Water discovered on a habitable planet

17 September 2019

Interview with 

Ingo Waldmann, UCL


Sunrise over the rim of a planet seen from space.


Scientists in the UK have tracked down what might be a habitable world orbiting a distant star. The planet’s called K2-18b, and Ingo Waldmann told Chris Smith what his team’s discovered and how they did it…

Ingo - We found a planet outside our own solar system which is rocky and it has an atmosphere which is potentially habitable. That means that it has the temperatures that may allow liquid water to exist on its surface, and it has an atmosphere, and that atmosphere has water vapour in it.

Chris - Sounds amazing. Where would I need to look in the night sky to able to see it where this is? And how far away is it?

Ingo - It's in the constellation of Leo, and it's 110 light years away from us. So it's not the closest, but it's also not the farthest exoplanet we've ever discovered.

Chris - And how did you find it?

Ingo - The Kepler mission discovered this planet, and the Kepler space telescope discovered the planet using the Transit Method. The Transit Method means that the planet orbits in our line of sight to the star. So it goes between us and the star. So that means that the light of the star dips slightly whenever it goes in between our line of sight and that's how you discover these planets.

Chris - That tells you it's there. How do you then interrogate what it's made of, the fact that you told me it's a rocky world, that you've got signatures of water there, how are you doing that?

Ingo - The fact that it's a rocky world we know from its density. So we know its mass, it's about 8 Earth masses, and its radius. We know from the Transit measurement, and that's about two radii. So that's about the density of Mars. So that tells us that there is a rocky core inside. Now, what we've done is that we took Hubble space telescope observations to measure the atmosphere of the planet, and the atmosphere is measured by observing the same transit as the planet goes in front of the star, but we observe the tiny little speck of stellar light that filters through the atmosphere while it goes in front of the star.

Chris - And how does looking at the light coming through the atmosphere tell you what chemicals are there?

Ingo - Different chemicals absorb light in different ways, so at different frequencies of light you have a very characteristic signature of the chemicals. So if we observe the transits at different wavelengths of light, we can identify what chemicals are in the atmosphere.

Chris - And when you total up what you see, what is the recipe of the atmosphere of this distant world?

Ingo - Well so far we've discovered water only. So we've discovered that there is an atmosphere around this planet which is a world first. So this is really exciting because we had never discovered an atmosphere around a habitable planet. And so far we've only seen water because the Hubble Space Telescope is only sensitive to the water feature. There's probably going to be more molecules such as methane, acetylene, ammonia and so on and so forth, but we're just not quite sensitive to that yet.

Chris - And how do you know the temperature? Because you're saying it's a habitable world, what would it be like on that planet?

Ingo - So the temperature you can quite easily calculate from the distance of the planet from the star. So the planet orbits its star, and it's a small red star, in 33 days. So every Earth-month is a year on Kepler-18b. That means that the radiation on the surface of that planet is just enough to keep water liquid. So the surface conditions are probably okay but we don't actually know what the surface is like at the moment. It could be a water world, or it could be entirely barren and rocky.

Chris - Given that you've got this set of findings, you now have a relatively small planet, quite similar to our own, with evidence of water. How does this shift the balance of probability that, when we look out into the universe, that there are other worlds like our own out there?

Ingo - Well, I mean that's the exciting bit, right? So this is hopefully only the first of many to be found in the next couple of years.

Chris - What about the thing that, I guarantee, everyone listening to this is gonna be thinking, well, if there's a habitable planet out there, could it be inhabited?

Ingo - That's the million dollar question, and we don't know whether there's a biosphere on that planet yet, because Hubble is not sensitive to the chemical fingerprints of life. Now, the James Webb Space Telescope will be sensitive to those and I bet you all my tiny little research salary that we will spend a significant amount of the James Webb time observing this planet. It might well be that we find an atmosphere which is, what we call, “out of equilibrium”. A chemistry that cannot be explained by natural processes other than our life. So in our Earth example if you look at the Earth from afar, what we would see is the existence of methane and oxygen and ozone. Now on that planet we don't know what that might be. It might be very similar because life may have evolved in a similar way but we just don't know yet.


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