if perpetual motion is impossible, why/how do planets orbit and spin continually?

Question

if perpetual motion is impossible, why/how do planets orbit and spin continually?

In: Physics

Answer 1

There is no friction* in space. Since the planets don’t have to do any work overcoming friction, they don’t lose energy and can continue orbiting.

*) in reality there’s a minuscule amount of friction from the tiny bits of debris in space, and planets are losing some tiny, tiny amount of energy due to gravitational waves, but these losses are so small it’s safe to ignore them for planets not orbiting in gas clouds.

Edit: a planet’s rotation around its own axis slows down due to tidal braking. That’s what happened to our moon, which is tidally locked to the earth.