Interstellar planet

A rogue planet (also termed an interstellar planet, nomad planet, free-floating planet, orphan planet, wandering planet, starless planet, sunless planet, or Planemo) is a planetary-mass object that orbits the galaxy directly. Such objects have either been ejected from the planetary system in which they formed or have never been gravitationally bound to any star or brown dwarf. The Milky Way alone may have billions of rogue planets.

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. A possible example is Cha 110913-773444, which might have been ejected and become a rogue planet, or otherwise formed on its own to become a sub-brown dwarf. The closest free-floating planetary-mass object to Earth yet discovered, WISE 0855−0714, is at 7 light years, though it may be a sub-brown dwarf.

Recent observations of a very young free-floating planetary-mass object, OTS 44, with the Herschel Space Observatory and the Very Large Telescope demonstrate that the processes that characterize the canonical star-like mode of formation apply to isolated objects down to a few Jupiter masses. Herschel far-infrared observations show that this young free-floating planetary-mass object is surrounded by a disk of at least 10 Earth masses, and thus eventually, can form a mini planetary system. Spectroscopic observations of OTS 44 with the SINFONI spectrograph at the Very Large Telescope reveal that the disk is actively accreting matter, in a similar way to young stars. In December 2013, a candidate exomoon of a rogue planet was announced.

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