Interview: Angelos Tsiaras discusses exoplanet K2-18b and the search for water on other worlds

The discovery of an Earth-like exoplanet with water in its atmosphere could point to similar worlds existing elsewhere in the cosmos.

Dr Angelos Tsiaras of University College London.

In September 2019 an exoplanet named K2-18b made headlines after a team of astronomers at University College London announced they had discovered water in its atmosphere. The presence of water on this Earth-like exoplanet put it as a forerunner in the search for worlds similar to our own planet.

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We spoke to one of the authors of this new study, Dr Angelos Tsiaras, who is a post-doctoral researcher at UCL working on exoplanetary atmospheres, about what it means for the future of exoplanet hunting.

You and your colleagues at UCL have been involved in a pretty exciting exoplanet discovery recently…

Yes, with exoplanet K2-18b. This is the first time that water has been found in the atmosphere of an exoplanet that has similar temperatures to Earth.

It is within what we called the ‘habitable zone’ of the star, where liquid water can pool. Of course, water has been found before on exoplanets, but they have all been gas giants.

This one is not a gas giant: it has a radius 2 times that of Earth, and weighs 8 times the mass of Earth, which puts it closer in the region of rocky planets.

How did the discovery come about?

The exoplanet was discovered by the Kepler Space Telescope in 2015, and was observed soon after with the Hubble Space Telescope.

The Hubble observations weren’t made by our team, but the data was available in the Hubble archive.

We used this data to analyse K2-18b by studying the stellar light that’s filtered through the exoplanet’s atmosphere.

We can measure this stellar light and look for the variations caused as it passes through the atmosphere to find out what ingredients are present around the exoplanet. In this case, we found water.

An artist’s impression showing exoplanet K2-18b, its host star and an accompanying planet in this system. Credit: ESA/Hubble, M. Kornmesser
An artist’s impression showing exoplanet K2-18b, its host star and an accompanying planet in this system. Credit: ESA/Hubble, M. Kornmesser

Are you able to use this same technique to discover other molecules and signatures in the atmosphere?

Unfortunately with the current instruments this is very difficult, especially on an exoplanet like this that has a lot of water.

Water is a very strong molecule and it absorbs a lot, so it’s hiding a lot of the other molecules.

Also, we need a very wide wavelength coverage so we need to observe different parts of the light spectrum to be able to observe different molecules.

What is the scientific importance of discovering an exoplanet with water in its atmosphere?

We expected this sort of exoplanet to exist already. We were looking for planets like this, orbiting in the habitable zones around their stars and with water present in their atmosphere.

But this is the first one. We couldn’t be sure that such planets were there, but we were expecting them.

The first one discovered is an indication that there are probably more, so we will start looking for more exoplanets with similar atmospheres containing water; especially those within the habitable zones around their stars.

Is this ultimately part of a search to look for signs of life on exoplanets?

We are still very far away from finding signs of life on exoplanets, but this is of course one small step in the right direction.

We are now looking for smaller and smaller exoplanets with atmospheres and water. We started with gas giants and now we are into the realms of super Earths.

And with different telescopes and different instruments we will be able to detect other molecules in exoplanets’ atmospheres and hopefully find what we call ‘biosignatures’.

These biosignatures may not be exactly what we expect, or what we know here on Earth, but this is what we are looking for: anomalies in the exoplanets’ atmospheres.

An artist's concept showing how exoplanet 55 Cancri e compares with Earth. 55 Cancri e has a mass 7.8 times and a radius just over twice that of our own planet, making it a 'super Earth'. Credit: NASA/JPL-Caltech/R. Hurt (SSC)
An artist’s concept showing super Earth exoplanet 55 Cancri and how it compares with Earth. 55 Cancri e has a mass 7.8 times and a radius just over twice that of our own planet, making it similar in size to K2-18b. Credit: NASA/JPL-Caltech/R. Hurt (SSC)

If we are looking for rocky bodies with water in their atmospheres that are located in habitable zones, are we ultimately looking for a second Earth?

In my personal opinion, I think we are very far away from finding a planet like that we could call a ‘second Earth’.

To characterise a planet as a second Earth there are so many requirements and I don’t think we will be able to verify all of them.

Right now we are pushing towards finding exoplanets that may have some similarities to Earth.

For example, we start with temperature. We could call potentially habitable exoplanets those planets with similar temperatures.

But if you talk to a biologist, they’ll tell you that doesn’t mean there is life! But we are trying to find more indications.

We could follow the logic that says there is one planet with life on it so there are probably many others. But our curiosity drives us and we need to know for sure.

A second similarity is water in the atmosphere of the exoplanet, which has never been found before in planets similar to Earth.

We could then look for further signs, such as oxygen, but this would be extremely difficult to find.

The only habitable planet that we know of is Earth, so unavoidably we are looking for signs in an exoplanet that make it look like Earth.

But of course life can evolve in very different ways and even on our own planet we see life surviving in very extreme environments; some of which we might have initially thought were inhabitable.

So you never know. But at the moment we are looking for properties that make an exoplanet similar to Earth. K2-18b is very far from being similar to Earth, but it’s the best candidate that we have right now.

When your discovery was announced there were a few voices saying we shouldn’t get too excited, and that K2-18b is more of a Neptune-like exoplanet, as opposed to Earth-like. What was your reaction to this?

Well I can speak with numbers! The planet has a radius 2 times that of Earth and a mass a bit more than 8 times that of Earth.

Of course this makes it much bigger than Earth, but at the same time it is much smaller and has a much higher density than a gaseous planet like Neptune.

Given the current models and the current data it is much closer to the model of a rocky planet or what we call a ‘water world’.

We don’t have super Earths in our Solar System so we don’t exactly know how they look or their composition, so we need to know more about their atmospheres.

This is the first one around which we’ve discovered an atmosphere. I think discoveries like this will always trigger a lot of discussion on the nature of similar bodies.

We shall see! But at the moment I would say it’s closer to what we would call a rocky planet.

An artist's conception of a hot Jupiter. These are exoplanets similar to Jupiter in our own Solar System, but orbiting much closer to their host star. Credit: NASA/Ames/JPL-Caltech
An artist’s conception of a hot Jupiter. These are exoplanets that are similar to Jupiter in our own Solar System, but are orbiting much closer to their host star. Credit: NASA/Ames/JPL-Caltech

One of the most interesting aspects of the study of exoplanets is that it has shown just how unique our own Solar System is.

Indeed. We have found many different kinds of planets that we don’t see in our own Solar System, like Jupiter-type planets orbiting very close to their stars or super-Earths: planets characteristically somewhere between Earth and Neptune. Actually we know now that these planets are the most common in the Galaxy.

However, we also have what we call observation bias. Our techniques are more sensitive to certain kinds of exoplanets and less sensitive to other kinds of exoplanets.

Finding a system like our own Solar System is quite difficult with our current detection techniques.

From what we understand right now, planets like Earth may not be the most common in the Galaxy, but they are quite common in terms of size at least.

But the conditions that you find on exoplanets is a completely different issue.

We shouldn’t be comparing the term ‘Earth-like’ for an exoplanet in terms of size and mass with ‘Earth-like’ in terms of conditions similar to Earth. Earth-like conditions are probably very rare.

Are you and your exoplanet-hunting colleagues excited about the next wave of exoplanet missions?

Yes we are all excited because we are waiting for new telescopes like the James Webb Space Telescope (JWST) and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) mission, because these missions will try to characterise exoplanets better.

As I said before, we need to look at different parts of the spectrum, and JWST and ARIEL will do this: a few in the case of JWST and 100s to 1000s in the case of ARIEL. So we will be able to look inside the atmospheres of more exoplanets.

In terms of detection, we currently have the Transiting Exoplanet Survey Satellite (TESS) mission, which is in operation and discovering more potential super Earths in habitable zones around cooler stars.

And there’s also ESA’s PLAnetary Transits and Oscillations of stars (PLATO) mission, which is due to launch within the decade starting 2020.

I think we will discover many more exoplanets to analyse and be able study molecules and atmospheric structures, because this is the obvious next step in understanding what exoplanets look like and their conditions.

Without studying atmospheres, we don’t really know.

NASA technicians hoist the James Webb Space Telescope’s primary mirror to move it to a clean room at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Credit: NASA/Desiree Stover
NASA technicians hoist the James Webb Space Telescope’s primary mirror to move it to a clean room at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Credit: NASA/Desiree Stover

If we consider how rare planets like Earth are, do you think this puts climate change into perspective?

We are looking for planets similar to Earth and we would like to find signs of life and, of course, habitable conditions, mainly because we want to know how rare Earth is, or how rare life is in the Universe.

Is life part of the Universe in general, or is it only on Earth?

We could follow the logic that says there is one planet with life on it so there are probably many others out there. But our curiosity drives us and we need to know for sure.

However, what ever we eventually find out there, it’s currently science fiction and I think it will still be science fiction for many decades to come.

So it’s much more efficient to look inwards and take care of our own planet instead of thinking about transforming other worlds so we can live on them. That would take a lot more energy and time.

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