[Titanscape (OP)]

Hi, I heard and saw a depiction of the Earth's early formation. And of a collision with a Mars sized planet. How is it then, that the Earth orbits the sun in a circular rather than an elliptical orbit?

[Soul Surfer]

The collision was probably quite a "gentle" glancing collision with an object going in much the same direction.  Most of the  collision energy probably came from the  mutual attraction of the two bodies so it is unlikely that the orbits would change very much and the orbit of the lightest object would change most.

[Airthumbs]

The Earth does not orbit the Sun in a circular motion.  Our orbit is elliptical!

http://en.wikipedia.org/wiki/Earth's_orbit

[Soul Surfer]

That's a bit pedantic.  When we talk about planets having circular orbits we always mean orbits with a small ellipticity there is essentially no such thing as a perfect circular orbit.

[Pmb]

That's a bit pedantic.  When we talk about planets having circular orbits we always mean orbits with a small ellipticity there is essentially no such thing as a perfect circular orbit.

There are always man made satelites which can change they're orbit with the equipment on board such as gas jets they use on satelites. But to be precisely and exactly zero is something that cannot be done. There are situations when no measurement can be exact in fact. Then again I know exactly how much money is in my wallet, which is not very much. All donations for me can be sent to me. Just e-mail me and we'll make arrangements. LOL

[damocles]

The solar system is a many-body system. But as far as any individual planet is concerned, its motion can be described (to a good approximation) in terms of just itself and the sun, because the sun has well over 90% of the total mass in the solar system.

So we say that planets orbit the sun in elliptical orbits, or, if the eccentricity is small enough, circular orbits.

But this is only a good first order approximation. What is the effect, over millions of orbital cycles, of perturbations due to other planets, or drag due to the very sparse interplanetary medium, or outward pressure due to the solar wind? I do not know about the detail of perturbation theory solutions for many-body systems of the solar system type, but I suspect that one property of the accumulated perturbations would be to gradually reduce the eccentricity of planetary orbits.

[Soul Surfer]

I am sorry to have to tell you Damocles that there is only really one stable solution to the orbits of three or more bodies under the influence of gravity.  That is orbits in an equilateral triangle (like the trojan asteroids).  All other orbiting systems are at best metastable.  Analysis of the solar system suggests that in its current configuration it has a long life but errors do not always fade out.

There are some recently publicised orbital structures that are stable but they are very peculiar and very precisely defined,  could not evolve naturally and of extremely limited stability.