Sub-brown dwarf

Comparison: the Sun, a young sub-brown dwarf, and Jupiter. As the sub-brown dwarf ages, it will gradually cool and shrink.

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 mass below the limiting mass for thermonuclear fusion of deuterium (about 13 M_J).^[1] Some researchers call them free-floating planets^[2] whereas others call them planetary-mass brown dwarfs.^[3]

Description

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]

Lower mass limit

The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 Jupiter mass (M_J).^[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 M_J candidate is described in a 2007 paper.^[8]

List of possible sub-brown dwarfs

Orbiting one or more stars

There is no consensus whether these companions of stars should be considered sub-brown dwarfs or planets.

• WD 0806-661 B
• DT Virginis c
• FW Tauri b
• HD 106906 b
• ROXs 42Bb

Orbiting a brown dwarf

There is no consensus whether these companions of brown dwarfs should be considered sub-brown dwarfs or planets.

• The 5–10M_J companion of 2MASS J04414489+2301513
• 2M1207b

Free-floating

Also called rogue planets:

• WISE 0855–0714 3–10 M_J about 7 light years away
• S Ori 52
• UGPS 0722-05 10–25 M_J 13 light years away
• Cha 110913-773444 5–15 M_J 163 light years away
• CFBDSIR 2149−0403 4–7 M_J 130 light years away
• OTS 44 11.5 M_J 550 light years away

See also

• Brown dwarf
• Fusor (astronomy)
• Giant planet
• Hot Jupiter
• Red dwarf
• Rogue planet
• Substellar object
• List of planet types

References