10. Exoplanets | Nobel Laureate Didier Queloz

Mejd Alsari (MA). In 1995 you dramatically changed the view we had about planet formation in the Universe with the discovery of the first giant planet outside our solar system. This discovery started a revolution in astronomy and in 2019 you shared the Nobel Prize in Physics with Michel Mayor for the discovery of an exoplanet orbiting a solar-type star. Can you summarise the key findings in the paper you published in Nature in 1995?[1]

Didier Queloz (DQ). We identified the first planet orbiting another star other than the Sun. That was a key discovery. It was a trigger for the field because, up to that day, people were hoping that there are planets somewhere orbiting other stars, but no one had really found one.[2] [3] [4] [5]

What came with the discovery was a lot of embarrassment as well because the planet wasn’t at all the way we expected it to be. We found a big planet (51 Pegasi b) which was the only one we could detect. In fact, due to instrumentation limits, we could only detect relatively large planets.[1]

The problem of that planet was its orbit, which was extremely close to its star, about 20 times closer to its star than the orbit of the Earth to the Sun, and that was really awkward. We call these planets hot Jupiters. The theories of planetary formations were not predicting such a planet.[6] In addition to the discovery of the first exoplanet orbiting a main-sequence star, we broke the theory.

That was really the main impact of this discovery almost 25 years ago.



MA. According to our former understanding of the formation of solar systems, 51 Pegasi b shouldn’t be where it is now. The past 30 years of discoveries tell us that our solar system is very unusual. Where are we now in terms of planetary system models? Can you compare between models back in 1995 and now?

DQ. This is a very interesting question. The first discovery was awkward, but all the other discoveries that came later on were awkward as well because we kept detecting planets that no one had predicted.[7] [8] [9] [10] [11] [12] [13] [14] [15] We have plenty of hot Jupiters but we also have objects we had no idea they would exist like hot Earths or super Earths[16] or hot mini Neptunes[17] . We have this kind of population of planets that we cannot directly compare to the ones in the Solar System.

We have a very detailed theory that is working pretty well to explain the formation and nature of our own system.[18] We have a lot of data on the planets of our Solar System, including remnant bodies from the early Solar System, which sometimes fall on Earth as asteroids.[19] There are lots of elements that we can put together. We know the atmospheric composition of at least one giant planet in the Solar System.[20] We have a good understanding of the telluric planets, maybe not Mercury, which is not very well-known.

I think discoveries of exoplanets don’t challenge our understanding of the Solar System, but tell us that this picture is one amongst many possibilities to form planets.

We have expanded our understanding of planetary system models by adding lots of ingredients. One of them is the fact that the planet can, in a way, move during the early stage a lot more than we thought. A planet can form at a certain location and then can move towards its star. We call this migration.[21] [22] [23] We can also have a multi-planetary system, where planets interact.[24] [25] [26] Due to this interaction, planets can move outwards or inwards in the planetary system. In this case, it becomes difficult getting a clear understanding of how to connect the end product to the initial stage of a planetary system evolution, because so many things can happen. Right now we are trying to retrieve as much data as we can from many planets in order to go back in time to reconstruct all the steps of planetary systems evolution.

This data includes parameters such as mass and size, but also information from the atmosphere of these planets.[27] [28] [29] This can tell us part of the story on the origin of the chemical constituents of a planet, whether they have been accreted by the planet or built-up in the planet.

This means that it’s not enough to detect them, get the mass, and the size. We really want to know more about the atmosphere of these planets as we’ve been doing in the Solar System when we studied the atmosphere of the giant planets. Our models tell us that the giant planets or the ones that look big enough to have a lot of gas, may have formed in the outskirts of the Solar System due to the fact that this is where the ingredients were in a kind of solid form that could have easily accreted on the planet.[30] [31] [32] This is known as the formation mechanism of giant planets beyond the ice line,[33] [34] [35] where by ice we indicate not only water but also any other gas that can solidify.

The general theory of planetary system formation is trying to connect the detailed data we have on the Solar System with the kind of loose data we have on other planets. We have not reached a complete agreement yet because we haven’t found enough planets that look like the ones in the Solar System, such as Earth or Venus.

When we go back to the Solar System, we ask ourselves: ‘Why the Solar System didn’t move?’ This is part of the key ongoing questions we are trying to solve.

There are several space missions that we hope will help addressing these questions. There is an operational space mission called TESS (Transiting Exoplanet Survey Satellite), which is trying to identify more transiting systems.[36] [37] Another mission that will be launched soon is CHEOPS (CHaracterising ExOPlanets Satellite mission), which will give a more detailed analysis of these transits.[38] [39] The James Webb Space Telescope (JWST or WEBB) will be launched soon to study the atmosphere of these planets.[40] [41] The European Space Agency (ESA) will launch ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) to study the chemical composition and thermal structure of transiting planets.[42] [43] [44]



MA. In these models, what sort of equations are you using?

There are several elements involved in the formation of a planet. We don’t understand all the details but we have a good global picture of it. The first element, on which most of the people would agree, is that planets form by an accretion mechanism.[45] In astrophysics, usually you don’t really have this mechanism. Typically, you have a collapse when you have something like a big cloud of gas. The cloud collapses by self-gravity and forms a star.[46]

Planets do not form in this way. They form inside out by gluing together small pieces, which collide, and glue further. When they become massive enough they start accreting material. When they become really massive, they accrete everything around them, typically gas.

This is what happens when a giant planet forms. The core builds up, it becomes a planetesimal, which accretes gas until it becomes a planet. In order to accrete material a planet needs to be within something that feeds it. We call this a protoplanetary disk.[47] We see them.[48] We have plenty of examples right now. The disc has also self-gravity. Therefore, there is an interaction between the planet and the disk. Practically the planet steers gravity waves into the disc and the disc reacts with a certain response time, a bit of a lag. This means that the disc itself will induce a gravitational effect on the planet and they’re not exactly balanced. This delay produces a tidal force and a torque, which affects the angular momentum of the planet causing the planet to move in all directions depending on the material around. If there are other planets around this planet, the gravitational effects dominate. Moreover if multiple planets are in orbital resonance (orbital periods related by a ratio of small integers) then these effects will be enhanced.[49]

The interaction with the disc wasn’t well understood early on. Some people predicted this for the solar system, for the formations of the satellites of the giant planet. However it was never thought to be an important factor for the formation of a planet. Right now these aspects are being considered seriously into the modelling of planetary systems formation.

So these are really the basic equations. If you really want to go into the details of accretion mechanisms, there’s a long list of theory, but there isn’t a well-defined theory explaining this.

As well as if you migrate the planet, how do you stop it?[50] When a planet starts migrating it shouldn’t stop. If the planet keeps going and if its angular momentum reduces, it will collide into its star. Therefore there has to be a way to hold the planet in a certain position. One mechanism is the decrease in the torque of a planet.[51] [52] Another way involves tidal effects between the star and the planet when they come very close.[53] There are also magnetic field effects.[54] So, there are lots of elements that come together, but nobody has a clear picture due to the difficulty of performing measurements of planetary systems during their formation. But we’re working on that.

There are bigger telescopes being built, which can see sharper, deeper, and in more detail. One of them is ALMA (Atacama Large Millimetre Array).[55] [56] It consists of an array of millimetric radio telescopes, which can be combined together.

Therefore the theory of the formation of planetary systems is a challenge for the community. But that’s one of the key focus of most programs that direct the efforts of the community right now.

Mejd Alsari (MA). You said there were antecedents to your discovery and you mentioned this in your paper published in 1995. But you said that the field wasn’t considered a serious topic. Was it because of resolution limits and false positives?

Didier Queloz (DQ). There is a long story in exoplanet science[1] [2] of false alarm or people claiming they found a planet and it was not a planet before we found the first one (orbiting a Sun-like star).[3] Some people may have thought ‘It’s again the same kind of stuff, it isn’t a planet!’

You have to realise that we setup the instrument and, one year after the first light, practically we had the first planet. So, it was extremely quick. Usually you have to establish some confidence with the machine. We were very confident but nobody was aware of this machine.

When the observations were confirmed by Geoffrey Marcy a couple of weeks later,[4] people understood that the data was fine, but then came up the big debate: ‘Can we explain the data?’

Maybe we just had a change in the radial velocity due to something other than a planet. We carefully studied that in the paper, but the topic was controversial. Some people suspected that maybe there were some effects, subtle effects, on the atmosphere of the 51 Pegasi star. They suggested an impossible precession (i.e. that the spectral-line variations were caused by a pulsating stellar surface), which is not observed on other stars, but that could apply to that one.[5] That was ruled out in our paper,[4] but I don’t think they understood the way we did it. Then for some time, people were arguing whether this alternative explanation was right or not.[6] Despite the fact that some people realised that the paper published in 1997 claiming that we did not detect a planet[5] was wrong,[7] [8] the paper is still there.

At that time the community was still not very sure. Well, people really working on the topic, there were maybe 50 people in the world, were pretty sure that the planet was real, but the community outside the field did not really know what to think about that and they all knew that the planet was awkward.[6]

When you have a planetary theory and data that doesn’t match this theory, usually the global physics attitude is being sceptical. If somebody tells you ‘I made an experiment and I found that this particle goes faster than the speed of light’,[9] you’ll say ‘Come on, this is a big theory’. If you do that, there are consequences.

In our case it was not that bad because I think formation theory is not like fundamental physics theory. But we had a nice picture of the solar system, and people were not really willing to trash it. ‘It works for the solar system, so maybe there’s something wrong’, they must have thought.

It took some time to swallow.

What I learned later on is that the bigger the discovery the longer it takes for the discovery to be understood. If you’re doing incremental science usually it’s fine. Everybody’s expecting it, you’re happy, you publish your paper, and everybody will be citing you. In our case, it was so awkward.

We were also trying to build up a theory. There were some attempts early on, but it was debated. New awkward planets kept coming. We had all these planets popping up but that weren’t exactly the way we were expecting them to be. It took some time to get this right.

For me it was certainly something difficult because I was just a PhD student. In a way I wanted to move forward and do something else. I was a bit tired. There were people challenging what I did, other people telling me ‘This is the greatest discovery ever’. ‘Oh my God, I started my career by making my best ever discovery, so what am I going to do next?’, I thought.

I decided to put that aside and move forward. I did what I could. We did the best we could.

It took me a long time to really understand the profound implication of this. I also think it took some time for the community to realise that a new field was really setting up. The public was enthusiastic way more than my colleagues. Fellow physicists were kind of reluctant. They also saw us like a little bit of a threat. ‘These people are asking a lot of resources, they want to do a lot of science, but my science is more serious than this kind of exoplanet stuff’, they said.

‘You are just fashion, it’s not going to last guys, it’s just trendy’, other people told us.

It’s interesting to see how things can mature. I know it’s a very serious business because you are flying spacecraft on this. I think it took about 10 years for the field to establish itself and to be understood and recognised as a very serious field of physics.

Right now I hope to be successful to move the field to being serious enough and go beyond this to the question of life in the universe, which is a very serious field, not only for physics, but also for chemistry and other sciences.

We made a new field of research here. That’s why we struggled.

MA. You mentioned that in summer 1994 your PhD advisor (Michel Mayor) gave you the keys of the observatory in south France. What’s the story behind that and how did you know what stars you needed to look for?

DQ. This is interesting because it tells a lot about what we were expecting. Nobody was expecting a planet like this one (51 Pegasi b).

The idea was to build the equipment and the equipment was clearly designed to get the right accuracy to find a planet. It was clearly the decision from the beginning to start the program.

My PhD advisor, Michel Mayor, told me ‘Start the program, I go in sabbatical, and nothing will happen but start the program. You should be aware that you are starting a long-term program and that you will not be able to continue it’. I said ‘fine’.

I built the equipment. I worked on it. I was happy to demonstrate that the equipment was working properly. So, I started the program. We decided to focus on about 100 stars. We selected different stars from the ones that our competitors in Berkeley, Geoffrey Marcy and Paul Butler, were looking at. We knew they were doing a similar study. We knew that it would have been a long race, a long run. So, I really went there and said ‘Let’s just start and wrap up my PhD and then I would do something else’.

I initially focussed on the 20 brightest stars out of these 100. I decided that these stars deserved a bit more measurements than the others, just to check the machine. I took two data points every week. I had one run of observations every two months.

51 Pegasi was one of the stars.

After my first two runs, I came back in January 1995. I kept observing that star and there was something awkward with the data. Data was not stable.

I started to panic here. I said ‘Maybe there is something wrong with the data processing of that star’. I really got a bit obsessed about that object. So I kept measuring it to try to understand, try to debug the system up to the point I realised that everything was fine, that my system was working properly. There were no mistakes because it worked well for the other stars. There might have been something special on that star. I looked at that star and I didn’t find anything special. It looked like an old star like the Sun.

I concluded that there must have been something orbiting that star and I tried to fit an orbit to the data. But I was so embarrassed. I wanted to come up with a ‘real’ explanation to Michel. But he was in Hawaii anyway. The communication was very loose because in those days the email was kind of starting.

‘I want to go to the end of this, I am going to find this, I am going to get the orbit’, I said.

When I finally fitted an orbit I said ‘OK, I think I found a planet!’.

It’s fascinating because I don’t think I realised what it really meant at that time. To me it was cool ‘I found a planet. Cool, yeah! A 4-days (orbit) planet!’

But I didn’t realise it was impossible.

When I mentioned this to Michel, he said, ‘Oh… yeah… maybe… let’s see’. I think it’s obvious he didn’t believe me.

He carefully explained ‘You know Didier, a planet like that could not exist because the formation theory says…’ and told me all the story.

‘Oh, I didn’t know that’, I replied.

I had these very naive perceptions of ‘Why not?’ In a way, it was a piece of luck because maybe if Michel was there, we might have discarded the observation and said ‘Something is wrong and we should not pay attention to that’. So, the fact that I didn’t know it was kind of good news because I got focused, obsessed. I got the orbit.

At the end, I explained what I did to Michel, and he had to admit that he had no other explanation and told me ‘Look Didier, we are not going to do anything with that. It’s so special. We have to leave it as it is, and we have to come back with the telescope. We are going to do a little bit of a game here. We are going to use the textbook of physics. The textbook of physics tells you, when you have a theory, whatever theory it is, the only thing that matters is not how nice is the theory or how revolutionary is the theory or how beautiful is the theory. What matters is, does the theory reflect the data?’

You can always find a theory matching data. It’s an easy game. You can twist the theory. But it’s much more difficult to make a prediction with the theory that would be matched by the data.

You might recall the famous experiment with the solar eclipse of 1919, which demonstrated the validity of Einstein’s theory of general relativity.[10] ‘We have to do the same. If this is a planet, your orbit should be valid’, Michel said.

‘When we come back to the telescope, later on in July, when the star will be back, your prediction should be matched’.

‘Okay, let’s do that’, I said.

In the meanwhile we realised that, because it was a bit edgy, we didn’t want to talk too much about that. So we just waited to go back to the telescope in July.

In July we went to the telescope and it was like a dream. You have the theory predicting something and all the data points exactly matching the prediction I made in February.

At that time I think Michel was getting convinced and he said ‘Oh my god that’s real, so let’s write a paper, because it’s so easy. The orbiting period is 4 days, maybe Geoffrey Marcy is doing it as well.’

We decided not to talk to anybody apart from our families. We immediately wrote a paper and we submitted it to Nature. The paper was sent to the reviewers to initiate the process.

‘I don’t believe you, it’s an alias’, was a comment from one of the referees.

‘What can we do? We get more data!’ I said.

So we went to the telescope to get more data in September and every day we were collecting data. I think we had two or three data points every night to demonstrate it was not an alias. Then we added the extra data points to the paper and resubmitted it right away.

We had to wait these impossible two months, during which I think Nature didn’t know what to do with our paper, because the other referee said ‘It’s fine’. There were two theoreticians that said ‘The theory they mention, migration, maybe it’s going to work’. The one dealing with the data said ‘I think it’s fine, I checked the data and I think this is the only possible orbit, the paper should be published’.

But Nature, delayed and delayed, up to a point where we said ‘We are going to announce this to the World, we cannot wait any more’.

‘You can do that, but don’t say that we’ve accepted the paper’, Nature said.

When we made the announcement in Florence, the world was very sceptical and we were lucky in a sense that our competitors in Berkeley, Geoffrey Marcy and Paul Butler, were observing at a telescope, and somebody told them ‘Look, there is a star that the Swiss guy pretends has a planet around, which is just 4 days period, could you have a look at that?’

‘Of course, we are going to check it’, they said.

Marcy was a well-known planet-killer in a way that in the past he had already killed lots of claims of exoplanet discoveries.

To me it was fascinating. The only thing I felt was under threat was the data not being confirmed. But one week after, Geoffrey Marcy sent this very nice email. ‘Look guys, I confirm everything you said. This is real. Congratulations. You found the first planet’.

To me that was such a relief because I said ‘He confirms the data, so everything is fine’. Then we got a bit lucky here because that helped strengthen the facts in the discovery. It turned out not to be enough for the community at large but it was enough for the people that were really interested in looking for the planet.

Nature at that point decided ‘Maybe we should accept the paper, because it’s too late to stop it’.

But we had been very close to being rejected I think by the editor – I am sure – because it was too big, and we were nobody.

After this some people told us sentences like ‘How come NASA is using billions to send spacecraft and they didn’t detect the first planet? These are two Swiss guys… somewhere… using a kind of telescope that was built in 1957 in an observatory that was about to close.’

I think this story is very fascinating in terms of sociology. I have just learned recently that our work is part of the ‘10 extraordinary Nature papers’.[11] So now they have changed their mind. We went from being barely accepted to the best 10.

Mejd Alsari (MA). Does the Drake Equation[1] [2] define a sort of roadmap for astronomers in their quest for understanding our universe when you read it from left to right?

Note: the Drake Equation predicts the number of extant advanced technical civilizations possessing both the interest and the capability for interstellar communication as N=R*×fp×ne×fl×fi×fc×L, where:[1]

• R* is the mean rate of star formation averaged over the lifetime of our Galaxy;
• fp is the fraction of stars with planetary systems;
• ne is the mean number of planets in each planetary system with environments favourable for the origin of life;
• fl is the fraction of such favourable planets on which life develops;
• fi is the fraction of such inhabited planets on which intelligent life arises during the lifetime of the local star;
• fc is the fraction of planets populated by intelligent beings on which an advanced technical civilization arises during the lifetime of the local star;
• L is the lifetime of the technical civilization.

Didier Queloz (DQ). No. I think the Drake Equation is based on a very simplistic concept of life; that the only possible life would be like the one on Earth. The problem with that equation is that it is only relevant for the Search for Extra-Terrestrial Intelligence (SETI) experiment, which looks at intelligent life as it is today on Earth on a global statistical way.[2] [3] [4] If you look at the history of life on Earth, it was invisible for most of the time. I think the Drake Equation is targeting the wrong public. It has given a very nice simplification to build up a program like SETI, looking for radio signals.[5] [6] Right now, this is not the way people are addressing the problem of extra-terrestrial life.

The problem is clearly not about the number of stars, the number of planets, etc. We know this. There are planets everywhere. The question that should be asked here is ‘What defines a planet that can sustain life?’[7]

There are two ways to do that. One can be lacking creativity and use the Drake Equation, which practically relates to Earth. Or one can try to be creative and say ‘There are many planets different from Earth that could produce life, which might be different from the one we are familiar with’. It might be different but it’s still chemistry, different kind of chemistry. Maybe life is not visible, almost invisible. Maybe there is life on Venus,[8] maybe there is life on Mars.[9] We are not very good at finding that right now. But maybe there is life there and we are not looking at the right stuff.

Given the diversity of the planetary systems we’ve discovered, I think it’s safe to hypothesize an extremely high diversity in the chemistry of these planets. Whether such a diversity in chemistry can lead to life, that’s open to debate. It’s something that can be tested. It’s a chemistry experiment that needs to be done to address these fundamental questions. What is the origin of the chemistry of life on Earth? How far can we divert from this chemistry on Earth to make another chemistry that leads to something that looks like life?[10] [11] [12]

Therefore, I think that from this perspective the Drake Equation does not tell you anything at all.

MA. What do you think of the SETI program?[5] [6]

DQ. The SETI program is targeted to a very specific question: ‘Could you find a civilization on the base of the signals they produce?’

SETI has been focussing mostly on radio signals, but there could also be light signals. I think there are no limits in science. If people believe this is what they want to look for, I am very happy for them to do that. The problem I have with the SETI program is that I would never do that because I think it’s an extremely frustrating program, in which you learn nothing from a null result. When you do an experiment you really have to acknowledge a null result. The null result should tell you as much as the result.

To me it’s a quest that is looking at something that you have an idea of. I think it’s a beautiful quest because I can understand the myth behind it, I can understand the impact. But if you find nothing you are still in the same situation.

In the field of exoplanets you can look for a planet, you can detect a planet, and ask questions about the origin of life in this planet. You measure the chemistry of this planet, you understand something about the chemistry, the geochemistry, the cycle of the chemistry, and possible imbalances in the chemistry of the atmosphere. Whether there is life or not, you can understand if there are the minimum conditions on these planets to support life. Then you can start elaborating on the likelihood of life developing on these planets.

The other problem I have with SETI is that, let’s imagine there is a civilization of fish. I don’t know how you can talk to fish. I think people working at SETI should first try with, for example, jellyfish. They could take a couple of jelly fish and try to exchange information with them here on Earth.

That’s exactly the kind of problem they are facing. I understand that SETI is a vision of the sixties.[5] I love it because it’s an easy try. It had to be tried. It’s easy. You can point to the stars with a radio telescope, and if you detect something you would know if that comes from an intelligent life.

People have been looking at lots of stars right now with the SETI program. Whether this program should continue or not, again, it depends on what they are really looking for. But then, at the end, if they inspect one billion stars and they get nothing, what did they learn about life in the universe? Nothing. They just learn that there is no intelligent life broadcasting signals outside Earth at the time they made the measurement. This does not mean that there wasn’t life in the remote past or that there will be life in the remote future on that planet.

These are the problems I have with SETI right now. But if there are people working on this, I am always curious to hear what they are doing. This is part of science, the diversity of science.

MA. Can you tell us about the work you’ve done in collaboration with a group at the Laboratory of Molecular Biology (LMB) here in Cambridge, where you talk about the origin of RNA precursors in exoplanets?[13]

DQ. Right now we are trying to define the way to make progress first with the origin of life on Earth and then with the possibility of life on other planets. There are lots of discussions on what is the best way to move forward. There are a couple of elements we all agree on. When you look for the origin of life (abiogenesis) on Earth, there is a limit in what you can get because we have no memory of the past on Earth. There is no way to know exactly how life emerged back then, except to look at what is the current structure of life.

The current mechanism of life is the result of an extremely complex evolution. Consider for example the first and last versions of the steam engine. There has been so much progress. In a similar way, life has optimised itself in the way we see it today. There may be some elements related to the primordial life in the current living mechanisms, but these are hard to detect.

In the work you mentioned, we decided to go as back as we could. We started with the fact that there is a limit in the number of amino acids that life is using. We asked ‘Can we start from that to define what the conditions for life are?’[14] [15]

That’s kind of going to the simplest possible assumption. It doesn’t mean that when we have amino acids we will get life. Nobody really has been able to connect the dots yet. There are people working on that. I am optimistic that at some point in a lab, here on earth, people will be able to do that, i.e. try to assemble the amino acids in a way that they produce some kind of replication mechanism.

Then we look at this and we ask the question ‘Could you do the chemistry, the science conditions, imagine a planet, and how does it work?’ We found out that the light from a star could act as an energy trigger. So we looked at the possibility of life from the stars. We tried to measure that. We compared it with the chemistry. We did chemistry experiments by bringing ‘astrophysics’ light, in a way, not just a lamp, but light that is properly calibrated.[15]

These are all the kinds of connections we are trying to establish right now. That’s the way the field is trying to evolve, which is trying to bring different pieces of knowledge together: the astrophysics knowledge from a planet, the atmosphere of a planet, the geophysics knowledge from Earth, and the Solar System. We also wanted to see how far we could learn about plate tectonics, volcanic activity, and magnetic fields on these exoplanets. This is part of the equation.

Some people assume that you absolutely need plate tectonics to recycle carbon.[16] [17] If this is the only way to recycle carbon, I guess we need that, otherwise we would end up like Venus. Venus may have looked like Earth half a billion years ago. It was experiencing some plate tectonic recycling. If you imagine the first inhabitants of Earth looking at Venus, they would have seen a kind of blue planet as well. When plate tectonics stopped in Venus,[18] carbon recycling stopped, leading to a runaway greenhouse effect.[17]

All these elements are very interesting and we can study them by combining all these fields. That’s the approach we’ve started here. We are not the only ones. There are a couple of people in Harvard.

There is a global initiative, which is supported by the Simons Foundation right now.[19] We are trying to connect all this together, which to me is extremely interesting. I’m trying to promote the idea and I hope that this Nobel Prize will help my voice to be heard that we are building a new field.

We are facing what’s called red tape effect, a bureaucratic effect, which is if you are an astrophysicist and you want to ask for money to do a chemistry experiment, you can’t. The astrophysics panel will tell you ‘Well, this is astrophysics you should ask chemistry’. The chemistry panel will tell you ‘You are an astrophysicist you shouldn’t ask us’.

So there is no funding. This is the kind of challenge you face when you do science between different scientific topics, which are usually completely separated.[20] We don’t even have the same science council for astronomers and chemists.

How can we assemble a sensible critical mass of people that will be able to operate together and come up with a real roadmap? I don’t think we have a clear roadmap right now but there are lots of elements, which are interesting. These are related to the investigation of the most basic elements of life as we know it, which is to find the conditions on other planets and then possibly explore deviations from that. If you don’t have the same chemistry, could you trick a little bit the chemistry and still have something, which could be different amino acids? You might come up with something similar, maybe not exactly the same, which will work as well. You might end up with a different kind of life, which is based on a different functioning mechanism that at the end leads to the same kind of global life kind of mechanism.

I think this is the adventure we are going to embark upon. I am pretty optimistic that, given all the efforts, all the brains, all the tools we have, we should get closer and closer to an answer. It may take a hundred years still, which in terms of science is nothing.

So that’s the kind of life roadmap that I would see while it is not yet clearly defined.

MA. In 2016 you discovered planets orbiting a star called TRAPPIST-1, what do we know about this system?[21]

Note: TRAPPIST is an acronym for TRAnsiting Planets and PlanetsImals Small Telescope-South.[22] [23]

DQ. That’s a great work that I did with Belgian colleagues and Michaël Gillon is the leader of this exercise. I think this is fascinating because we are trying to trick the system here. We are using smaller stars, cool stars, M stars,[24] [25] to compensate the fact that we haven’t found a true Earth, but we have plenty of planets similar to Earth. The idea is that if you look for an Earth-sized planet very close to a star and you want that planet to be potentially habitable you need to make sure the planet is not roasted by the star. By looking for cooler stars, M stars, you have the right distance to have the same kind of flux coming from the star that we experience here on Earth.

We found the TRAPPIST-1 system and there are two or three planets, which are clearly like Earth in terms of mass and size. These planets are in a region that we call the habitable zone. It’s a region where if you took Earth as it is and you moved it within this habitable zone it would maintain liquid water on its surface.[16] [21]

The assumption is that within the habitable zone you would have atmosphere and geophysics similar to the one on Earth. We don’t have such information for the TRAPPIST-1 system. But the fact that those planets are within the habitable zone gives some boundary conditions.

There is a real interest to go a little bit deeper about these planets and to try to detect their atmosphere. Low mass stars emit a different kind of radiation from the Sun. These stars are very active when they are young (superluminous pre-main-sequence phase).[26] [27] The Sun slowed down very quickly and then it became nice; although you would still have eruptions from time to time. M-stars are very different and took long time to cool down. There is a lot of activity, lots of X-rays. So we don’t really like M stars because of this. They emit a lot of high energy particles.[28] [29]

It is not very clear whether an atmosphere in a planet orbiting an M-star would survive. But let’s find out.[21] Assuming the atmosphere would survive, how would it look like?[30]

Here we are talking about another kind of star, with a very close planet. So nobody knows well about this. But again let’s have a look.

We have the James Webb Telescope (JWST) and maybe it could do something.[30] [31] [32]

The ambiguity of this program is that if the atmosphere is very thick like for example 200-500 km it will be detected by the JWST. However if the atmosphere is like the one on Earth, 10-20 km, we won’t see anything.[32] So we should be able to know whether there is an atmosphere, which doesn’t look much like Earth, because we don’t have 500 km atmosphere on Earth. Or we might detect no atmosphere, which doesn’t tell that there is no atmosphere. It tells that we haven’t detected anything, but we could still have a 10 km atmosphere or no atmosphere at all like on Mars.

TRAPPIST-1 is an interesting system because if you detect a thick atmosphere in any of its planets, you can almost reject life on it, but if you don’t detect atmosphere there is still hope that there is life there. In this case you wait for the next generation of more sensitive instruments. So it’s science building up on the past.

TRAPPIST-1 is a fascinating system. There may be others. We are looking for other similar systems. Who knows what kind of surprises we will be able to find. I think it’s great because for the first time we can really try something and we can at least get some data. It is not about talking and imagining something. We have a target, we have the tool, let’s find out!

I think it’s a fantastic adventure we are living right now.

MA. The closest potentially habitable exoplanet is Proxima Centauri b that was discovered in 2016.[33] If we decide to send a probe there, with our current propulsion systems, it would take some 50,000 years. So what are the most viable candidates for new propulsion systems that are currently under study?[34] [35] [36] [37] [38] [39]

DQ. I don’t really like the idea of sending a probe. I think it’s a sci-fi dream. I think it’s a misuse of funds right now. You can study a lot of stuff just by looking. I think it would be much better to design the kind of equipment necessary to retrieve as much data as we can before sending a probe. I think we are a hundred years behind sending any probe at that distance. If you send a probe with the current technology, in 10,000 years we might have a technology to go much faster and we would still have this probe on its way when the new one passes it.

I know that there are people dreaming about this. There are private sponsors that would like to do that.[40] [41] I personally regret that they are not talking to astronomers about this. I love the idea to look for life on Proxima Centauri b, it’s a very good program. But sending a probe is not a good idea. We should build equipment to just look at it. With that we should be able to tell a lot already without physically going there.

Now, if you want to go there, the problem we have to face, is that you better go fast because otherwise it takes too long. The speed has to be quite significant. When you start reaching 1% of the speed of light the mass becomes bigger.[42] [43] Moreover how will you slow down the probe? If you just have a flyby of one second around the planet, what’s the purpose?

I think we should try to engage better with the people that would like to have this program. I would really like to invite people interested in this topic, especially private sponsors, to talk to astronomers and try to come up with a realistic program.

This idea of sending a probe to Proxima Centuari, I think, is just a dream right now. We shouldn’t spend money in this way. You can still dream, it’s fine, but spending money on that is just a mistake to me.