The 2019 Nobel prizes in physics

The awards for 2019 were announced today and half went to P. J. E. (James) Peebles for his theoretical work in physical cosmology and the other half jointly to Michel Mayor and Didier Queloz for the first discovery in 1995 of an exoplanet orbiting a solar-type star. By now more than 4,000 exoplanets have been found.

Directly observing a planet orbiting a star other than the Sun is not easy since stars are distant and planets are ‘dark’ (i.e., not primary sources of light). We directly see stars but not planets. Mayor and Queloz tried to see if they could detect the existence of a planet by the fact that due to gravity, the star and the planet orbit around their common center-of-mass. As the star moves towards us during its orbit, the light is blue-shifted and when it is moving away it is red-shifted and it is this ‘wobble’ that they were looking for. But since stars are so much more massive than planets, this wobble is usually tiny. For it to be significant, the planet’s mass should be large but usually large mass planets have large orbital periods, making the detection of variations in light frequency hard. For example, the largest planet in our Solar System is Jupiter that has an orbital period of almost 12 years.

When Queloz and Mayor set up the search it was with low expectations of finding anything because any planets massive enough to create a measurable Doppler shift were expected to have such long orbits that the wobble would take years to detect. Surprisingly, though, they found a huge planet sitting extremely close to its host star, with an orbit of just four days.

“Because it was so near in to its star, no one really believed it,” said Queloz, adding that it took several years to convince the world that the finding was real.

Since then, astronomers having since found more than 4,000 exoplanets, with an incredible range of sizes, forms and orbits. Learning about these strange and varied world’s beyond our solar system has transformed our understanding of how planets formed and given new focus to the question of whether there could be alien life out there somewhere.

Queloz said the sheer numbers of planets out there makes it hard to believe that our own planet is the only one to host life.

“We may find out that life is extremely rare. We know life is special, but we may not acknowledge how special or rare it is,” he said. “It’s not impossible that in the next 20 to 30 years there will be new kinds of equipment that would be able to answer this question. Whether they will find something is open.”

For those who want more scientific detail about the work being recognized this year, this article published on the Nobel committee website has good explanations.

As early as the fall of 1994, they found that the radial velocity of the star 51 Pegasi in the constellation Pegasus had a periodic variation of just about four days.

This was surprising because based on the only data point available at that time – our own Solar System – a Jupiter-mass companion ought to have a much longer period. A period of only four days would put the Jupiter-mass companion to 51 Pegasi at a distance of only 0.05 astronomical units (AU), one-hundredth of the distance between Jupiter and the Sun.

On the other hand, the short period gave Mayor and Queloz opportunities to study several full cycles. Another advantage with the very short period was that it could be checked very quickly by other radial velocity groups.

As with many experimental discoveries, the award given to Mayor and Queloz is not just for finding something hitherto unknown but also for innovations in detection techniques that enabled them to see things that were formerly undetectable.

In collaboration with André Baranne at the Marseille Observatory and colleagues from the Haute-Provence Observatory, they built the ELODIE spectrograph [11], an updated version of CORAVEL, which had been in use for more than a decade at the Haute- Provence Observatory. In order to survey more than just very bright stars, Mayor and collaborators chose a solution that did not include an absorption cell and a slit. Instead, they had an optical fibre–fed echelle spectrograph with the clear intention of avoiding the disadvantage with a cell, for which suitable objects are limited to bright stars in the vicinity of the Solar System. The intention with ELODIE was to expand the number of objects for which precision Doppler spectroscopy could be applied.

As for the other award, Peebles is a name that will be familiar to anyone who has ever looked into cosmology at all because his work has been highly influential in so many areas in elucidating the nature of the universe and indeed was key to interpreting the meaning of the cosmic microwave background (CMB) radiation that was discovered back in 1965.

A key contribution is a paper by Peebles alone from the same year, 1965 [22]. He had already submitted it to The Astrophysical Journal on 8 March 1965, revised 1 June and published 15 November. The first sentence of the abstract states: “A critical factor in the formation of galaxies may be the presence of a black-body radiation content of the universe.” This work, together with other contributions by the late Russian cosmologist Yakov Zeldovich [23], can be viewed as the starting point of physical cosmology, where the laws of physics are applied to the Universe at large. This is the moment when cosmology embarks on its way to become a science of precision and a tool to discover new physics.

Peebles went on to do other important work in his long career, such as on things like cold dark matter and to connect anisotropies in the CMB to the large-scale structure of the universe. As the Nobel article states:

Physical cosmology, with its interplay between observations and theory, is a tremendous success story that over the past half century has changed the way we view our Universe. Once, cosmology was a subject full of unfounded speculations and little data. It is now an exact mathematical science, where evermore accurate observations play a key role. The era of discovery is not over. As the measurements become more precise, new and unexpected phenomena are likely to be discovered. Physical cosmology will have more surprises in store, and Peebles is the one who has shown us the way to discover them.

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