A rogue planet, also known as an interstellar planet, nomad planet, free-floating planet or orphan planet, is a planetary-mass object that orbits the galaxy directly. They have either been ejected from the planetary system in which they formed or never been gravitationally bound to any star or brown dwarf.
Some planetary-mass objects are thought to have formed in a similar way to stars, and the IAU has proposed that those objects be called sub-brown dwarfs. The closest free-floating planetary mass object to Earth yet discovered, WISE 0855–0714, is around 7 light years away.
|An artist's conception of a rogue planet|
Most methods of detecting exoplanets rely on periodicity of the planet orbiting a host star and thus cannot be used to detect rogue planets. Two methods to detect rogue planets still can be used: gravitational microlensing and direct imaging.
When a planetary-sized object passes in front of a background star, its gravitational field causes a momentary increase in the visible brightness of the background star. This is known as microlensing. Microlensing cannot be observed continually, but it allows the detection of older and lower-mass planets than is possible through direct imaging.
There are nearly two rogue planets for every star in the Milky Way. Other estimations suggest a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way. In November 2012 astronomers discovered a rogue planet around 100 light-years from Earth.