15 upvotes, 2 direct replies (showing 2)
View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
Besides life, is it possible to exist other improbable feature in the cosmos? Like infinite energy? Or a planet so small that it can fit in my hand? Is there anything impossible, out there?
Comment by WallyMetropolis at 26/04/2023 at 17:15 UTC
34 upvotes, 1 direct replies
A planet is *defined* in part based on size. So *by definition* you can't have a planet that fits in your hand. However, that doesn't say anything about what is physically possible. There are countless objects in space that are the size of an orange. Many, for example, in the asteroid belt. But to be a planet an object must:
1. Orbit a star
2. Have sufficient mass for it's own gravitational forces to cause it to be spherical
3. Have sufficient mass that it's gravity clears it's orbit of other objects as large as it
I suppose a small black hole could meet each of these definitions. But otherwise, nothing that could fit in the palm of your hand would.
Comment by uselessscientist at 26/04/2023 at 18:36 UTC
4 upvotes, 0 direct replies
Neutron stars seem to fit that description pretty well. They are 'impossibly' dense for a physical system.
Obviously, black hole singularities are more 'impossible', but we know less about them