Q&A WITH AN EXOPLANET EXAMINER
We know that life on Earth creates waste gases that unbalance our atmosphere. Now astronomers are seeking similarly off-kilter worlds in the hunt for alien life
Ezzy Pearson interviews Professor Jayne Birkby
How are you using exoplanet atmospheres to look for life?
Life impacts our atmosphere on Earth. The amount of oxygen in our atmosphere is in a disequilibrium state; there’s something constantly replenishing it in our atmosphere – and that’s life. If all life on Earth was to suddenly disappear, the oxygen in our atmosphere would disappear. The idea is that if we can see the same signature of disequilibrium elsewhere, that starts to become a signature that maybe life is causing that.
What molecules are you looking for?
There are four main molecules that we focus on when talking about Earth-like life: oxygen, water, carbon dioxide and methane. There are other less abundant things out there that might signal life as well, molecules that can only be made by life. But the former are the easiest things for us to look for. It’s the abundance that matters – on Earth, we have 21 per cent oxygen. The molecule itself also makes it easier to observe, as it has a strong absorption line in its spectrum [which makes it easier to detect].
How do you observe planetary atmospheres?
As a planet orbits, it causes its star to wobble back and forth because of the gravitational pull of that star. The star is wobbling at a few centimetres or metres per second. Meanwhile, the planet is whizzing around at kilometres per second. That means if you were to observe the spectra of the planet you would see it Doppler-shifting back and forth, while the star by comparison would look almost stationary. Remove everything that isn’t moving in wavelength over time and what you’re left with – buried in a lot of noise – is a spectrum. If we do this at a high resolution, there are many lines, so it’s a very robust detection.
What kind of planets do you observe using this technique?
The important thing about this technique is that it works on non-transiting exoplanets. That means it makes the nearest planets accessible. With our current facilities, telescopes in the eight-to-10-metre class, we’re looking in detail at hot Jupiters. We’re starting to push towards the mini-Neptunes – we really don’t know much about these planets at all. About 75 planet
