A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a planetary mass, therefore by definition below the limiting mass for thermonuclear fusion of deuterium (about 13 MJ). [1] Some researchers call them rogue planets [2] whereas others call them planetary-mass brown dwarfs. [3] They are sometimes categorized as Y spectral class brown dwarfs.
Sub-brown dwarfs are formed in the manner of stars, through the collapse of a gas cloud (perhaps with the help of photo-erosion) but there is no consensus amongst astronomers on whether the formation process should be taken into account when classifying an object as a planet. [4] Free-floating sub-brown dwarfs can be observationally indistinguishable from rogue planets, which originally formed around a star and were ejected from orbit. Similarly, a sub-brown dwarf formed free-floating in a star cluster may be captured into orbit around a star, making distinguishing sub-brown dwarfs and large planets also difficult. A definition for the term "sub-brown dwarf" was put forward by the IAU Working Group on Extra-Solar Planets (WGESP), which defined it as a free-floating body found in young star clusters below the lower mass cut-off of brown dwarfs. [5]
The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 Jupiter mass (MJ). [6] This is because to collapse by gravitational contraction requires radiating away energy as heat and this is limited by the opacity of the gas. [7] A 3 MJ candidate is described in a 2007 paper. [8]
There is no consensus whether these companions of stars should be considered sub-brown dwarfs or planets.
At around 2022 the IAU working definition of an exoplanet excludes these objects as planets. [9] The only fitting label would be as sub-brown dwarfs, but they are more often referred as planetary mass objects. Other definitions, like from the NASA Exoplanet Archive or the Extrasolar Planets Encyclopaedia would include these as exoplanets. There is no consensus whether these companions of brown dwarfs should be considered sub-brown dwarfs or planets.
WISE J0336−0143B, orbits a brown dwarf or sub-brown dwarf. The primary has a mass of 8.5 to 18 MJ and secondary has a mass of 5-11.5 MJ. This object does not fit the IAU working definition of an exoplanet. [9] This definition requires a mass ratio of about q<0.04, but the mass ratio of WISE J0336−0143AB is q=0.61±0.05. [10] It also does not fit the definition of a rogue planet, because it is gravitationally bound to a brown dwarf (or possibly sub-brown dwarf). Therefore it only fits the definition of sub-brown dwarf.
2M1207b orbits around a young brown dwarf with a circumstellar disk and itself is likely surrounded by a circumstellar disk. The mass ratio is well above the upper limit of q=0.04 for exoplanets. [11]
Other examples of planetary-mass objects orbiting brown dwarfs and with MB<13 MJ and q>0.04:
Also called rogue planets:
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. As of 1 April 2024, there are 5,653 confirmed exoplanets in 4,161 planetary systems, with 896 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.
A giant planet is a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. There are four known giant planets in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar giant planets have been identified as orbiting other stars.
Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main-sequence stars. Their mass is approximately 13 to 80 times that of Jupiter (MJ)—not big enough to sustain nuclear fusion of ordinary hydrogen (1H) into helium in their cores, but massive enough to emit some light and heat from the fusion of deuterium (2H). The most massive ones can fuse lithium (7Li).
A rogueplanet, also termed a free-floating planet (FFP) or an isolated planetary-mass object (iPMO), is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf.
2M1207b is a planetary-mass object orbiting the brown dwarf 2M1207, in the constellation Centaurus, approximately 170 light-years from Earth. It is one of the first candidate exoplanets to be directly observed. It was discovered in April 2004 by the Very Large Telescope (VLT) at the Paranal Observatory in Chile by a team from the European Southern Observatory led by Gaël Chauvin. It is believed to be from 5 to 6 times the mass of Jupiter and may orbit 2M1207 at a distance roughly as far from the brown dwarf as Pluto is from the Sun.
An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.
HD 114762 is a triple star system approximately 125 light-years (38.2 pc) away in the constellation Coma Berenices. It consists of a yellow-white F-type main-sequence star (HD 114762 A) and two red dwarf companions (HD 114762 Ab & HD 114762 B) approximately 0.36 & 130 AU distant. Both are low-metal subdwarfs. Planets around such metal-poor stars are rare. A telescope or strong binoculars are needed to view the primary. HD 114762 had been used by scientists as a "standard star", one whose radial velocity is well established, but with the discovery of the spectroscopic companion HD 114762 Ab its usefulness as a standard has been called into question.
XO-3b is an exoplanet with about 11.79 times the mass of Jupiter, and it orbits its parent star in about 3.2 days. The radius of this object is 1.217 times that of Jupiter. Astronomers announced their discovery on May 30, 2007, at the American Astronomical Society in Honolulu, Hawaii. Its discovery is attributed to the combined effort of amateur and professional astronomers working together on the XO Project using a telescope located on the Haleakala summit in Hawaii.
HD 100546, also known as KR Muscae, is a pre-main sequence star of spectral type B8 to A0 located 353 light-years from Earth in the southern constellation of Musca. The star is surrounded by a circumstellar disk from a distance of 0.2 to 4 AU, and again from 13 AU out to a few hundred AU, with evidence for a protoplanet forming at a distance of around 47 AU.
HR 8799 is a roughly 30 million-year-old main-sequence star located 133.3 light-years away from Earth in the constellation of Pegasus. It has roughly 1.5 times the Sun's mass and 4.9 times its luminosity. It is part of a system that also contains a debris disk and at least four massive planets. Those planets, along with Fomalhaut b, were the first exoplanets whose orbital motion was confirmed by direct imaging. The star is a Gamma Doradus variable: its luminosity changes because of non-radial pulsations of its surface. The star is also classified as a Lambda Boötis star, which means its surface layers are depleted in iron peak elements. It is the only known star which is simultaneously a Gamma Doradus variable, a Lambda Boötis type, and a Vega-like star.
A planetary-mass object (PMO), planemo, or planetary body is, by geophysical definition of celestial objects, any celestial object massive enough to achieve hydrostatic equilibrium, but not enough to sustain core fusion like a star.
SIMP J013656.5+093347 is a brown dwarf or planetary mass object at 19.9 light-years from Earth in the constellation Pisces. It belongs to the spectral class T2.5 and its position shifts due to its proper motion annually by about 1.24 arcsec in the right ascension.
WISE 0855−0714 is a sub-brown dwarf 2.28±0.01 parsecs from Earth, therefore the fourth-closest star or (sub-) brown dwarf system to the Sun, the discovery of which was announced in April 2014 by Kevin Luhman using data from the Wide-field Infrared Survey Explorer (WISE). As of 2014, WISE 0855−0714 has the third-highest proper motion after Barnard's Star and Kapteyn's Star and the fourth-largest parallax of any known star or brown dwarf. It is also the coldest object of its type found in interstellar space, having a temperature of about 285 K.
VHS J125601.92–125723.9 is a young triple brown dwarf system located in the constellation Corvus approximately 69.0 light-years from the Sun. The system consists of the equal-mass binary VHS J1256–1257AB and the distant planetary-mass companion VHS 1256–1257 b. In 2022, a continuous radio emission from the radiation belts surrounding VHS J1256–1257 was detected.
A circumplanetary disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a planet. They are reservoirs of material out of which moons may form. Such a disk can manifest itself in various ways.
WD 0806-661 B, also formally named Ahra, is a planetary-mass companion of the white dwarf star WD 0806−661, or Maru.
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