Question: Do planets have to form around stars?
I was just wondering if a planet had to form near a star or not. If it didn't orbit one, would it be considered an asteroid?
Only around secondary stars.
We already know of planets orbiting pulsars:
Presumably other objects may initiate planetary accretion.
The problem is that our previously well honed theories of planetary formation had to be discarded after the results of the Kepler mission came in. This illustrates the danger of building "theories" that are not based on data.
The ubiquitous instability of planetary systems suggests strongly that interstellar space might be populated by a great many "rogue planets" that have been flung out of planetary systems.
You might also ponder the ramifications of the solar system's unique stability, and the likelihood that it has been just luck that has prevented a catastrophic encounter with a rogue planet during the solar system's four billion year existence.
By IAU definition, to be a planet in this solar system, it must orbit a star, in this case, the sun.
If you had a ball of gas or rock at hydrostatic equilibrium (spherical) just off orbiting a large planet, you get a moon.
If you have a body that does orbit a star but is not large and massive enough to have come to hydrostatic equilibrium, but does orbit the star, it will be an asteroid, a comet, or something else similar.
There is the idea of a "rogue planet," a planet that has satisfied all of the requirements but is for some reason moving through interstellar space rather than orbiting a star; presumably the star failed, or it was hit by something and knocked out of its solar system like a billiard ball.
Note that while it seems unlikely, if you're *outside* of the solar system, the requirement of actually orbiting a star no longer directly applies. You might have a planet somehow orbiting a black hole, for instance.
However, every planet that we know of, both in our solar system and all the 550+ extrasolar planets that we have discovered, all orbit a central star.
a planet needs to orbit a star to be considered a planet. but a star does not need to have planets orbiting it. ******
yes, a planet (as far as we know) must form around a star. As a star is forming, the debris surrounding the star forms a disc and materials on the outer edge of the disc begins to coalesce into asteroids. Over some of these asteroids are pulled together to form planets (note this is how rocky planets are formed; gas giants are formed simply from lighter materials like hydrogen and helium pulled together from the outer edge of the disc around the star). These planets may then be eventually flung out into space to become rogue planets but they still had to form around a star first. This is the modern theory on planet formation.
Although there is lots of debris around other large gravitational bodies like black holes, the debris is being bombarded with energy and smashed apart so it is hardly the type of environment a planet could form in.
No, but to be considered planets they must have a object with extreme gravity to revolve around.