in shape. For spinning black holes, it will likely be an oblate spheroid.

Most galaxies are only spherical when they are forming. Their angular momentum usually causes them to flatten out over time. However, one part of galaxies, called the "halo", is spherical.

Wormholes are purely hypothetical, so I won't really entertain that.

• As an extra note on galaxies in particular, I found this question on the Physics site which explains why you can get some galaxies that aren't spherical.
– Dean
Mar 23, 2016 at 19:55
• @Dean Most galaxies aren't spherical. Mar 23, 2016 at 21:09
• @RobJeffries I know, I was just making the point that they aren't necessarily "round" as the OP says. I wasn't claiming to make a reference to the number of spherical vs non-spherical galaxies with my comment.
– Dean
Mar 24, 2016 at 11:03