Rogue planet

A rogue planet (also termed an interstellar planet, nomad planet, free-floating planet, unbound planet, orphan planet, wandering planet, starless planet, or sunless 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.[1][2][3] The Milky Way alone may have billions of rogue planets.[4]

This article is about a type of astronomical object. For other uses, see Rogue planet (disambiguation).
Rogue Planet. Artist's impression by A. Stelter

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.[5] 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.[6]

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.[7] Spectroscopic observations of OTS 44 with the SINFONI spectrograph at the Very Large Telescope have revealed that the disk is actively accreting matter, in a similar way to young stars.[7] In December 2013, a candidate exomoon of a rogue planet was announced.[8]

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

Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics and the Optical Gravitational Lensing Experiment collaborations, published their study of microlensing in 2011. They observed 50 million stars in the Milky Way by 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 Jupiter-mass rogue planets for every star in the Milky Way.[9][10][11] One study suggested a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way, though this study encompassed hypothetical objects much smaller than Jupiter.[12] A 2017 study by Przemek Mróz of Warsaw University Observatory and colleagues, with six times larger statistics than the 2011 study, indicates an upper limit on Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star in the Milky Way.[13]

Nearby rogue planet candidates include WISE 0855−0714 at a distance of 7.27±0.13 light-years.[14]

Retention of heat in interstellar space

Interstellar planets generate little heat and are not heated by a star.[15] 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.[16]

During planetary-system formation, several small protoplanetary bodies may be ejected from the system.[17] 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.[16] 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,[16] 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.[16] 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.[18] 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 force heating.[19]

Known or possible rogue planets

The table below lists rogue planets, confirmed or suspected, that have been discovered. It is yet unknown whether these planets were ejected from orbiting a star or else formed on their own as sub-brown dwarfs. Whether or not exceptionally low-mass rogue planets (such as OGLE-2012-BLG-1323 and KMT-2019-BLG-2073) are even capable of being formed on their own is currently unknown.

Exoplanet Mass (MJ) Age (Myr) Distance (ly) Status Discovery
OTS 4411.5~0.5–3554Likely a low-mass brown dwarf[20]1998
S Ori 522–81–51150Age and mass uncertain; may be a foreground brown dwarf2000[21]
Cha 110913-7734445–152~529Candidate2004[22]
SIMP J013656.5+09334711-13200~20-22Candidate2006[23][24]
UGPS J072227.51−054031.25–4013Mass uncertain2010
[MPK2010b] 44502–3325Candidate2010[25]
CFBDSIR 2149−04034–7110–130117–143Candidate2012[26]
MOA-2011-BLG-2624~May be a red dwarf2013
PSO J318.5−225.5–821–2780Confirmed2013[27]
2MASS J2208+292111–1321–27115Candidate; radial velocity needed2014[28]
WISE J1741-46424–2123–130Candidate2014[29]
WISE 0855−07143–107.1Age uncertain; may be a brown dwarf2014[30]
2MASS J12074836–390004311–137–13200Candidate; distance needed2014[31]
SIMP J2154–10559–1130–5063Age questioned[32]2014[33]
SDSS J111010.01+011613.110–12110–13063Confirmed2015[34]
2MASS J1119–11374–87–13~90Candidate2016[35]
WISEA 11475–137–13~100Candidate2016[36]
OGLE-2012-BLG-13230.007245–0.07245Candidate; distance needed2017[37][38][39]
OGLE-2017-BLG-05601.9–20Candidate; distance needed2017[40][41][42]
KMT-2019-BLG-20730.19Candidate; distance needed2020[43]
gollark: If slow.
gollark: It should actually be fairly easy to make it mostly work.
gollark: No, Milo does.
gollark: It appears that I'll have to implement some incredibly accursed code to emulate an introspection module using the turtle API.
gollark: Can Milo be modified to not need introspection modules?

See also

References
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Bibliography
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