# Low-eccentricity orbits and the Plane of the Elliptic

1. Aug 29, 2014

### GofG

So I've been playing some Kerbal Space Program, and it's taught me a couple things. Primarily, that circularizing an orbit (bringing its eccentricity down to near 0) is a matter of extremely precise engineering, and that matching planes with another satellite's orbit is also extremely nontrivial.

How is it that an inordinate amount of the stuff orbiting Sol all shares the same plane of the elliptic? All of the planets, all the asteroids, most of the transneptunian junk... I scoured Wikipedia for an explanation, but couldn't find one. Is it normal for star systems to have a single plane of the elliptic for most of their satellites? What caused this?

Secondly and more importantly, why do the orbits of all the planets and junk have such low eccentricity? In KSP, I can spend hours with a high-ISP engine and kilotons of rocket fuel trying to get my eccentricity below 0.10, and yet all of the planets in our system seem to have accomplished this feat without even any course corrections! How did this happen? I would expect Earth's orbit to be circular á la the anthropic principle, but why do the other planets have this property as well?

I suspect the answer is simple and I'll feel foolish when I hear it, but I'm still quite eager to hear it.

2. Aug 29, 2014

### Staff: Mentor

According to computer models and the hundreds of planetary systems studied so far, yes.

During formation of a stellar system, the cloud of dust and gas becomes a disk (a disk is the only "stable" configuration without permanent collisions). And in the very long run, other orbits have problems with stability. That's nothing Kerbal Space program can show.

Long-term stability is also important for the eccentricity. In a planetary system as dense as ours, any highly eccentric orbit would be very unstable due to the influences of other planets.

3. Aug 30, 2014

### Ophiolite

To reinforce what mfb has said: you have spent hours trying to achieve a result. The planets spent a few million, or even a few scores of millions of years to achieve that low eccentricity.

A high eccentricity planet will eventually collide with another planet, plunge into the sun, or through gravitational interaction be ejected from the system. Highly eccentric orbits are rarely going to be stable over billions of years.

Note that the Trans-Neptunian objects have higher eccentricities and stray further from the ecliptic, while that is even more the case for Oort cloud comets - there is just so much more space out there (and the objects are that much smaller) that they can 'get away with' being eccentric.