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

Our orbits are pretty circular. The most elliptical is Mars with a value of 0.2, with 1 being the most elliptical.

But over the history of the solar system things like merging gas masses at different orbits, impacts of protoplanets into each other, planets passing through clouds of gasses… Pretty much anything that could affect the speed of the planet would lead to it being less circular in its orbit and those effects add up over time.