Interview: Michel Mayor, Nobel prize-winning astrophysicist
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How did a theoretical physicist transform into a man whose fascination with instrumentation led to the discovery of the first extrasolar world and last year’s Nobel Prize for Physics? E&T sat down for a conversation with astrophysicist Michel Mayor.
Scientists, philosophers, and science fiction writers have long speculated about distant worlds. Through the 19th and 20th centuries, scientists grasped at scraps of evidence of planets beyond our solar system (exoplanets), but it was not until the 1990s that instrumentation had come far enough to confirm their existence.
As a young Swiss student in the late 1960s, Michel Mayor was utterly uninterested in instrumentation. He had studied theoretical physics and then embarked on a similarly theoretical PhD on the dynamics of stars in spiral galaxies, exploring “how matter can organise itself into these beautiful forms”.
After finishing his PhD, however, Mayor became interested in collecting some observations of radial velocities to test the theory he had been working on.
“I started doing theoretical physics, the interaction of particles with large spin and things like this, then density waves in spiral galaxies,” Mayor told E&T, speaking via video call from Geneva. He is warm, unpretentious, and frequently punctuates his sentences with laughter. “I was so happy to start developing instrumentation. It’s crazy because when you are a young PhD student, evidently what is most elegant is the theory, and instrumentation is some kind of…”
“Engineering?” I suggested. He laughed riotously. “Yes, exactly!”
Stars move relative to our own solar system; the component of velocity along our line of sight is radial velocity. Radial velocity of distant stars can be calculated using a spectrometer to measure the shift in their spectra caused by the Doppler effect (change in frequency as source and observer move relative to each other).
Unfortunately for Mayor, there had been little historic interest in measuring radial velocities. As a consequence, the instrumentation available was limited, with massive errors in radial velocity measurements of around 1,000m/s. This was far from adequate for taking the observations that he was interested in.
A few years before, however, Roger Griffin at the University of Cambridge had demonstrated the feasibility of the cross-correlation spectrograph for radial velocity measurements for the first time; this effectively allows thousands of spectral lines to be used simultaneously, improving illumination and concentrating information. Measuring radial velocity using this technique would allow for much more precise measures of radial velocity to be taken compared with existing photographic techniques.
Incorporating Griffin’s work, Mayor and his colleague André Baranne set out to design a new spectrometer at Marseille Observatory. Their state-of-the-art photoelectric spectrometer, COREVAL, was mounted on the 1m Swiss telescope at Haute-Provence Observatory in the South of France in 1977. COREVAL allowed for readings with smaller errors of around 300m/s, opening up a universe of possibilities for the astrophysicists.
With this new instrument at their service, Mayor and his colleagues made rafts of discoveries, including that some stars which had been presumed to be binary stars may in fact be single stars with planetary companions.
Good as COREVAL was, it was still not precise enough to detect the slight shift in radial velocity (‘wobble’) which is caused by a planet as it orbits its star. Jupiter causes the Sun to change velocity by just 12m/s over its 12-year period, for instance, while Earth causes a change of just 0.1m/s.
In 1988, the director of the observatory asked Mayor and Baranne to design a successor to COREVAL, which would be adapted to its 1.93m telescope. Their new spectrometer, ELODIE, was equipped with a CCD camera, an echelle grating for high-order diffraction (essentially improving clarity), a dual optical fibre delivery system, and automated data processing. ELODIE was implemented at the observatory towards the end of 1993. It could take radial velocity measurements with an accuracy of around 10m/s, finally placing other worlds within Mayor’s sights.
“When we have a new instrument like this, [we say] “okay, what can we do best?” Mayor said. “It was just the level to do a planet search! We were driven by technology.”
Mayor was granted one week of observation time every two months to search for companions to solar-type stars. Beginning in spring 1994, Mayor used ELODIE to study 142 stars with his PhD student Didier Queloz. They found that an unassuming star 50 light years from Earth (51 Pegasi) was wobbling in a fashion which indicated it was being orbited by a Jupiter-like planet.
There was a catch however; this planet had a 4.2-day orbital period. At the time, the theory of formation of gas giants suggested that it should have a period of at least 10 years (theoretically having to orbit at a certain minimum distance from its star in order to have accreted the ice particles necessary to reach its size). This was troubling for Mayor, who wanted to avoid becoming the latest astrophysicist to fall into the trap of a false detection.
“The history of the detection of extrasolar planets is completely full of false detections. Already in 1943 [there were] two announcements of detection of planets by different techniques, and then after we have other detections. So it was some kind of badly-renowned domain,” Mayor explained.
He and Queloz decided not to speak to anybody about their discovery, and arranged for more observation time when 51 Pegasi could next be seen in the skies. Following a week of observations at Haute Provence in July 1995, they ended up with the same measurements as before. According to Mayor, that was the moment that “everything changed”. They celebrated over a bottle of champagne.
“We had exactly the same period, the same amplitude, the same phase […] so we decided to publish. We submitted the paper to Nature at the end of August. We announced the discovery in a conference in Florence in early October and that was the start of the crazy period. The rumours started to spread,” Mayor said. Faxes started arriving from the world’s largest newspapers.
Since that discovery, Mayor has remained active in improving the precision of spectrometers, thrilled with how each technological advance reveals other worlds in more detail than before. After ELODIE, he and his colleagues developed a new cross-correlation spectrometer called HARPS. The tool – which was implemented on a 3.6m telescope at La Silla, Chile – has an even better resolution of 1m/s, and Mayor used it to search the Southern sky for more exoplanets over five years.
Since then, an even better spectrometer has been implemented on an 8m telescope at La Silla with a precision of 0.1m/s. This spectrometer is the first to reach the precision necessary to search for tell-tale wobbles caused by Earth-like planets orbiting their parent stars.
“What for me is absolutely magic, what is fascinating, is that from the first instrument implemented in 1977 we had 300m/s and today, something like 40 years later, we arrive at 0.1m/s,” he said. “It’s a factor of 3,000 of improvement, it’s technological progress.”
More than 4,000 more exoplanets have been discovered since 51 Pegasi, many of which were found with the same ‘wobble’ technique Mayor used. Some are rocky Earth-like planets in habitable zones which could contain water and sustain life. While in the 1980s and 90s there were just a handful of groups around the world searching for exoplanets, there are now thousands of scientists discovering other worlds.
The importance of Mayor’s discovery was recognised with the highest honour in physics last year when Mayor, Queloz, and theoretical cosmologist Jim Peebles were awarded the Nobel Prize in Physics.
“I knew I was nominated already for many, many years. But what is the meaning of nomination? Some people propose your name, but there are so many physicists working on so many beautiful discoveries around the world […] every year a few hundred people are nominated so I decided immediately that I would never consider the possibility [of winning],” Mayor said. At the time that the 2019 prize was announced, he was not waiting hopefully for the announcement but relaxing on holiday in San Sebastian: “It is only by chance that with my computer I saw what people were saying. I got the information from the computer.”
While Mayor is buoyant at having received the prize, he said that it was only won due to the impact of his discovery on his field of research. He maintains that many better physicists, such as Arnold Sommerfeld, missed out on the honour year after year.
“If you consider colleagues doing science in many different fields, they are much better physicists. It’s not about the quality of the people. We have to keep our feet on the ground,” he says.
Like many other distinguished scientists who have gained a public profile, Mayor is serious about engaging the public with his work. In particular, he is motivated by what he perceives as a low level of scientific understanding among politicians.
“If you are looking at the big issues of global warming, nuclear power, and so on, all the people in parliament don’t have a background in science. This is at the level of parliament but it is also at the level of the general population,” he said. “So, I believe [science engagement] is extremely important. At the present time, you see with the coronavirus pandemic you have such crazy ideas. To have some kind of scientific background is very important.”
Among other outreach activities – he is currently preparing his Nobel lecture – Mayor is a regular at the Starmus festival: an annual celebration of science and music. Starmus features daytime lectures from distinguished scientific figures (past guests include Stephen Hawking, Roger Penrose, Richard Dawkins, and Neil Armstrong) followed by evenings of music. Mayor smiles as he talks about Starmus; he fondly recalls how the Norwegian royal family arrived at the 2017 Starmus festival to open the event, but ended up staying for the whole afternoon to listen to the lectures.
The next event (Starmus VI) will be held in Armenia upon the invitation of its president Dr Armen Sakissian, a former theoretical physicist and computer scientist.
Mayor believes that scientists have an important public role to play. He says that this is not just about sharing scientific knowledge, but encouraging critical thinking and inspiring the next generation of scientists.
“In the past people were afraid of comets. Now [we consider them] nice and interesting but they’re not frightening,” he said. “I believe science has some kind of sociological role to educate people in a very general way to allow people to have a more critical view of what is going on.”
“And personally, I would say science is curiosity-driven. It’s exciting, and it’s so diverse!”
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