Looking for life light years away

Can we measure the atmospheric composition of exoplanets?
25 September 2014

Interview with 

Eleanor Bacchus, University of Cambridge


How can we tell if a distant planet is potentially habitable when it's light years away across the Milky Way? Figure 1: An artist's view of a planet around a red dwarf starOne way is by looking in the Goldilocks zone - this is the planetary zone around a star where it's not too hot or too cold - and measuring how the spectrum - or colour - of light from the star changes when it passes through the atmospheres of any planets. This can tell you what's in those atmospheres, as Cambridge astrophysicist Eleanor Bacchus explained to Chris Smith.

Eleanor - So certain molecules you have, or certain elements, they absorb and re-emit different colors of wavelengths, so if you imagine looking at a white light source and you split this light up into different colors like you would if you're seeing a rainbow. We're looking for colors that are missing or colors that are more intense than we might otherwise expect them to be and that will give us an indication of what sort of elements or compounds are in the atmospheres of these planets.

Chris - I mentioned the light coming through the atmosphere. Do we only look in that way or could we look at the reflected light from an orbiting planet and do the same thing?

Eleanor - Yes. So that's a lot more difficult but what you basically have here are two different methods of looking at a spectrum of a planet. The one where you have the light coming through the atmosphere is known as transiting, so you're looking at the planet as it passes in front of the star, and then the other method that was mentioned is actually what I work on which is called direct imaging and we're looking for the light that's coming specifically from the planet and this light, in the cases where the planet's habitable, will be light reflected off the exoplanet's atmosphere that's actually coming from the star.

Chris - How far away are the objects you're studying?

Eleanor - So the nearest star is maybe about four light years away and we do think there's a planet around there but we can't, sort of maybe... ten light years at most at the moment. So, it's really very, very close by in terms of the size of the galaxy, let alone the size of the universe.

Chris - How do you do these experiments? Because I would think that the light that's coming from the star is tremendously bright, compared with anything that's reflected off a little planet next door, so how do you get away the light from the star compared with the planet itself?

Eleanor - With an awful lot of difficulty really.

Chris - You're shaking your head.

Eleanor - Yeah, it's incredibly hard. So, it's this very, very, very complicated instrumentation involved which to be honest it took me about a year to get my own head around. But fundamentally we can focus the light from the star into a specific region on telescope instead of channel it away, and then hopefully, after an awful lot of post-processing, and complicated computer algorithms, get like a couple of pixels which is a tiny blob and you point at that, and you go, "That's probably a planet, maybe, we hope."

Chris - So there's a little way to go at the moment you're sort of saying.

Eleanor - Yes.

Chris - But if I were to look at a planet, how would I tell whether the molecules in the atmosphere are indicating a planet which could be home to life or could provide a hospitable environment from one which wouldn't? What sort of parameters are you looking at?

Eleanor - I mentioned earlier something called bio-signatures and these are really the important things we're looking for and in terms of looking at exoplanets, we're looking for specific gases really in the atmosphere that can only really be created by life. So the common examples that people often use are things like oxygen and methane in our atmosphere where they're sort of metabolic byproducts and we can't produce them in the labs that we see in our atmosphere by any sort of geological or photochemical processes, they have to be produced biologically.

Chris - There was some controversy on Mars though, wasn't there? With these methane seeps. People said, "Oh look! This is perhaps evidence of subsurface microorganisms or something like that." They've largely backed down from that theory, so that still leaves you in the same sort of situation doesn't it? Because all you can see is literally the signature of a chemical, you can't distinguish between, say different heavier or lighter forms which a life scientist could do on Earth to distinguish a life-giving process versus a non-life generating process.

Eleanor - Yeah. It's incredibly difficult, so a lot of  work at the moment is going into coming up with what possible biosignatures we might have and to be honest, I don't think we have any at the moment that are definitively, if you saw this you would know there was life there. All of them have a kind of false-positive scenario which we would have to sort of try and rule out by just looking at maybe other elements that we can see in the atmosphere, or trying to find out sort of more fundamental parameters about the planet, so, where it is in the habitable zone.


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