Massive Jupiter-sized Brown Dwarf
A 64 Jupiter-mass brown dwarf has recently been discovered transiting its parent red dwarf star. The star system, TOI-2119, is about a billion years old and is located just over 100 light years away. The brown dwarf companion, TOI-2119b, is located in quite a close but eccentric orbit around its parent star.
Because it routine cross in front of its star (as seen from Earth, astronomers have been able to study its size using the Transiting Exoplanet Survey Satellite (TESS) mission. They have determined that this brown dwarf (BD) companion isn't much larger in size than Jupiter:
"TOI-2119b has a radius of about 1.08 Jupiter radii, while its mass is estimated to be 64.4 Jupiter masses. It orbits its host star every 7.2 days, at a distance of some 0.06 AU from it. The orbit of this brown dwarf has an eccentricity at a level of 0.337 and is inclined 88.4 degrees. ...the authors of the paper concluded that TOI-2119b may have formed in an eccentric, close-in orbit, or it may currently be undergoing a quick inward migration from a farther out formation distance that will take billions of years to complete." (1)
Because the star system is over a billion years old,
the brown dwarf has had time to burn off its nuclear
fuel, and so cool down considerably from its early
bright youth. This will have led to
contraction as it did so:
"We generally expect BDs to
contract with age as they cool over time as they
lack a mechanism to initiate any increase in radius
like stars do." (2)
Even so, it's striking how dense this object is. It's not much bigger than Jupiter in size, but 64 times as massive! Although not discussed in the paper (beyond its temperature (1750 Celsius), I can only assume that this brown dwarf is particularly active - a seething and turbulent world of extreme heat and weather. Given what we know about Jupiter's aurorae and X-ray emissions, one can only imagine what this BD is capable of - despite being a 'failed' star.
Written by Andy Lloyd, 1st March 2022