WHAT I REALLY WANT TO KNOW IS…
How can we detect life on other planets?
Professor Sara Seager is searching for gases in alien atmospheres that could tell us if they are inhabited
Planets are extremely common. We know of thousands and almost every star seems to have some kind of planetary system. Our nearest star, Proxima Centauri, has what might be a rocky planet. For years people wondered about worlds beyond our Solar System. The fact that there’s one right next door is amazing.
My research led to the first detection of an atmosphere on an exoplanet. I had described how one might detect the atmosphere of a planet if it passed in front of its star. Starlight shines through the atmosphere and the atmospheric features of the planet get imprinted on the star. By subtracting the light from a star when a planet is not transiting from that when it is, you’re left with information about the planet’s atmosphere.
I’m interested in the signals in that data which might tell us if the planet is habitable or even inhabited. We call such signals biosignature gases. The subject is hotly debated, but everyone pretty much agrees that water vapour on a small rocky planet is an indicator of habitability because all life as we know it needs liquid water.
A massive enough planet will have water vapour in its atmosphere without there being oceans. We’ve seen that in giant planets already. Earth has oceans because the water evaporates, rises to form clouds and then it rains. But on some planets hotter than Earth that water will evaporate and keep rising into the atmosphere. On other planets hotter than Earth, that water vapour rises so high in the atmosphere that, instead of turning to rain, the hydrogen is stripped out and escapes into space. A planet needs to be small and rocky like Earth to have oceans.
The puzzle of oxygen
Finding certain gases in an atmosphere could also indicate whether a planet has life. A simple lifeform, cyanobacteria, transformed Earth’s atmosphere billions of years ago when these organisms figured out how to harness energy from the Sun. But in doing so they created oxygen as a byproduct, which was poisonous to everything around them. They nearly killed their world but totally reengineered our atmosphere, so that it now has 20 per cent oxygen by volume. If we detect oxygen on another planet we’ll have to make sure that it can be attributed to life and not another environmental scenario. We can’t simply find oxygen and be excited about it. On Earth, microbes create all sorts of chemicals. You can walk into a swamp and it smells terrible, or into a pine forest and it smells beautiful. Both of those scents are entirely due to the gases produced by life. Astronomers will be looking for oxygen first and then methane, nitrous oxide and maybe hydrogen sulphide. If we recognise a gas that doesn’t belong, or seems to be there in greater quantities than expected, that would be interesting. How will we search for biosignature gases? Important tools will be NASA’s James Webb Space Telescope, plus the new generation of ground-based telescopes such as the Giant Magellan Telescope, the European Extremely Large Telescope and the Thirty Meter Telescope. They’ll study M-dwarf stars in different wavelengths and with different techniques. We might like to study Sun-like stars but their brightness would overpower the faint signals we’re searching for. But to find a twin for Earth, we need to go into space and block out the starlight so that we can see the planet directly. Even the new, ground-based telescopes can’t block out starlight to the one part in 10 billion needed to spot another Earth. My favourite space-based mission concept is the Starshade, a giant specially shaped screen that blocks out starlight so a distant telescope will only detect the planet’s light. My personal ambition is to find another Earth. But when might we get an answer to whether there is life out there? It really could be at any time. Once the James Webb launches, and if planets and life are extremely common, then we could get incredibly lucky. But if life is not there around M-dwarf stars, we will have to wait a lot longer.