What is a rogue planet?

A rogue planet is a planetary-mass object that does not orbit a star directly. Such objects have 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 to trillions of rogue planets, a range which the upcoming Nancy Grace Roman Space Telescope will likely be able to narrow down.

Some planetary-mass objects may have formed in a similar way to stars, and the International Astronomical Union has proposed that such objects be called sub-brown dwarfs. A possible example is Cha 110913-773444, which may have been ejected and become a rogue planet, or formed on its own to become a sub-brown dwarf.

Astronomers have used the Herschel Space Observatory and the Very Large Telescope to observe a very young free-floating planetary-mass object, OTS 44, and demonstrate that the processes characterizing the canonical star-like mode of formation apply to isolated objects down to a few Jupiter masses.

Herschel far-infrared observations have shown that OTS 44 is surrounded by a disk of at least 10 Earth masses and thus could eventually form a mini planetary system. Spectroscopic observations of OTS 44 with the SINFONI spectrograph at the Very Large Telescope have revealed that the disk is actively accreting matter, similarly to the disks of young stars. In December 2013, a candidate exomoon of a rogue planet was announced.

Interstellar planets generate little heat and are not heated by a star. However, in 1998, David J. Stevenson theorized that some planet-sized objects adrift in interstellar space might sustain a thick atmosphere that would not freeze out. He proposed that these atmospheres would be preserved by the pressure-induced far-infrared radiation opacity of a thick hydrogen-containing atmosphere.

During planetary-system formation, several small protoplanetary bodies may be ejected from the system. An ejected body would receive less of the stellar-generated ultraviolet light that can strip away the lighter elements of its atmosphere. Even an Earth-sized body would have enough gravity to prevent the escape of the hydrogen and helium in its atmosphere. 

In an Earth-sized object that has a kilobar atmospheric pressure of hydrogen and a convective gas adiabat, the geothermal energy from residual core radioisotope decay could maintain a surface temperature above the melting point of water, allowing liquid-water oceans to exist.

These planets are likely to remain geologically active for long periods. If they have geodynamo-created protective magnetospheres and sea floor volcanism, hydrothermal vents could provide energy for life.

These bodies would be difficult to detect because of their weak thermal microwave radiation emissions, although reflected solar radiation and far-infrared thermal emissions may be detectable from an object that is less than 1000 astronomical units from Earth. 

Around five percent of Earth-sized ejected planets with Moon-sized natural satellites would retain their satellites after ejection. A large satellite would be a source of significant geological tidal heating.

In October 2020, OGLE-2016-BLG-1928, an Earth-mass rogue planet, was discovered in the Milky Way.

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