Rogue 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.[1][2][3] The Milky Way alone may have billions of rogue planets.[4]

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.[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] 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.[7] 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.[7] In December 2013, a candidate exomoon of a rogue planet was announced.[8]

Observation

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

Most methods of detecting exoplanets rely on periodic motion of the star caused by the planet in orbit. Since a rogue planet has no star, such a method cannot be used to detect rogue planets. Two methods used to detect rogue planets are gravitational microlensing and direct imaging. 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, with no glare of a host star, an isolated planet can be observed more easily, once found. When a planetary-mass object passes in front of a star, the object's gravitational field causes momentary changes in the visible brightness of the background star. This is termed microlensing. Such effects cannot be observed continually because the planet is in motion relative to the background star, but it briefly allows the detection of older brown dwarf 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.[9][10][11] Other estimates suggest a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way.[12] In November 2012, astronomers discovered a rogue planet at 100 light-years.[13]

Retention of heat in interstellar space

Interstellar planets generate little heat nor are they heated by a star.[14] In 1998, David J. Stevenson theorized that some planet-sized objects adrift in the vast expanses of cold interstellar space might 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.[15]

It is thought that, during planetary-system formation, several small protoplanetary bodies may be ejected from the forming system.[16] 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 atmosphere, composed predominantly of hydrogen and helium, would be easily confined even by an Earth-sized body's gravity.[15]

It is calculated that, for an Earth-sized object with a kilobar atmospheric pressure of hydrogen, in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will maintain a surface temperature above the melting point of water.[15] 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 there is a geodynamo-created protective magnetosphere, with possible sea floor volcanism (hydrothermal vents) providing an energy source for life.[15] Thus theoretically, humans might 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 such an object were to pass within 1000 AU of Earth.[17] A study of simulated planet ejection scenarios has suggested that around 5% 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 force heating.[18]

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

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.

Exoplanet Mass (MJ) Age (Myr) Distance (ly) Status Discovery
OTS 44 ~15 0.5–3 160 Likely a low-mass brown dwarf[19] 1998
S Ori 52 2–8 1–5 1150 Age and mass uncertain; may be a foreground brown dwarf 2000[20]
Cha 110913-773444 5–15 ~2 163 Candidate 2004[21]
UGPS J072227.51-054031.2 5–40 13 Mass uncertain 2010
[MPK2010b] 4450 2–3 325 Candidate 2010[22]
CFBDSIR 2149-0403 4–7 110–130 117–143 Candidate 2012[23]
MOA-2011-BLG-262 ~4 May be a red dwarf 2013
PSO J318.5-22 5.5–8 21–27 80 Confirmed 2013[24]
2MASS J2208+2921 11–13 21–27 115 Candidate; radial velocity needed 2014[25]
WISE J1741-4642 4–21 23–130 Candidate 2014[26]
WISE 0855−0714 3–10 7.1 Age uncertain; may be a brown dwarf 2014[27]
2MASS J12074836–3900043 11–13 7–13 200 Candidate; distance needed 2014[28]
SIMP J2154–1055 9–11 30–50 63 Age questioned[29] 2014[30]
SDSS J111010.01+011613.1 10–12 110–130 63 Confirmed 2015[31]
2MASS J1119–1137 4–8 7–13 94 Candidate; distance needed 2016[32]
WISEA 1147 5–13 7–13 94 Candidate; distance needed 2016[33]

Rogue planets in fiction

The 1977 George R. R. Martin novel Dying of the Light is mostly set on a rogue planet named Worlorn that has been temporarily made habitable as it passes a nearby star system.

The American science-fiction series Star Trek: Enterprise aired an episode on March 20, 2002 titled "Rogue Planet", where the crew encounters a rogue planet in deep space. Due to geothermal venting along the equator, enough heat and oxygen exist allowing the humans to explore that region of the rogue planet with no need for EVA suits.

The 2012 novel Dark Eden by British author Chris Beckett is set on a rogue planet known as "Eden" to the descendants of a woman and a man stranded there almost two hundred years earlier. Remote from any star, Eden is heavily ice-bound, though with temperate oases where native life occurs. This life is driven by tree-like organisms that tap heat from the planet's warm interior, and which feed a complex ecosystem that the stranded humans have become a part of.

In Lars von Trier's film Melancholia, the destruction of the Earth by a rogue planet is a plot point.

See also

References

  1. ↑ Shostak, Seth (2005-02-24). Orphan Planets: It's a Hard Knock Life. Space.com, 24 February 2005. Retrieved on 2009-02-05 from http://www.space.com/searchforlife/seti_orphan_planets_050224.html.
  2. ↑ Lloyd, Robin (2001-04-18). Free-Floating Planets – British Team Restakes Dubious Claim. Space.com, 18 April 2001. Retrieved on 2009-02-05 from http://www.space.com/scienceastronomy/astronomy/free_floaters_010403-1.html. Archived 13 October 2008 at the Wayback Machine.
  3. ↑ Author unknown (2001-04-18). Orphan 'planet' findings challenged by new model. NASA Astrobiology, 18 April 2001. Retrieved on 2009-02-05 from "Archived copy". Archived from the original on 22 March 2009. Retrieved 9 February 2009..
  4. ↑ Neil deGrasse Tyson in Cosmos: A Spacetime Odyssey as referred to by National Geographic
  5. ↑ Working Group on Extrasolar Planets – Definition of a "Planet" Position Statement on the Definition of a "Planet" (IAU) Archived 16 September 2006 at the Wayback Machine.
  6. ↑ Rogue planet find makes astronomers ponder theory
  7. 1 2 Joergens, V.; Bonnefoy, M.; Liu, Y.; Bayo, A.; Wolf, S.; Chauvin, G.; Rojo, P. (2013). "OTS 44: Disk and accretion at the planetary border". Astronomy & Astrophysics. 558 (7). Bibcode:2013A&A...558L...7J. arXiv:1310.1936 Freely accessible. doi:10.1051/0004-6361/201322432.
  8. ↑ A sub-Earth-mass moon orbiting a gas giant primary or a high-velocity planetary system in the galactic bulge
  9. ↑ Homeless' Planets May Be Common in Our Galaxy Archived 8 October 2012 at the Wayback Machine. by Jon Cartwright, Science Now, 18 May 2011, Accessed 20 may 2011
  10. ↑ Planets that have no stars: New class of planets discovered, Physorg.com, May 18, 2011. Accessed May 2011.
  11. ↑ [T. Sumi; et al. (2011). "Unbound or Distant Planetary Mass Population Detected by Gravitational Microlensing". arXiv:1105.3544v1 Freely accessible [astro-ph.EP].
  12. ↑ "Researchers say galaxy may swarm with 'nomad planets'". Stanford University. Retrieved 2012-02-29.
  13. ↑ (BBC) (Astron. & Asrophys.)
  14. ↑ Sean Raymond (9 April 2005). "Life in the dark". Aeon. Retrieved 9 April 2016.
  15. 1 2 3 4 Stevenson, David J.; Stevens, CF (1999). "Life-sustaining planets in interstellar space?". Nature. 400 (6739): 32. Bibcode:1999Natur.400...32S. PMID 10403246. doi:10.1038/21811.
  16. ↑ Lissauer, J.J. (1987). "Timescales for Planetary Accretion and the Structure of the Protoplanetary disk". Icarus. 69 (2): 249–265. Bibcode:1987Icar...69..249L. doi:10.1016/0019-1035(87)90104-7.
  17. ↑ Dorian S. Abbot; Eric R. Switzer (2 June 2011). "The Steppenwolf: A proposal for a habitable planet in interstellar space". arXiv:1102.1108 Freely accessible.
  18. ↑ Debes, John H.; Steinn Sigurðsson (20 October 2007). "The Survival Rate of Ejected Terrestrial Planets with Moons". The Astrophysical Journal Letters. 668 (2): L167–L170. Bibcode:2007ApJ...668L.167D. arXiv:0709.0945 Freely accessible. doi:10.1086/523103.
  19. ↑ Luhman, Kevin L. (10 February 2005). "Spitzer Identification of the Least Massive Known Brown Dwarf with a Circumstellar Disk". Astrophysical Journal Letters. 620 (1): L51–L54. Bibcode:2005ApJ...620L..51L. arXiv:astro-ph/0502100 Freely accessible. doi:10.1086/428613.
  20. ↑ Zapatero Osorio, M. R. (6 October 2000). "Discovery of Young, Isolated Planetary Mass Objects in the σ Orionis Star Cluster". Science. 290: 103. Bibcode:2000Sci...290..103Z. doi:10.1126/science.290.5489.103.
  21. ↑ Luhman, Kevin L. (10 December 2005). "Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk". Astrophysical Journal Letters. 635: 93L. Bibcode:2005ApJ...635L..93L. arXiv:astro-ph/0511807 Freely accessible. doi:10.1086/498868.
  22. ↑ Marsh, Kenneth A. (1 February 2010). "A Young Planetary-Mass Object in the ρ Oph Cloud Core". Astrophysical Journal Letters. 709: L158. Bibcode:2010ApJ...709L.158M. arXiv:0912.3774 Freely accessible. doi:10.1088/2041-8205/709/2/L158.
  23. ↑ Delorme, Philippe (25 September 2012). "CFBDSIR2149-0403: a 4-7 Jupiter-mass free-floating planet in the young moving group AB Doradus?". Astronomy & Astrophysics. 548A: 26. Bibcode:2012A&A...548A..26D. arXiv:1210.0305 Freely accessible. doi:10.1051/0004-6361/201219984.
  24. ↑ Liu, Michael C. (10 November 2013). "The Extremely Red, Young L Dwarf PSO J318.5338-22.8603: A Free-floating Planetary-mass Analog to Directly Imaged Young Gas-giant Planets". Astrophysical Journal Letters. 777 (1): L20. Bibcode:2013ApJ...777L..20L. arXiv:1310.0457 Freely accessible. doi:10.1088/2041-8205/777/2/L20.
  25. ↑ Gagné, Jonathan (10 March 2014). "BANYAN. II. Very Low Mass and Substellar Candidate Members to Nearby, Young Kinematic Groups with Previously Known Signs of Youth". Astrophysical Journal. 783: 121. Bibcode:2014ApJ...783..121G. arXiv:1312.5864 Freely accessible. doi:10.1088/0004-637X/783/2/121.
  26. ↑ Schneider, Adam C. (9 January 2014). "Discovery of the Young L Dwarf WISE J174102.78-464225.5". Astronomical Journal. 147: 34. Bibcode:2014AJ....147...34S. arXiv:1311.5941 Freely accessible. doi:10.1088/0004-6256/147/2/34.
  27. ↑ Luhman, Kevin L. (10 May 2014). "Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun". Astrophysical Journal Letters. 786: L18. Bibcode:2014ApJ...786L..18L. arXiv:1404.6501 Freely accessible. doi:10.1088/2041-8205/786/2/L18.
  28. ↑ Gagné, Jonathan (10 April 2014). "The Coolest Isolated Brown Dwarf Candidate Member of TWA". Astrophysical Journal Letters. 785 (1): L14. Bibcode:2014ApJ...785L..14G. arXiv:1403.3120 Freely accessible. doi:10.1088/2041-8205/785/1/L14.
  29. ↑ Liu, Michael C. (9 December 2016). "The Hawaii Infrared Parallax Program. II. Young Ultracool Field Dwarfs". Astrophysical Journal. 833: 96. Bibcode:2016ApJ...833...96L. arXiv:1612.02426 Freely accessible. doi:10.3847/1538-4357/833/1/96.
  30. ↑ Gagné, Jonathan (1 September 2014). "SIMP J2154-1055: A New Low-gravity L4β Brown Dwarf Candidate Member of the Argus Association". Astrophysical Journal Letters. 792: L17. Bibcode:2014ApJ...792L..17G. arXiv:1407.5344 Freely accessible. doi:10.1088/2041-8205/792/1/L17.
  31. ↑ Gagné, Jonathan (20 July 2015). "SDSS J111010.01+011613.1: A New Planetary-mass T Dwarf Member of the AB Doradus Moving Group". Astrophysical Journal Letters. 808: L20. Bibcode:2015ApJ...808L..20G. arXiv:1506.04195 Freely accessible. doi:10.1088/2041-8205/808/1/L20.
  32. ↑ Kellogg, Kendra (11 April 2016). "The Nearest Isolated Member of the TW Hydrae Association is a Giant Planet Analog". Astrophysical Journal Letters. 821 (1): L15. Bibcode:2016ApJ...821L..15K. arXiv:1603.08529 Freely accessible. doi:10.3847/2041-8205/821/1/L15.
  33. ↑ Schneider, Adam C. (21 April 2016). "WISEA J114724.10-204021.3: A Free-floating Planetary Mass Member of the TW Hya Association". Astrophysical Journal Letters. 822 (1): L1. Bibcode:2016ApJ...822L...1S. arXiv:1603.07985 Freely accessible. doi:10.3847/2041-8205/822/1/L1.

Bibliography

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