Saturday, April 09, 2011
There's a lot of disinformation about brown dwarfs and their suitability for life so hopefully this post will help out for those doing a search on the subject. The short answer is that we don't know (since we know about very few brown dwarfs), but the key to whether life can exist on planets around brown dwarfs is mostly to do with the brown dwarf's Roche limit. The Roche limit is basically a line that cannot be crossed by an orbiting body, because if it gets to close to the planet, star, etc. it orbits, it will be torn apart by tidal forces and turn into a ring. Wikipedia shows us an object outside the limit:
then an object closer in as it begins to be deformed by the tidal forces:
and then after it crosses, it is no more:
This is, of course, over long periods of time - objects do not simply explode due to crossing the Roche limit.
Why the Roche limit matters is this: the closer a moon gets to its host planet the more tidal forces exerted upon it, and more tidal forces = more heat. The Galilean moons of Jupiter (Europa for example) are heated with tidal forces, and Io's volcanism is a result of this. Tidal forces make it possible for liquid water to exist in an area that would otherwise be far too cold. Brown dwarfs are very cool stars, so the best bet for a moon orbiting one to stay warm is tidal heating.
So, the question with brown dwarfs is thus: can a moon orbiting a brown dwarf outside its Roche limit receive sufficient tidal heating to sustain liquid water, and maintain this for long enough for life to develop?
The tentative answer to this seems to be: yes, but the more massive the brown dwarf the better. This paper (PDF) shows that a brown dwarf with a mass 2% of the sun would have a habitability zone very close to the Roche radius, but at about 6% mass and above it moves considerably farther away, giving a much larger zone of potential habitability.
There are other possibilities for liquid water on a moon around a brown dwarf. Even rogue planets that careen through space all on their own for example are thought to be possible places for life to exist because there is no solar wind to strip away their atmosphere and any heat generated from within the planet could be retained by a thick enough atmosphere. However, a quick calculation on tidal heating is one of the easiest ways to make a rough guess.