Rogue planets

Rogue planets are planetary masses which are not gravitationally bound to any star and hence wander freely through the cosmos. Due to this nature of them, they are also termed as free floating planets or also as isolated planetary-mass object.

These are not 'feed' by a star in terms of a 

They were first discovered by UK team Lucas & Roches with UKIRT in the Orion Nebula.

Formation:

There are two ways they are believed to be formed.

The first suggests that perhaps they formed independently by the gas and dust, like a star or something.

The other suggest that perhaps they got 'kicked out' of their solar system.  Most of the rogue worlds ore believed to be ejected out of their solar systems during the early stages.

What happens when a rogue planet enters a solar system?

Rogue planets are not very uncommon. Our galaxy alone has trillions of them. So a question comes to mind. 

What would happen if it entered a solar system? Now we come up with three possibilities. First, it could remain unbound of its gravity. Second, it could get weakly bound. or third, it could 'kick out' another planet and take its place, like one it had been ejected.

Poetic justice, isn't it?

Anyways, now a threat we can think of is that what would happen if one entered our solar system?

The same could happen. But good for us that the probability of a rogue planet entering out solar system is one in one trillion. This is so low because there are countless obstacles in the way of a star.

Detection:

How could things like these be detected. We can detect them by two means. 

First is by their own light. For smaller planets which do not emit any light, we would have to use the a method called gravitational microlensing, which is going to take another post in itself.