The universe is just 5% visible matter, something that Princeton University cosmologist James Peebles foresaw. 

NASA/ESA/S. Beckwith (STScI)/HUDF Team

Physics Nobel honors architect of modern cosmology, discoverers of other worlds

This year’s Nobel Prize in Physics honors the human desire to understand both the fundamental nature of the universe and its planetary particularities. Half of the $900,000 prize goes to Princeton University cosmologist James Peebles, for laying the foundations of modern-day cosmology and predicting the basic ingredients of the universe. The other half will be split between astronomers Michel Mayor and Didier Queloz. In 1995, at the University of Geneva in Switzerland, they discovered the first planet around another sunlike star—opening the floodgates to the discovery of thousands more exoplanets of every description.

Peebles’s many theoretical predictions have proved prescient. For starters, in 1965 he predicted the big bang nearly 14 billion years ago should have left an afterglow, radiation that would have stretched to microwave wavelengths as the universe expanded. That cosmic microwave background (CMB) was discovered the same year and has proved invaluable for deciphering the universe. “He was the guy in the early days,” says Joseph Silk, a cosmologist the University of Oxford in the United Kingdom. He “put the physics into cosmology.”

Others had suggested an afterglow; Peebles laid bare the details. He showed how its temperature is pinned down by the abundance of light elements in the cosmos. He also predicted that the sloshing interplay between radiation, ordinary matter, and dark matter—the invisible stuff that was already thought to hold the galaxies together—would cause the temperature of the CMB to vary from point to point across the sky. Those tiny fluctuations were eventually spotted by NASA’s Cosmic Background Explorer satellite, which launched in 1989.

To get the size of those fluctuations right, however, Peebles had to include in this theoretical work not only dark matter, but also a stretching energy in empty space. He revived Albert Einstein’s repudiated idea of a cosmological constant, a form of dark energy woven into space itself. That move was bolstered with the 1998 discovery that the expansion of the universe is accelerating, as if driven by dark energy.

In the past 2 decades, ever-more-precise space missions to study the CMB, such as NASA’s Wilkinson Microwave Anistropy Probe (WMAP) and Europe’s Planck spacecraft, have refined cosmologists’ recipe for making the universe: 5% ordinary matter, 26% dark matter, and 69% dark energy. “When I first saw the WMAP result my one-sentence summary was ‘Jim Peebles is right,’” says Princeton cosmologist David Spergel, who worked on that mission.

The gas giant 51 Pegasi b (artist’s illustration) was the first planet to be discovered around a sunlike star.

ESO/M. Kornmesser/Nick Risinger

Ironically, Peebles says that as a graduate student at Princeton in the late 1950s he had his doubts about entering the field of cosmology at all. “I was very uneasy about going into cosmology because the experimental observations were so modest,” he recalled by phone during the press conference announcing the prize. “The field grew and I grew with it.” Chuck Bennett, a cosmologist at Johns Hopkins University in Baltimore, Maryland, notes that Peebles’s prize is “more a lifetime achievement award than an award for one master stroke.”

Mayor and Queloz, in contrast, changed astronomy with a master stroke—though at the time, even they wondered whether their discovery could be real. It was a planet that no one thought was possible: a gas giant half the mass of Jupiter, but orbiting so close to its star that it seemed to be virtually skimming the surface. At the time, astronomers thought large planets could only exist farther out in a solar system. “They took a long time to convince themselves that they had actually found a planet in that orbit,” says astronomer Nikku Madhusudhan of the University of Cambridge in the United Kingdom.

Now, less than 2.5 decades later, exoplanets have gone “from obscure, fringe, and laughable to Nobel-worthy,” says astronomer Sara Seager of the Massachusetts Institute of Technology in Cambridge, who calls the prize “a huge tribute to people all around the world making exoplanets real.” Astronomers now know that almost all the stars in the Milky Way have a coterie of planets. And the most common type of planet is one not found in our solar system: something between the size of Earth and Neptune.

Astronomers had been looking for planets around other stars for almost as long as they have had telescopes. But the challenges are huge: A star can be billions of times brighter than the planets around it, and at interstellar distances their apparent separation is tiny. In the 1980s, astronomers began to try to detect them indirectly using precision spectroscopy. The gravity of a large planet, as it orbits, can tug its star around. Astronomers can thus look for periodic Doppler shifts in the wavelengths of the star’s light as it moves to and from Earth.

In April 1994, Mayor and Queloz began to observe 142 sunlike stars for the telltale wobble using a new spectrometer from the Haute-Provence Observatory in France. Their sensor could detect stellar movements as slow as 13 meters per second, slightly faster than sprinter Usain Bolt. Several stars appeared to show movement but one star, called 51 Pegasi, offered a clear signal. From the period and amplitude of the oscillation and the mass of the star, they calculated that the planet, a gas giant called 51 Pegasi b, was a “hot Jupiter,” orbiting at about one-seventh the distance Mercury is from the sun. “Planet formation theory [at that time] didn’t predict such a planet,” Madhusudhan says.

Other teams began to find hot Jupiters around wobbling stars, and astronomers began to suggest that giant planets may form far from their stars but can migrate inward. Then, in 2000, astronomers found a new way to discover planets: by watching for periodic dimmings in a star’s light, a sign that a planet had passed in front of the star, or “transited.” Whereas wobbles have revealed several hundred exoplanets, the transit method, deployed with particular aplomb by NASA’s Kepler satellite, has bagged several thousand. Those two techniques reveal a planet’s mass and radius, respectively, so that its density can be calculated—and astronomers can infer whether the distant world is rocky or gaseous.

They can also probe exoplanet atmospheres by looking for the absorption patterns left by gas molecules in the spectrum of starlight that has grazed the planet. The technique—applied to only a few exoplanets so far—will get a boost with the launch of the James Webb Space Telescope in 2021.

Mayor and his team discovered about 200 more planets. Though formally retired, he continues to observe. Queloz, now at Cambridge, continues to hunt exoplanets and is involved in the Search for habitable Planets Eclipsing Ultra-cool Stars (SPECULOOS) project, a collection of telescopes to detect planets around the smallest stars using the Doppler method. According to SPECULOOS chief Michaël Gillon of the University of Liège in Belgium, Queloz was in a project meeting today when he excused himself to take a phone call. Minutes later they all watched a webcast announcement of the prize awarded to their absent colleague. “It’s really well deserved,” says Gillon, who did a postdoc fellowship in Geneva with Mayor and Queloz. “The next decade will be superexciting for the field.”