QUESTION:
Why can't two planets share one orbit?

ANSWER from Dr. Thomas L. Roellig on May 10:
Actually, two  planets can go in the same orbit, although it would be 
very unlikely for this to happen naturally. It would take very precise 
positioning of the planets if this were to work. Nature is not usually 
that precise when it formed the planets. I will explain later below 
the instance where this has happened in our solar system. With our 
rockets man can achieve the precision that is necessary to achieve 
such careful orbit placement. For example, we frequently launch 
satellites around the earth that share the same orbit. Some examples 
of satellites like this are the communication satellites that are used in 
sending telephone calls and television programs. In most cases these 
orbits are what we call unstable, which means that eventually the 
satellite will move out of this orbit and go elsewhere, although it may 
take a long time before this happens. 

There is one example in nature where Jupiter and two groups of 
objects share essentially the same orbit. The groups of objects are 
known as the Trojan asteroids. An asteroid is a chunk of rock that is 
orbiting the sun like a planet, but is not big enough to be considered 
one of the planets. Some of the asteroids orbiting around the sun are 
bunched up so that they are located in the same orbit as Jupiter, with 
one group ahead of Jupiter in its orbit and the other group behind 
Jupiter in its orbit. According to the laws of gravity these asteroid 
groups are in what is known as stable positions. The more accurate 
name for this type of position is a "stable Lagrangean position." So 
you see that in some cases planets or planet-like objects can be in 
the same orbit.


Dr. Thomas L. Roellig
Astrophysicist