What would happen if a planet had no interia?

Answer 1

If a planet had no intertia, it would stop orbiting its star, it would stop spinning on its axis, and it would simply sit there, unmoving relative to its solor system, except when it was hit by a moving object that did have inertia. Even the smallest particle with inertia would cause our planet to be pushed ahead of it until our planet rolled off of the particle to one side or the other.

Since your question did not indicate an absence of gravity, too, then objects on the planet including its atmosphere would remain there. If there was life on the planet, all the life forms dependent on day/night cyles would wither and die.