Could Brown Dwarf Sub-Dwarf Companions Host Habitable Moons?

[|Glitch|]

The majority of our focus for exoplanets have been around main sequence stars, understandably so. However, one place we have not looked very closely is for exoplanets around brown dwarfs (or would it be exomoons?). Brown dwarfs are not quite planets and not quite stars, so I am not certain whether to refer to an orbiting object around a brown dwarf as either a moon or a planet. For the purposes of this post, I will reference it as a "sub-dwarf companion."

Whatever the reference of such an object may be, it raises some interesting possibilities. For example, does a deuterium fusing brown dwarf generate solar winds? It is unlikely that such an object would produce solar flares, although it should still have a very strong magnetic field. In many respects brown dwarfs might be more conducive for life than red dwarf main sequence stars, given the right conditions.

Another consideration is the number of moons in our solar system. It would appear that the larger the planet, the more moons it is likely to have. Jupiter has ≈62 moons, therefore it is not unreasonable to presume that a brown dwarf might also have a large number of sub-dwarf companions.

Obviously one of those conditions has to be whether or not the sub-dwarf companion is within the habitable zone of the brown dwarf. The temperature and atmospheric pressure has to be within the triple point of water in order to have liquid water on its surface. Unfortunately Kopparapu et al. (2014) cannot be used because it depends upon luminosity of the main sequence star. However, the Stefan-Boltzmann Law can be used, if one factors in Albedo and radiative forcing.

It just so happens that there is a brown dwarf binary system only 1.9980 ± 0.0004 parsecs (6.516 ± 0.001 lightyears) from Sol - Luhman 16AB. Furthermore, there may be a sub-dwarf companion orbiting one of these two brown dwarfs. The mass given for Luhman 16A is between 0.04 and 0.05 M_☉ (47 ± 5 M_J) with an effective surface temperature of 1,350°K. The mass given for Luhman 16B is between 0.03 and 0.05 M_☉ (42 ± 10 M_J) with an effective surface temperature of 1,210°K. Given their size it is unlikely that either brown dwarf is fusing lithium. These binary brown dwarfs are separated by an estimated 3 AU.

Therefore, applying the Stefan-Boltzmann Law, and factoring in an Albedo of 30% and radiative forcing of +21.77°K, we get a habitable zone range of between 202,546 km and 532,442 km for Luhman 16A, and a habitable zone range between 144,635 km and 380,209 km for Luhman 16B.

Both habitable zones would seem to be extraordinarily close to their parent brown dwarf, and indeed most likely cause any sub-dwarf companion to be tidally locked. Any sub-dwarf companion would undoubtedly be influenced by the strong magnetic field of the brown dwarf as well, but unlikely to experience any loss of atmosphere due to solar flares or solar winds.

If anyone has any reason why such a scenario is not plausible, I would be interested in hearing from you.

Surface map of Luhman 16B recreated from VLT observations ​

Sources:
Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass - The Astrophysical Journal Letters, Volume 787, Number 2, May 15, 2014 (free issue)
Resolved Near-Infrared Spectroscopy of WISE J104915.57-531906.1AB: A Flux-Reversal Binary at the L dwarf/T dwarf Transition - The Astrophysical Journal, Volume 772, Number 2, July 16, 2013 (free issue)
Possible astrometric discovery of a substellar companion to the closest binary brown dwarf system WISE J104915.57–531906.1 - Astronomy & Astrophysics, Volume 561, January 2014 (free issue)
On the Nearby Binary Brown Dwarf WISE J104915.57-531906.1 (Luhman 16) - arXiv 1303.5345

 

[Ratman]

The timescale for deuterium fusion in brown dwarfs is up to several million years, which is short not only for the emergence of life, but for planetary accretion. From the end of fusion the dwarf "cools down like a rock" which obviously takes some time, but in a geologic time scale the luminosity decreases rapidly and habitable zone soon gets too tight for a potential orbiting world to stay warm. Moreover your calculation put the habitable zone very close or even at the Roche limit, so such unlucky satellite could be torn apart by tidal forces OR prevented from accreting in the first place. And some young brown dwarfs emit X-ray flares...

 

[|Glitch|]

The timescale for deuterium fusion in brown dwarfs is up to several million years, which is short not only for the emergence of life, but for planetary accretion. From the end of fusion the dwarf "cools down like a rock" which obviously takes some time, but in a geologic time scale the luminosity decreases rapidly and habitable zone soon gets too tight for a potential orbiting world to stay warm. Moreover your calculation put the habitable zone very close or even at the Roche limit, so such unlucky satellite could be torn apart by tidal forces OR prevented from accreting in the first place. And some young brown dwarfs emit X-ray flares...

Thanks for the feedback.

I was aware that brown dwarfs in the 60 to 80 Jupiter mass range fused all their lithium in approximately 100 million years, but I could not find any information on the rate in which brown dwarfs in the 13 to 60 Jupiter mass range fused their deuterium. Perhaps you could provide a source.

The Roche limit of Luhman 16A for an Earth radius and mass object is 197,904 km. That would put it just outside (4,642 km outside) the habitable zone inner limit. However, the Roche limit of Luhman 16B for an Earth radius and mass object is 190,285 km. Which would put it about 45,650 km inside the habitable zone inner limit. That would not be a good thing. Of course, reducing the size of the sub-dwarf companion (making it smaller than one Earth radius and mass) could solve that problem. A Mars radius and mass sub-dwarf companion, for example, would give Luhman 16B a Roche limit of 134,325 km. Well outside (10,310 km) the habitable zone inner limit.

The only example I could find of a "brown dwarf" emitting an x-ray flare was LP 944-20 in December 1999. While it is indeed referenced as a "brown dwarf" its spectral type is actually an M9 main sequence star. So it is not really a brown dwarf. I could find no examples of an object that fuses deuterium releasing a flare of any kind.

I would very much like to know the rate at which brown dwarfs in the 13 to 60 Jupiter mass range fuse their deuterium, if anyone has that information. Thanks.

 

[glappkaeft]

I would very much like to know the rate at which brown dwarfs in the 13 to 60 Jupiter mass range fuse their deuterium, if anyone has that information. Thanks.

It's complicated and unless someone know of a overview paper the only recourse is to dive into the relevant papers.
Here is one for instance: https://inspirehep.net/record/866788/

 

[glappkaeft]

And I should add that deuterium fusion is a thought to be a very brief event on astronomical time scales especially for smaller brown dwarfs.

 

[|Glitch|]

It's complicated and unless someone know of a overview paper the only recourse is to dive into the relevant papers.
Here is one for instance: https://inspirehep.net/record/866788/

Thanks. It is more information than I had before.

 

[Ratman]

My main reference on red and brown dwarfs is New Light on Dark Stars by Reid & Hawley, 2nd ed. As for the X-rays there are others: for example a strong flare was detected from Gl 569 Bab, otherwise a soft X-ray emitter old enough to have used up all the deuterium. LP 944-20 has estimated mas of 7% solar and lithium lines in the spectrum, so it is a brown dwarf. The spectral class is not a discriminant as there are BDs of spectral class M as well as true hydrogen-fusing stars of spectral class L, though only a handful.

The companion of our hypothetical BD still has to form inside this narrow band of orbits in a short time period. If our Solar Systems' giant planet systems are representative, the large companions would form much farther from the dwarf leaving only little chunks of rock, or rings, inside the HZ. If we consider a migration scenario, the companion has to settle on a low-eccentricity orbit in a narrow zone (e<0,45 for Luhman 16 A or B), and avoid falling into the dwarf.