What can a circle be in space?

Answer 1

Nothing, really. Circles cannot exist in a three-dimensional world, as a circle only

has two. If by circle, you mean sphere, then eliptical galaxies, stars, and other

celestrial bodies are usually spheres.

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If no 2-dimensional entity can exist in a 3-dimensional world, then certainly

1-dimensional ones can't either, and there's no such thing as a straight line.

Any closed gravitational orbit lies entirely in a 2-dimensional plane, and it can

be a circle, although almost none of them are. Probably the closest thing to a

circular orbit is the ones that geostationary satellites are put in. Each of those

is torqued and tweaked by ground controllers for weeks or months, until the

orbit is finally circular enough to be usable.