The search for signs of life on other planets has an unlikely new target. The atmospheres of brown dwarfs – gaseous objects too big to be a planet and too small to be a star – might have the capacity to support life.
Brown dwarfs aren’t massive enough to sustain nuclear fusion, the process that powers stars. But they do produce heat early in their lives. Once their fuel is exhausted they begin to cool.
“One reason we’re interested in brown dwarfs is that they’re much bigger than Earth, and the volume of the atmosphere is much bigger,” says Manasvi Lingam at Harvard University. They also cool very gradually, he says, which means they can sustain temperatures suitable for life for a long period.
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He and his colleagues calculated that the habitable volume of the atmosphere of a typical brown dwarf – places that remain at temperatures and pressures conducive to life – may exceed the habitable volume of Earth-like planets by a factor of 100.
This includes zones where the temperatures are between -23°C and 76°C. In these pockets, where the atmosphere is cool enough to allow for water clouds, Lingam says the pressure would be about 0.1 to 1 bar. The high end of that range is roughly the pressure of Earth’s atmosphere at sea level.
“We find evidence for microbes at different layers in Earth’s atmosphere, which made us wonder whether something similar could happen in the atmospheres of cool brown dwarfs,” he says.
It is true that bacteria can act as seeds for cloud droplets to form in Earth’s sky. But Paul Byrne at North Carolina State University points out that these bacteria come from the ground or the lowest reaches of the atmosphere. “When they’re up there, the bacteria don’t do much. They’re in a dormant state or they’re dead, until they fall back down.” It’s hard for life to persist up there principally because of the lack of water vapour and exposure to damaging ultraviolet rays.
Rory Barnes at the University of Washington says he is sceptical about finding life in brown dwarf atmospheres. “I think the prospects for life to originate and survive in a fairly gaseous environment is pretty close to nil,” he says. “Somehow a freak event could occur where life is able to hop from raindrop to raindrop, but we have to ask ourselves, what is the likelihood of seeing that?”
Lingam and his colleagues also point out that the key chemical ingredients for life as we know it can be found in brown dwarfs. This includes carbon, hydrogen, oxygen, nitrogen, phosphorous and sulphur.
But Abel Méndez at the University of Puerto Rico at Arecibo says there may too little of these on brown dwarfs for life. “You wouldn’t have the concentrations needed to maintain a full cycle of life, even with microbial life that doesn’t need much,” says Méndez.
However, we know some brown dwarfs are orbited by planets. It may be more likely that life could crop up on these worlds, rather than on the brown dwarfs themselves.
Lingam and his colleagues also investigated the habitable zones for these planets, and found that they’re unlikely to host life if the brown dwarf they are circling is less than 30 times the mass of Jupiter. But they calculate that there may be as many planets that could potentially host life around the larger brown dwarfs as there are around larger stars, so these environments could still be a target for finding an exoplanet with signs of life.
“This is far more likely,” says Méndez. “Terrestrial planets around brown dwarfs have a more stable surface. At least for maintaining microbial life, it’s more stable than the atmosphere of any planet.”
The Astrophysical Journal
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