Why do planets move in an elliptical orbit instead of a circular orbit?

Question

And how exactly did we find out how they move?

In: Planetary Science

Answer 1

If you have a perfectly circular orbit and a speck of dust collides with the planet then it’s no longer a perfectly circular orbit. It stays an elliptic orbit. A circular orbit is a special case of an elliptic orbit. The orbits of the planets are not that far away from a circle, but there is no reason why they should be exactly circular.