Eric Nolan asked:
Would a rogue planet (a planetar) be visible? So, planets that get booted out of their own system, then wander aimlessly through space. Can this happen, how does it happen and would we be able to see them as presumably they don't produce any light?
Dominic - Yes, we think there are quite a lot of objects like this. We think solar systems form, and then often many of the smaller planets get thrown out by the larger planets. And one reason we think that is, is that a lot of the solar systems we have seen around other stars have Jupiter sized planets very close in to their host stars. It's absolutely impossible to form these Jupiter sized planets there, so these planets must have formed in the outer parts of these solar systems and then spiralled in. As they spiralled in they will have thrown out any smaller planets that they spiralled past. So there probably are a large number of lonely planets just lying out there in outer space, not near any stars. Unfortunately they are virtually impossible to see and we have no idea how many there are because there's nothing to light them up - they're cold and dark.
Chris - And let's hope they're not heading this way!
Yes, planets can be ejected from their parent systems, and even their parent galaxies, and end up travelling alone through space. Given this then it's possible that one could be heading towards us.
There are efforts to estimate the number of these small dark bodies by watching lots of reasonably distant and stable stars and looking for the characteristic light variation caused by the gravitational lensing that these small dark gravitating bodies can do. I believe that so far a few likely events have been observed. Soul Surfer, Fri, 10th Dec 2010