Cosmologist James Peebles (left) and exoplanet astronomers Didier Queloz and Michel Mayor (right) share the 2019 Nobel Prize in Physics.Credit: Torbjörn Zadig/University of Cambridge/ Inamori Foundation

Cosmologist James Peebles and astronomers Michel Mayor and Didier Queloz have won the 2019 Nobel Prize in Physics for discoveries about the evolution of the Universe and Earth’s place in it.

In 1995, Mayor, at the University of Geneva, Switzerland, and his then-student Queloz announced1 the first discovery of a planet orbiting a Sun-like star — launching a field that has become one of astronomy’s hottest. They detected the exoplanet through its tiny gravitational pull on its star, 51 Pegasi, a technique that is now used to study some of the more than 4,000 exoplanets known to exist.

The exoplanet files

Peebles, who is at Princeton University in New Jersey, developed a theoretical framework that the Nobel Committee says forms “the foundation of our modern understanding of the Universe’s history, from the Big Bang to the present day”.

Peebles helped to lay the theoretical foundations for the cosmic microwave background (CMB), the ‘afterglow’ of the Big Bang, and to establish the current ‘standard model’ of the evolution of the Universe. In this model, the mysterious substance known as dark matter plays a central part in assembling large-scale structures of the cosmos, such as galaxies and clusters of galaxies.

Mayor and Queloz, who are both Swiss and were born in 1942 and 1966, respectively, share one-half of the prize, worth 9 million Swedish kronor (US$910,000). Peebles, who was born in Winnipeg, Canada, in 1935, will receive the other half.

Surprise world

Mayor and Queloz’s discovery “started modern exoplanet science”, says Guillem Anglada-Escudé, an astronomer at the Institute for Space Sciences-CSIC in Barcelona, Spain.

Researchers had discovered exoplanets orbiting spinning cores of dead stars known as pulsars, but not around stars similar to our own, which could host habitable planets.

The pair’s discovery came as a surprise to the astronomy community. The planet they detected, called 51 Pegasi b, is a gas giant, a type that astronomers had expected would orbit the outer reaches of a solar system. But the pair found it orbiting around ten times closer to its star than Mercury is to the Sun — an early sign that other planetary systems might not be like our own.

Several groups were making similar observations at the time, but Mayor and Queloz were first to make the detection because they cast a wider net, says exoplanet astronomer Francesco Pepe. “When they started their programme at the Observatoire de Haute-Provence — by which they found 51 Pegasi b — they didn’t focus on trying to discover planets like those in our Solar System,” says Pepe, who is the head of the astronomy department at the University of Geneva. In particular, their observations enabled them to see Jupiter-sized planets with very tight orbits. “This made the difference.”

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The finding was remarkable for being almost completely unambiguous and quickly confirmed, says Anglada-Escudé. The Swiss team almost immediately began to survey the sky in earnest, he says, which kicked off a “cold war” in exoplanet hunting with a team at the University of California, Berkeley, that was using a similar technique to detect planets and that was the first to confirm the duo’s findings. The Berkeley team’s leader, Geoffrey Marcy, was previously also tipped as a Nobel contender. He stepped down from his position at Berkeley in 2015, after a university investigation ruled that he had violated sexual-harassment policies.

Both Mayor and Queloz, who splits his time between the University of Cambridge, UK, and the University of Geneva, are famed for their talent at building exquisitely precise instruments. They developed spectrographs able to observe the tiny shifts in the frequency of light as a star wobbles under the influence of a planet’s gravity. Known as the radial velocity technique, the method can detect exoplanets and estimate their mass. Today, it is just one of several methods that astronomers use to find exoplanets and study their atmospheres and potential exomoons. Some of those planets have Earth-like qualities, including signs of water in the atmosphere of a planet twice the size of our own.

The pair are “superb scientists” who, together with an ever-growing community of researchers, paved the way for a completely new research area, says Christiane Helling, an exoplanet researcher at the University of St Andrews, UK. Queloz “has always worked to develop and support the community as a whole, rather than only for his own success”, she adds.

Probing first light

Meanwhile, Peebles’ theories have helped cosmologists to understand much more about the CMB and the Universe’s beginnings.

“Were it not for the theoretical discoveries of James Peebles, the wonderful high-precision measurements of this radiation over the last 20 years would have told us almost nothing,” said Mats Larsson, a molecular physicist at Stockholm University and chair of the 2019 Nobel physics committee, when he announced the prize.

The quest to unlock the secrets of the baby Universe

“This is such a long-deserved recognition,” says François Bouchet, an astrophysicist at the Institute of Astrophysics in Paris, of Peebles’ prize. He says Peebles has left his mark on nearly all branches of cosmology, helping to put the field on a sound basis with calculations that are rooted in physics. In particular, in 19702, Peebles was a pioneer in predicting the shape of the CMB. Bouchet was a leading researcher on the European Space Agency’s Planck mission, which in this decade has made the most precise mapping yet of the CMB sky.

Peebles helped to develop a model of the Universe’s evolution known as the ‘cold dark matter’ theory3, which describes how cosmological structures formed as the Universe expanded and cooled from its hot, dense beginning. Together with the later addition of ideas about the enigmatic force known as dark energy, this has become the standard framework of modern cosmology.

Although the precise nature of dark matter is yet to be understood, several high-precision surveys of the Universe have lent support to the theory — these include studies of the CMB and the mapping of galaxies across large swathes of the sky. The theory suggests that if dark matter consists of subatomic particles, these must be massive and relatively slow-moving; observations have now largely excluded models in which lighter but faster-moving particles would dominate dark matter.

Planck and other projects have found features in the CMB that Peebles had predicted. “These are in the sky — they really exist, it’s not just theory,” says astrophysicist Rashid Sunyaev, one of the Soviet astrophysicists who performed similar calculations in 1970 and is now at the Max Planck Institute for Astrophysics in Garching, Germany.

“The establishment that our Universe is evolving was in part a theoretical discovery, but that was meaningless without the evidence — very firm evidence now — that it really did expand from a hot, dense state,” said Peebles, speaking by telephone to journalists after the announcement of the prize. “We depend on that interaction of theory and observation.”

It is unusual for extrasolar planets and cosmology to be paired up in the same prize, but both lines of work “give a fresh perspective of the place humans have in the cosmos”, Bouchet says.