[hinckley buzzard]

Interesting. One would think that after billions of years they would have been captured by the gravity of some great star as they passed by.

[Abin Sur]

One would think that after billions of years they would have been captured by the gravity of some great star as they passed by.

Space is just too big for this to be likely. Even over the course of billions of years, the chance of such a planet wandering close enough to a star to be gravitationally captured is very small.

[Vince Ferrer]

Interesting. One would think that after billions of years they would have been captured by the gravity of some great star as they passed by.

If they are traveling through any particular solar system at a speed faster than the escape velocity of the system, then they would not be captured. Only if they are already going slow, or if they are slowed through a "slingshot" effect, could they be captured. And if they were created out beyond the orbit of any star, in the galactic spaces, they would be traveling that fast.

[buwaya]

Just as likely to have been perturbed away from an orbit around a star, and gone off into interstellar space.

[Abin Sur]

One would think that after billions of years they would have been captured by the gravity of some great star as they passed by.

Someone just did the math: http://blogs.discovermagazine.com/badastronomy/2011/05/19/are-we-in-danger-from-a-rogue-planet/

It seems the chance of one of these rogue planets wandering close enough to us to be a danger is almost zero.