The nominal trajectory of interstellar asteroid A/2017 U1, as computed based on the observations of October 19, 2017 and thereafter. Note the differing orbits of the planets (fast and circular), the Kuiper belt objects (elliptical and roughly coplanar), and this interstellar asteroid. Image credit: Tony873004 of Wikimedia Commons.

Ask Ethan: Why Don’t Comets Orbit The Same Way Planets Do?

Instead of nearly-circular ellipses, comets are extraordinarily elongated, or even on an exit path. Why so different?

When you look at how the planets orbit in our Solar System, the correct answer was given hundreds of years ago: first by Kepler, whose laws of motion described it, and then by Newton, whose law of universal gravitation allowed it to be derived. But comets, both the ones originating from our Solar System and the ones coming from far beyond it, don’t move in those same, nearly circular ellipses at all. Why is that? Rajasekharan Rajagopalan wants to know:

Why [do] comets orbit the Sun in a parabolic path, unlike planets which orbit in an elliptical one? Where do comets get the energy to travel such a long distance, from the Oort cloud to the Sun & back? Also, how could interstellar comets/asteroids come out of their parent star [system] and visit other ones?

We can answer this, but there’s an even bigger question we can answer: why do all objects orbit the way they do?

The planets of the Solar System, along with the asteroids in the asteroid belt, orbit all in almost the same plane, making elliptical, nearly circular orbits. Beyond Neptune, things get progressively less reliable. Image credit: Space Telescope Science Institute, Graphics Dept.