Are planets common around brown dwarfs? We aren’t yet in a position to say, but the question is intriguing because some models suggest that the number of brown dwarfs is comparable to the number of low-mass main sequence stars. That would mean the objects — ‘failed’ stars whose masses are below the limit needed to sustain stable hydrogen fusion — could be as plentiful as the M-dwarfs that far outnumber any other type of star in the galaxy. If planets form around brown dwarfs, then we have to add them to our list of possible abodes for life.
Evidence for Brown Dwarf Planet Formation
But first, to the planet question. We can find suggestive analogs to planet formation around brown dwarfs in nearby space. The star Gl 876, some fifteen light years away, is not a brown dwarf, but this M-dwarf is only 1.24 percent as luminous as the Sun, with most of its energy being released at infrared wavelengths. We now know that at least three planets, two of them gas giants similar to Jupiter, orbit the star. Among brown dwarfs themselves, we have cases like 2M1207b, MOA-2007-BLG-192Lb and 2MASS J044144. In fact, the planet orbiting the second of these brown dwarfs is one of the smallest exoplanets known at 3.3 Earth masses.
As Andrey Andreeschchev and John Scalo (University of Texas) noted in a 2002 paper (thanks to Centauri Dreams regular ‘andy’ for the tip), we can extrapolate from what we find in our own Solar System to lower-mass stars, with simulations indicating that terrestrial-mass planets can form around low-mass objects like these as long as sufficient disk material is available. The authors study whether or not such planets can be habitable, noting this key fact about brown dwarf evolution: The brown dwarf is continually fading as it releases gravitational potential energy. As the object fades, its habitable zone moves past any worlds in it.
Time, Tides and Habitability
Is there time, then, for life to form on such a planet? When andy sent the pointer to this paper, he added an intriguing comment of his own:
It’d be interesting to come up with some scenarios for evolution on such a planet whose star decreases in luminosity as it ages (as opposed to more conventional stars that brighten as they age) – perhaps life might begin in the cloud layers of an initially Venus-like planet, moving to the surface as the atmosphere cools and the oceans rain out of the atmosphere, and finally moving to a more Europa-like state with the oceans frozen under an ice layer.
Now that’s a chewy science fiction scenario for the writers who frequent these pages to work on. Andreeshchev and Scalo note that a brown dwarf planet will be within the tidal lock radius, meaning the planet will always present one side to its star even when the brown dwarf is young, but we do have some studies showing that atmospheres can remain viable in such settings, so this may not rule out life. A bigger question is just how long the habitable zone will remain habitable and how, as andy notes, life might adapt. Clearly, evolutionary time-scales on a brown dwarf planet could be much different from those on Earth, but the paper notes that a habitability duration of less than 0.1 billion years would present real issues about the viability of complex life.
I can’t get Andreeshchev and Scalo’s diagram reproduced well enough to display well here, but they study the duration of residence in the evolving habitable zone as a function of the planet’s distance from the brown dwarf, assuming a circular orbit. They find that much depends on how we set limits on the habitable zone, but in general habitability durations of a billion years are possible for planets within 2-3 Roche radii for brown dwarfs above 0.03 solar masses. The Roche limit defines how close a planet can be to its host star before being torn apart by tidal forces. A habitable zone duration of up to 4 billion years is possible only close to the Roche limit, but could theoretically occur for brown dwarfs as small as 0.04 solar masses.
In fact, if you push these numbers to their upper limits, you can work out a habitable zone that has a duration of up to 10 billion years for a brown dwarf with a mass of 0.07 solar masses, as long as you’re willing to skirt the Roche limit about as close as possible. The authors are working, by the way, with a habitable zone definition that involves liquid water at the surface, the classic formulation of habitable zone rather than more recent extensions of the idea.
Temperature and Intelligence
This is a short but fascinating paper, and here’s something that catches the eye:
…if development of intelligence is partially driven by cooling episodes, as suggested by Schwartzman & Middendorf (2000), then on BD planets cognitive evolution may be expected to contain a stronger continuous component than on Earth.
I leave it to the science fiction writers to come up with depictions of the societies that may result. And I’ll end with the thought that if we do decide brown dwarf planets are not uncommon, and that complex life may find ways of evolving on such worlds, then nearby space may be littered with astrobiologically interesting destinations that are largely unknown to us. Or will be until infrared surveys like WISE tell us just how common brown dwarfs really are in our stellar neighborhood.
The paper is Andreeshchev and Scalo, “Habitability of Brown Dwarf Planets,” Bioastronomy 2002: Life Among the Stars. IAU Symposium, Vol. 213, 2004 (abstract).