We think of planetary orbits as one to a customer, but they don’t always have to be that way. When you have on body orbiting another, like a planet around a star or even a nice heavy moon around a planet, you get these nice big spots of gravitational equilibrium, 60 degrees ahead and behind the orbiting body.
These are called the Lagrange points, what you might have seen called L-4 and L-5 in science fiction. If you have a much lighter body than either of the first two and it is placed in either L-4 or L-5 it maintain an orbit there. This is the gravitational equilibrium.
We are fairly sure that a Mars sized object struck the early Earth and in the resulting collision our Moon was thrown off and achieved an orbit around the Earth. One of the theories as where this object came from is that it was a co-orbiting planet with the Earth and then had its equilibrium disturbed or was too big in the first place for true equilibrium to form.
This theory has received a huge boost as the Kepler Observatory has found the first observed star system to have two planets in the same orbit. The system, named KOI-730, has four planets but one three orbital tracks.
The planets (as with most of the ones we’ve been able to identify visually) are very close into their star. They have a year (on orbit) of only 9.8 Earth days or 235.2 hours. It is this fast passing between KOI-730 and the Earth that has made them easier to find by timing the flickering of the star (Kepler is an orbiting observatory, so there is no atmosphere to provide the flickering).
One of the reasons that scientists like the idea of a second planet forming at the L-4 or L-5 and then hitting the early Earth is that the impact would have had to have been relatively slow for the outcome that we observe. The question that we all want to know is the one that was asked by theNew Scientist staff reporter;
Will KOI-730's co-orbiting planets collide to form a moon someday? "That would be spectacular," says Gott. That may be so, but simulations by Bob Vanderbei at Princeton suggest the planets will continue to orbit in lockstep with each other for the next 2.22 million years at least....
So, unless you have some super longevity pills neither you nor I will be around to see such and event. But that is okay, as these are very early days for observing extra-Solar planets and as our instruments and techniques improve, we may find a system like this that is a little more likely to have two planets in the same orbit that are closer to bumping into each other.
For now we should just enjoy the majesty that is our observable universe and wonder at what else we might find as we peer into the deep dark and the deep past.