rogue planet, in astronomy, planets that do not orbit a star but instead wander freely through space. Astronomers believe that most rogue planets were forced out of their original star systems via gravitational encounters with other celestial bodies.

Rogue planets are objects smaller than brown dwarfs, objects that are intermediate in size between planets and stars. Many astronomers draw the line between brown dwarfs and planets at the lower fusion boundary of about 13 Jupiter masses, or 0.013 times the mass of the Sun. Objects heavier than this burn deuterium in their cores.

The first rogue planets were discovered in 2000 and were at the upper end of this mass range, larger than about five Jupiter masses. They were found in optical and infrared observations of regions where stars are being born, and thus these rogue planets were likely formed in a similar manner to stars through the collapse and fragmentation of a molecular cloud core.

Rogue planets much smaller than Jupiter are too small to be seen directly in optical and infrared telescopes. However, such planets can be indirectly observed through gravitational microlensing, which happens when a celestial body such as a star or planet aligns almost perfectly with an object behind it. A kind of natural lens is then created when the gravitational field of the object in front bends the light emitted by the object in back and thus the object in back seems to grow brighter. Rogue planets as small as between 0.3 and 2 Earth masses have been observed with microlensing.

Rogue planets are quite numerous. Extrapolating from the results of years-long microlensing surveys gives a galactic population of about two trillion rogue planets, with small, Earth-sized bodies more numerous than large, Jupiter-sized objects. There are even six times as many rogue planets as planets that orbit stars.

Because rogue planets do not orbit a star, most are believed to be icy and incapable of sustaining life. However, it has been conjectured that rogue planets that retain their original hydrogen-helium atmospheres and have internal radioactive heating could have liquid water on their surfaces and thus suitable conditions for life. It has also been conjectured the moons of rogue planets might be more hospitable to life thanks to the presence of water that could survive under a thick carbon dioxide atmosphere that would trap the heat generated by tidal friction, and, if a moon was large enough, it could heat its planet through tides.