WE ARE already taking the first steps toward learning if there could be life on TRAPPIST-1’s newly discovered planets – and what that life might look like.
Last week, a team led by Michaël Gillon at Belgium’s University of Liege announced that TRAPPIST-1, a small, faint star some 40 light years away, has four more rocky planets to join the three we already knew about.
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All are less than 20 per cent bigger than Earth, and all orbit well within the distance at which Mercury circles our sun. Despite this closeness, the planets may be candidates to search for life. That’s because TRAPPIST-1 is much smaller and dimmer than the sun, so three of the planets may be cool enough to host liquid water on the surface, putting them in the habitable zone (see diagram).
But small, red stars like TRAPPIST-1 can emit powerful flares of X-rays and UV radiation. Too much UV radiation could leave a planet lifeless – and since they orbit so close to their star, TRAPPIST-1’s planets are at particularly high risk.
So astronomers are puzzling out whether life could find a way, and if so how we could spot it.
Lisa Kaltenegger and Jack O’Malley-James, both at Cornell University in New York, examined how well different atmospheres could keep harmful radiation from reaching the surface of those planets in the habitable zone. They found that an atmosphere similar to present-day Earth’s would provide enough protection for life as we know it to survive there, even if the star turns out to be one of the most active of its type.
An atmosphere one-tenth as thick as ours would be less effective, letting through similar amounts of UV radiation to what Earth received 2 billion years ago – harsh, but still potentially amenable to life.
An even thinner atmosphere, similar to what Earth had when life arose 3.9 billion years ago, would only allow for life as we know it if TRAPPIST-1 turns out to be calm. If the star flares, any life would have to take shelter under the surface, in oceans or in caves. Such hidden life would be safe, but nearly impossible for us to detect (arxiv.org/abs/1702.06936).
Other means of protection could provide a shining beacon for us to spot. Some corals absorb and re-emit UV light at longer, less-harmful wavelengths, resulting in a blue-green glow. Kaltenegger and O’Malley-James say that similar mechanisms could help organisms survive on planets like the ones around TRAPPIST-1 (arxiv.org/abs/1608.06930).
“Your planet would light up, but so would the ones that you see in the night sky,” says Kaltenegger. “Wouldn’t that be a beautiful thing to see?”
In another study, Kaltenegger and her Cornell colleague Ramses Ramirez found that volcanic activity that releases lots of hydrogen into the atmosphere could keep a chilly planet warm, potentially extending TRAPPIST-1’s habitable zone out to its most distant planet (The Astrophysical Journal Letters, doi.org/b2c3).
Researchers are already using the Hubble Space Telescope to look for possible life-supporting atmospheres around six of TRAPPIST-1’s planets. Hubble’s successor, the James Webb Space Telescope, should be able to follow up with a closer look at their potential atmospheric chemistry.
The planets’ coordinated orbits and proximity could make it easy for meteorites bearing water or even life to jump between them. “You would only be required to have biological material on one planet, which would then be shared to the others,” says Stephen Kane at San Francisco State University.
This article appeared in print under the headline “Seeking life on new rocky worlds”
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