Rogue planet

This article is about a type of astronomical object. For other uses, see Rogue planet (disambiguation).

This video shows an artist's impression of the free-floating planet CFBDSIR J214947.2-040308.9.

A rogue planet, also known as an interstellar planet, nomad planet, free-floating planet, orphan planet or starless 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 have never been gravitationally bound to any star or brown dwarf. There may be billions of rogue planets in the Milky Way.

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 (an example of this is Cha 110913-773444, which may be an ejected rogue planet or may have formed on its own and be a sub-brown dwarf). The closest free-floating planetary-mass object to Earth yet discovered, WISE 0855−0714, is around 7 light years away.

Recent observations of a very young free-floating planetary-mass object 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 the young free-floating planetary-mass object OTS 44 is surrounded by a disk with a mass of at least 10 Earth masses and can, therefore, eventually 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 similar to young stars.

In December 2013, a candidate exomoon of a free-floating planet was announced.

Observation

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.

Artist's conception of a Jupiter-size rogue planet.

Direct imaging allows astronomers to observe rogue planets continuously. However, only young and massive rogue planets can be observed this way because they are the only ones that emit enough radiation to be detected. On the other hand, without the glare of the host star, the planet itself can be observed more easily once found.

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. Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations, carried out a study of microlensing which they published in 2011. They observed 50 million stars in the Milky Way using the 1.8-meter MOA-II telescope at New Zealand's Mount John Observatory and the 1.3 meter University of Warsaw telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that 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.

Retention of heat in interstellar space

In 1998, David J. Stevenson theorized that some planet-sized objects drift in the vast expanses of cold interstellar space and could possibly sustain a thick atmosphere that would not freeze out. He proposes that atmospheres are preserved by the pressure-induced far-infrared radiation opacity of a thick hydrogen-containing atmosphere.

It is thought that, during planetary-system formation, several small protoplanetary bodies may be ejected from the forming system. With the reduced ultraviolet light that would normally strip the lighter components from an atmosphere, due to its increasing distance from the parent star, the planet's predominantly hydrogen- and helium-containing atmosphere would be easily confined even by an Earth-sized body's gravity.

It is calculated that, for an Earth-sized object at a kilobar hydrogen atmospheric pressures in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will be sufficient to heat the surface to temperatures above the melting point of water. Thus, it is proposed that interstellar planetary bodies with extensive liquid-water oceans may exist. It is further suggested that these planets are likely to remain geologically active for long periods, providing a geodynamo-created protective magnetosphere and possible sea floor volcanism which could provide an energy source for life. Thus humans could theoretically live on a planet without a sun, although food sources would be limited. The author admits these bodies would be difficult to detect due to the intrinsically weak thermal microwave radiation emissions emanating from the lower reaches of the atmosphere, although later research suggests that reflected solar radiation and far-IR thermal emissions may be detectable if one were to pass within 1000 AU of Earth.

A study of simulated planet ejection scenarios has suggested that around five percent of Earth-sized planets with Moon-sized natural satellites would retain their satellites after ejection. A large satellite would be a source of significant geological tidal heating.

Proplyds of planetars

Recently, it has been discovered that some exoplanets such as the planemo 2M1207b, orbiting the brown dwarf 2M1207, have debris disks. If some large interstellar objects are considered stars (sub-brown dwarfs), then the debris could coalesce into planets, meaning the disks are proplyds. If these are considered planets, then the debris would coalesce as satellites. The term planetar exists for those accretion masses that seem to fall between stars and planets.

Known or possible rogue planets

There is no current way of telling whether these are planets that have been ejected from orbiting a star or were originally formed on their own as sub-brown dwarfs.

Planet	Mass (MJ)	Distance (ly)	Status	Discovery
WISE 0855−0714	3–10	7.1	May be a brown dwarf	2014
S Ori 52	2–8 (or brown dwarf)		Mass not constrained
UGPS J072227.51-054031.2	5–40	13	Mass not constrained	2010
Cha 110913-773444	5–15	163	Mass not constrained	2004
CFBDSIR 2149-0403	4–7	130±13	Candidate	2012
PSO J318.5-22	6.5	80	Confirmed	2013
MOA-2011-BLG-262	~4		May be a red dwarf	2013
OTS 44	6–17	160	Confirmed	2013

See also

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References

• Article by Stevenson similar to the Nature article but containing more information, titled: "Possibility of Life Sustaining Planets in Interstellar Space"

External links

• Definition of a "Planet" (Resolution B5 - IAU)
• Strange New Worlds Could Make Miniature Solar Systems Robert Roy Britt (SPACE.com) 5 June 2006 11:35 am ET
• The IAU draft definition of "planet" and "plutons" press release (International Astronomical Union) 2006