Sub-brown dwarf

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

Summarize

Perspective

Orbiting one or more stars

There is no consensus whether these companions of stars should be considered sub-brown dwarfs or planets. Some authors agree these objects should be considered sub-brown dwarfs, since they likely formed on their own, like a "failed star".^[9]

2MASS J0441+2301 Bb

Nonetheless, the IAU working definition of an exoplanet ignored formation mechanism as a criterion, and based on it these objects would be considered planets.^[10]

Orbiting a brown dwarf

At around 2022 the IAU working definition of an exoplanet excludes these objects as planets.^[10] 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^[11] 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

WISE J0336−0143B, orbits a brown dwarf or sub-brown dwarf. The primary has a mass of 8.5 to 18 M_J and secondary has a mass of 5-11.5 M_J. This object does not fit the IAU working definition of an exoplanet.^[10] 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.^[12] 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). It could be considered a planet according to alternative definitions, but according to the IAU it only fits the definition of sub-brown dwarf.

2M1207b

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 according to the IAU.^[13]

Others

Other examples of planetary-mass objects orbiting brown dwarfs and with M_B<13 M_J and q>0.04:

CFHTWIR-Oph 98B, with M_B=11.6+0.4
−0.8 M_J and q=0.509+0.017
−0.023^[14]
2MASS J0249-0557ABc, with M_c=11.6+1.3
−1.0 M_J and $q={M_{c} \over M_{AB}}=0.13$ ^[15]

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
PSO J318.5−22 6–8 M_J about 80 light years away