Best Astronomy: Detecting Exoplanets

How can you see planets outside our solar system? Astronomer Beth Biller explains all...
23 April 2019

Interview with 

Beth Biller, University of Edinburgh


Image of the Sun's halo as moon passes by


Could you spot a firefly next to a lighthouse? That's a similar issue astronomers face when trying to see planets outside our solar system (exoplanets). Astronomer Beth Biller from the University of Edinbrugh explains to Chris Smith how they get around that issue as the best thing in Astronomy. 

Beth - I'm going to go with detecting and characterizing exoplanets. So again, exoplanets are planets orbiting stars other than our sun. And I want to talk a little bit about how we know what they're like. So as we saw earlier for planets in our own solar system, our initial attempts at characterizing them went kinda disastrously wrong. There are no canals on Mars as we know.

Most of the planets we've detected around other stars today, we've done so indirectly because of their slight tug on their star or because they pass in front of the star and dim it just a little bit. If we want to understand what planets are actually like, we need light from the planets themselves. If we have light from the planet itself we can get a sense for how warm it is, how cold it is. We can get a sense of what it's made out of, at least what its atmosphere is made out of.

So one technique to do this, the one that I work on in particular and hence the one that's nearest and dearest to my heart, is direct imaging of exoplanets which is when we just try to take a picture of an exoplanet next to a star. Obviously this is really challenging, right, cause stars are really bright and planets are not so bright so it's kind of like trying to take a picture of a firefly next to a lighthouse. If the lighthouse and the firefly were in Dublin and we're here in Edinburgh.

Chris - That's quite some way. So is there a way of potentially masking the sun or the star so that that light can't bleach out the fireflies. You can still turn really sensitive equipment on your firefly and therefore see it.

Beth - Exactly. That's what you need to do to get the contrast necessary to actually image your firefly next to your lighthouse. So first of all you have to do something about the atmosphere. One solution to this is to put your telescope in space. Once you've done that, well, the next obvious step is let's block out the light from the star, right. The instrument to do this is something called a chronograph.

Chris - How does it work?

Beth - The same way as if you see the sun is very in the sky you block, put your hand in front of it and block it out.

Chris - So you can put something in the field of view of the telescope which just covers the star but doesn't cover the tiny speck of light that you hope is there. There's the planet you want to see.

Beth -  Exactly. So we've used this already to detect a number of planets. These are planets very unlike our own. They're what I like to refer to as baby Jupiter so they're like Jupiter but much younger. There are maybe 10 to 100 million years old. And yes I realize that's only young in the world of astronomy, but because they're young, they're a lot warmer than they are at later ages and hence brighter. So that's what we've done so far. The hope is eventually to push these technologies down to lower mass planets and cooler planets, eventually imaging planets like our own.

Chris - Can we also not, by looking at the light coming through the would-be atmosphere of the planet, learn something about what's in that atmosphere, because you can assume that if the lights go to go through the atmosphere and there are chemicals in there that might soak up some of the light, and different chemicals soak up different colors of light, if you look at different colors you could tell what's in the atmosphere and therefore you could spot and you can learn quite a bit about the weather and the climate and what sort of environment of that particular planet may be without actually ever having to go there.

Beth - Yeah exactly. So this is the power of spectroscopy, right. Looking for the fingerprints of different atoms and in particular when you're talking about cool planetary atmospheres, molecules in your atmosphere. So for instance there's already been detections of water and methane in exoplanet atmospheres, by looking at their spectra from the light of the star passing through the atmosphere. So that's one technique that's used. Also by actually getting your directly image spectra of the planet itself.

Chris - Thanks Beth.


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