Interview to Didier Queloz — ‘From 51 Pegasi to the search for life around small stars’

Interview to Prof. Didier Queloz at University of Cambridge/UK, by Guillem Anglada-Escude

In the early 90’s the search for extra-solar planets was not even a research topic. What can you tell us about those first days?

At the end of the 80’s and early 90’s, exoplanets were not fashionable at all. I was involved in the design and building of a new type of instrument specifically designed to find planets around other stars. Our team were very successful in making key design decisions, so as soon we had the instrument on the telescope, we quickly identified one with quite a different variability from the others. It was 51 Peg.

The spectrograph concept was developed by a team under the direction of Prof. Michel Mayor. Who created the optical design? I heard that a French professor called Andre Baranne was a key person at that stage…

Yes, in any instrument, there is always an expert in precision optics. The person for that project was Prof. Andre Baranne. He was the creator of the so-called ‘white-pupil’ design, which is now adopted by most high resolution spectrometers. Before Andre’s work, spectrometers were huge, photon-eating devices. Thanks to that improvement, instruments became compact and efficient. He was close to retirement but he became very active in the project. The spectrometer was build at Observatoire de Haute Provence (OHP). In those days they had very sensitive cameras for faint objects, but a lot of telescope time could not be used because of background contamination by the moon. This is when Michel Mayor came forward offering a high resolution spectrometer for stellar astrophysics that, at the same time, would be able to detect radial velocities with unprecedented precision. Because it was a joint effort of Micheal’s team and the observatory, quite a lot of people were behind the design of the numerous subsystems.

The ELODIE spectrograph ready for operation at the 193 cm Telescoep of l'Observatoire de Haute Provence. Image credit : CNRS / OHP
The ELODIE spectrograph ready for operation at the 193 cm Telescoep of l’Observatoire de Haute Provence. Image credit : CNRS / OHP

You and Micheal Mayor were at the Geneva Observatory at the time but the spectrograph was made by OHP?

Yes, OHP built it but most participating astronomers were from Geneva. Michel already had a working instrument at OHP called CORAVEL, so it was a natural choice for him to to build the new one with them. The deal was the following; OHP would build two spectrometers, and the second one would be installed at the Swiss telescope at la Silla in Chile (CORALIE). For a number of reasons, the OHP one -ELODIE- was at the telescope first, which is where I spent most of my PhD time testing the new hardware, detectors, optical fibres, wavelength calibration using Thorium-Argon lamps and simultaneous tracking. These are obvious things to do today, but they were completely new concepts at the time. ELODIE was the first of a series of instruments that led to HARPS.

World-renowned Swiss astronomers Didier Queloz and Michel Mayor of the Geneva Observatory are seen here in front of ESO’s 3.6-metre telescope at La Silla Observatory in Chile. The telescope hosts HARPS, the world’s leading exoplanet hunter. Image credits : L. Weinstein/Ciel et Espace Photos/ESO
World-renowned Swiss astronomers Didier Queloz and Michel Mayor of the Geneva Observatory are seen here in front of ESO’s 3.6-metre telescope at La Silla Observatory in Chile. The telescope hosts HARPS, the world’s leading exoplanet hunter. Image credits : L. Weinstein/Ciel et Espace Photos/ESO

So what was the key element that made possible the breakthrough of finding the first planet in 1995?

Two really important things. We had enough telescope time to look at a meaningful sample of stars. And second, of course, we also had the machine to do it. We could regularly obtain data with a precision better than 10 m/s, which had not been possible before… and the signals were just there. Once you have done the really hard work of getting that kind of precision, the planets come for free (‘almost’). The previous precision was 50–100 m/s with instruments similar to CORAVEL, and even some first results reported by G. Marcy’s team , were in the 20–30 m/s level. When Marcy & Butler managed to get down to 5–10 m/s level, the planets started to show-up in their data too. The same for us. This new machine started delivering better than 10 m/s since the beginning, so with all this hard work done you can only start finding those planets.

How was finding 51 Peg, and more importantly, how sure were you that it was a planet? Lots of people were skeptical those days, arguing that it was an instrumental error? astrophysical artifact?..a binary?

In a sense, people were right to be skeptical. We were as well. You have to realize there were no known exoplanets in those days. It was a rather special situation. Today is very different. You can now publish, or claim detections of planets, even if you are not 100% sure because there are many of them so one more or less is not that transcendental. That was not the case back then. You REALLY needed to be sure. In our case, it was a new instrument and nobody was expecting to find a planet at such short period. I was the first not to expect it, and the same for Michel Mayor. Michel was on sabbatical, so I started the observation program more or less alone. Quite early on I picked up a strange object. It was weird, that star was clearly not stable above those 10 m/s, but it was known to be a very non-active sun-like star too. I kind of felt responsible for the operation of the spectrograph and all the software, so I became completely obsessed with it. I observed 51 Peg much more often than was planned. Consequently I found that there was a periodicity to the signal. Then I took quite some time to convince myself first that the signal was a planet without telling anybody. Convincing myself implied reviewing all the data-processing, the way the velocity was measured, that the period was not related to some instrumental issue and review the other stars in the sample. Once there was no more to check, I sent a fax to Michel who was in Hawaii. “Michel, I think I have found a planet with this period”. Michel responded “Yes, ok… maybe, I’ll see when I come back”. He was really puzzled. We then reviewed everything from the start again, thinking there might be a bug somewhere… even what we knew from the star itself; star-spots on it could create a signal.

Radial velocity curve of 51 Peg as measured by ELODIE. The radial velocity variations follow an amplitude of 59 m/s and have a period of 4.23 days. Source : OHP
Radial velocity curve of 51 Peg as measured by ELODIE. The radial velocity variations follow an amplitude of 59 m/s and have a period of 4.23 days. Source : OHP

It’s kind of funny for me, because most of what has been done later—looking at activity features and comparing it to the orbits of the possible planets—we did all these in that first paper too. I suspect nobody understood the reason for all those tests and complexity (read about the reasons in X. Dumusque’s article here). The detection of the signal was the easy part! The hard part was to be completely sure that it was a planet, and nothing else. When we had all this, we submitted the paper, and it barely got accepted. It felt a bit like magic because it was shaking the currently held theory. In a way, when we announced it at the Florence meeting, we were lucky that G.Marcy & P. Bulter were at the telescope at that very moment. G. Marcy later confessed that he thought the signal was a complete fraud, so they were also really surprised when they could confirm the signal after only a few days. This was kind of the key point of my PhD, and a big relief. That meant that the data was fine, the spectrograph was working and the period was also fine. Then we had to struggle a lot with the community. For example, many argued this could not be a planet but the atmosphere of a star changing over time. In science when you make a big claim you typically get heavily attacked, and if you survive you come back even stronger. So it took us a couple of years to convince everybody, but the final blow came in 2000 when the first of these planets was found to transit in front of the star.

51 Peg, and the planets we familiarly know as hot-Jupiters, are still a mystery and a challenge. We know a lot about these hot-Jupiters, we probe their atmospheres, we can see if their orbits are aligned with the star. But it is still a mystery how they fit in the big picture of how we think planets should form. We now know that those planets are relatively rare (about <2% of the stars have them). But with these odds, you pick up 50 stars at random and this is what you get. True enough, there was only one hot-Jupiter in our sample. In a sense, you need to be lucky to find a planet. You need the right instrument and the right strategy, and the planet needs to be there.

But one needs to push his luck…

Sure, what we were really ‘lucky’ about is that the other team didn’t get it first! Geoff Marcy started 2 years before so they could have found it two years earlier.

There were issues with resources if you ask Paul Butler (see story here!)… Are there other discoveries after 51 Peg that you feel proud of as well?

Well, I think the discovery of 51 Peg was the key to this threshold—it changed the whole game, it opened up the field of exoplanets. So I came out in this strange situation, my best ever result and highest impact paper is that first one. I mean, we created the field with 51 Peg in 1995. Before it was a weird topic, after ’95 it was a scientific topic, and the theme has been made broader because it is related to the search for life in the universe. 51 Peg was key. Of course, I have been doing lots and lots of other things, and working on other techniques like transit searches and astrometry.

What is driving your research these days?

Oh, this is simple. We have a long list of questions now. 51 Peg was the entry point. There are numerous scientific questions to answer, and a handful that are really important and deep ones. For example, the formation of our Solar System in the context of other planetary systems. We need to detect lots of planets and characterize their atmospheres to understand how planetary systems form and evolve…

…but the real question that is driving my efforts is looking for life in the universe. After finding the first planet, this is the next big thing. From a practical point of view; can we define a robust and affordable strategy to do this? I am getting more and more convinced that a step-by-step process is realistic, but it will require out-of-the-box thinking in terms of support of the science. So now I invest a lot of time to try to explain to people that the Victorian division of the sciences like Chemistry, Physics, Astrophysics, Biology doesn’t make sense in this context anymore. The question of life in the universe is a multi-disciplinary problem that needs to be tackled in a different way. I try to convince agencies, and the universities, that all the work I have been doing is about promoting this new kind of work. I might not be doing it myself because I am getting too old, but I really think that the task of the next generation of scientists won’t be searching for the planets, it will be about figuring out whether there is life on these planets.

From all the proposals to search for evidence of life around exoplanets, do you have a favorite one?

There are plenty of ways to look for evidence of life on other planets. The difference is in the practicalities. It will be enormously difficult to detect and characterize an Earth-analog around a star like the Sun. It will be done, I am pretty sure, we will eventually have pictures of such a world, we will see continents, rotation… that will happen, I am confident, nothing will stop. It is just that, being realistic, the technology we need is not there. As scientists we want to think big and far, but we also need to look at what the technology of today can achieve. Along these lines, there are a number of experiments that allow us to push pretty far in the understanding of exoplanets (post by Don Polacco). The transit technique gives potential access to the atmospheres, so we need to work on that. And the direct imaging method has finally made great progress and soon will be providing abundant information about the atmosphere of planets (gas giants first).

Can we do well enough to be able to find life? This is where we need to go back to the books. People have been thinking about this for a long time. What would an Early-Earth atmosphere look like. What about the early UV and X-ray fluxes? All the assumptions made so far were very simplistic and the habitable zone concept much tied to the Earth’s… you add some hydrogen into the atmosphere and the possible climates change completely. We need studies at telescopes, but also in the lab. My idea is being as open-minded as I can. The real drive of the field has been finding and reporting the unexpected. We really need to get away from being over-simplistic.

Today, there are kinds of stars where we might be able to do it, because it is easier. These are very very small stars (like Proxima). With the available technology of today, there are realistic chances of finding the first hints of life in planets around them. This is an amazing field of research. It is extremely exciting to begin the transition from exoplanet detection towards the search for life. These planets must be very different than Earth. Nobody has thought much about taking an Earth and putting it so close to the star. The amount of UV fluxes, tidal interactions, the nature of the atmosphere and climates… all can be so different! We have to go to the drawing board and broaden our expectations. In this sense, I think Pale Red Dot is the kind of project that is opening up where these planets are, it can lead to the new science that will explode soon. There will be some chance of seeing hints of organic activity, but let’s make it more simple… let’s look for something that tells us that an atmosphere is out-of-balance. Life takes the Earth atmosphere out of balance. This is something that cannot happen without an active agent on the planet surface. So, let’s search for signs of these atmospheres being out of balance. This will be a new big window that can potentially open the field as the first planet did. I’m willing to invest time enabling this new era.

We all have high hopes of that… so how do you see the mid-term future? Do you see a large class mission in space anytime soon?

I have experience with space missions. Careful! Space business is about minimizing risk. Space missions and agencies run away from doing new technology. On the other hand, you can do many more technological cycles from the ground. The low-mass stars can be done from the ground. And this is the problem. There is no big experiment systematically preparing to investigate planets around these very low-mass stars. There are small attempts but we really need more. The one program I am aware of is SPECULOOS, and there can be many more of these programs. But these are on small class telescopes and the goal is finding them, not characterizing them… Is there a plan for the big telescopes? No, there isn’t! We can do it and we should do it. Infrared, stabilized spectrographs on the VLT do not need a 100M investment. So a lot can be done from the ground.

Technical sketch of the SPECULOOS bservatory. Source :
Technical sketch of the SPECULOOS bservatory. Source :

Space is great, but space is not the place for innovation and development. You need to first to have the technology, show that it will definitely work, and setup long and expensive technology development programs. The European Space Agency (ESA) is not good at that. The budget is really limited compared to larger agencies like NASA. For example, ESA could not launch something like JWST. Given that this is our working framework, we should be promoting and strongly developing our ground based facilities. We could be world-leading, and we are not doing that. There are exoplanet detection programs attached to some instrument developments but, given the weight and influence of the field, we don’t have enough. We are not investing enough to go for the big challenge that is the search for life. I will be happy to change my mind if a revolutionary idea (and resources) show up. But we need to be very careful in thinking that space is the solution to all our needs.

For example, look at the gravitational wave experiments. It took 30 years to build up and refine the experiments needed to finally be successful, and they might also get a space mission. We are now in a similar situation. I think we need a bit more progress. We should be looking for life around these low-mass stars. Once we find it (or evidence for it), that will completely change the field (as 51 Peg did) . The current designs of big missions are not appropriate to search for evidence of life. People designed the missions to detect planets orbiting G,K and early M-stars. That is not what is needed in the most immediate time-frame to move forward in the search for life. My hope? When we start detecting and investigating these planets around low-mass stars, we will realize we haven’t built the right instrument and we will react to it.

A paradigm change then…

Yes, I think with experiments like Pale Red Dot and SPECULOOS it will become obvious these planets are probably there in large numbers; and then we won’t be looking for the planets themselves, we will start looking for life. The experiments and the field become different. I don’t want to minimize the importance of other questions like origins and formation of planetary systems. It is crucial to understand how the solar system started and put it in context. But if you really want to look ahead, the goal is to search for life, nothing else. By finding hints of life around these small stars, the argument will become strong and solid enough to promote and narrow-down the design of THE space mission that will address the question of life in the universe in a broader context.

Taken during the European Southern Observatory 50th anniversary gala, held in the Residenz, Munich, on October 11, 2012. Image credits : M.McCaughrean (ESA)/ESO
Taken during the European Southern Observatory 50th anniversary gala, held in the Residenz, Munich, on October 11, 2012. Image credits : M.McCaughrean (ESA)/ESO

About the author. Didier Queloz was a Ph.D. student at the University of Geneva when he and Michel Mayor discovered the first exoplanet around a main sequence star. Queloz performed an analysis on 51 Pegasi using radial velocity measurements (Doppler spectroscopy). He worked on and lead several large instrumentation projects including ground based interferometers and space-missions. He was appointed as faculty member at Geneva University in 2003, and in 2008 he became full professor. During his career he has received numerous awards and recognition and he has recently taken a Professor position at the Cavendish Laboratory at University of Cambridge (UK), where he is also a fellow member of the prestigious Trinity College.