One Big Question: What could life be like in the TRAPPIST-1 system?
Despite being 39 light years from Earth, the TRAPPIST-1 star system has been very much in the news recently. This is not only because the red dwarf has no less than seven Earthlike planets, but three of these are potentially habitable. To find out what life might be like on a TRAPPIST planet, as part of our regular series called "One Big Question," we asked director Lisa Kaltenegger and research associate Jack O'Malley-James at the Carl Sagan Institute at Cornell University to weigh in.
Lisa Kaltenegger: For thousands of years, humanity has wondered if we are alone in the universe, and now we are on the verge of finding out. Now we have the technology to look. So, it's not question of whether I think yes, or I think no. We have the technology to find out. And one of the prime examples of a great place to look is the newly discovered TRAPPIST-1 system.
That system's star is much smaller than our own Sun. The star is very small and doesn't shine very bright, and all the planets are all really close to it, but that's fine because close to a very small star there's enough light and warmth on the surface of a planet to have liquid water.
The key thing in the system is that we have seven planets that are more or less like the Earth. Of those seven, three of them are in the part of the system where it's not too hot and not too cold. If you get too close to the star, the water boils off. If you go too far away from the star, you become so cold that you don't get any warmth from the star and you're outside of the temperate or habitable zone.
TRAPPIST-1 has three planets in this region, which is exciting. In our Solar System, we have Earth and Mars, but Mars is too small, so it loses its atmosphere because it's not massive enough. What that means is that it can't hold on to the greenhouse gases that could warm the planet, so even though we have Earth and Mars in the habitable zone, only Earth is habitable. But in the TRAPPIST system, the planets are so placed that they could hold on to an atmosphere.
But the devil, as always, is in the details. TRAPPIST-1 is a very dim star. It's only five percent as bright as our Sun but it's incredibly active, which means there are storms and eruptions on the star that spew out UV radiation and solar winds that hits those very close planets that could pull away some of the atmosphere.
Jack O'Malley-James: When it comes to what life looks like on these planets, we try to figure out what kinds of environments we expect. We know that the planets are being bathed in biologically harmful UV radiation. This kind of radiation is filtered out by the atmosphere on Earth, which means that the animals we see around us every day are not adapted to UV radiation.
We have to look at what life is like in extreme environments to get some idea of what life is like on these TRAPPIST environments, where they could be bathed in a very high UV flux. Extremophiles, these microbes on Earth that live with radiation, are things like the water bears or tardigrades that you can expose to these really harsh conditions and these really high UV rays and they still manage to survive.
Lisa Kaltenegger: What's really cool about this is that life has much more tolerance than previously thought. We find it at the bottom of the oceans and in really weird holes in the desert. So even though life might exist where you and I couldn't go comfortably, it means that there are an amazing range of conditions where life as we know it can survive.
Jack O'Malley-James: We're constrained by life as we know it, but every year we learn of new extremes that life could cope with and that expands the range of biosignatures that we can pick up and try to fit to the kind of environments we can predict. That's what all this work on the TRAPPIST planets is about. It's to predict the kind of UV environment that these planets can have, which depends a lot on the atmospheres.
If a planet had an atmosphere like Earth with an ozone layer, then the atmosphere could filter out the biologically harmful UV, which means you could have more familiar kinds of life living on the surface. But because this is a very active star spraying out a lot of radiation, the atmosphere may have been eroded away, so we could end up with worlds where the atmosphere is a lot thinner. This could have a thinner ozone layer and allow more UV to get to the surface.
This is a more realistic case, and tells us we must start to consider UV tolerance when we start picturing what kind of life could live there. If not enough oxygen is being produced to create an ozone layer, then the full flux of UV radiation would be similar to what you would get in space and that would affect what kind of things could live on that planet.
You could protect yourself by living underground or underwater. The problem is that if life is doing that and it's not on the surface, then it's much harder for us to detect if it's there and what it's doing. It could be happily living underground or underwater and we'd have no idea that it was there.
Lisa Kaltenegger: In a way, this is where some other work that we did earlier comes in. What about this UV hitting life? Let's see what Earth life does when this happens. There is a form of coral that when you shine UV light on it, it starts to fluoresce. It does this because it lives with algae that can deal with a lot of UV radiation and it protects the coral. If you spin this idea forward, then you have a completely alternative way of finding such life.
Think about an ocean world with biospheres everywhere, then when that UV hits that coral, it would start to glow in different colors depending on which UV frequency hits and what kind of biosphere it inhabits. It could be red or blue or green, and it could be a very detectable signature by the next big telescopes we're building.
What's even cooler, and this is all highly hypothetical, there could be planets where the life evolved to favor fluorescence because it would allow you to live on the surface where the nutrients are. Going back to TRAPPIST-1, one of the planets potentially has conditions where, if you look up into the sky, you could see the other TRAPPIST planets and some of them would be twice the size of the Moon as seen from Earth. Just imagine the biosphere glowing in all these different colors. You could have a beautiful spectacle of light in that night sky.