James Peebles, Michel Mayor, and Didier Queloz will receive the 2019 Nobel Prize in Physics for helping to understand our place in the universe through advances in theoretical cosmology and the detection of extrasolar planets, the Royal Swedish Academy of Sciences announced on Tuesday. Peebles is a theoretical cosmologist at Princeton University who helped predict and then interpret the cosmic microwave background (CMB) and later worked to integrate dark matter and dark energy into the cosmological framework. Mayor and Queloz are observational astronomers at the University of Geneva who in 1995 discovered 51 Pegasi b, the first known exoplanet to orbit a Sunlike star. Peebles will receive half of the 9 million Swedish krona (roughly $900 000) prize; Mayor and Queloz (who also has an appointment at the University of Cambridge) will share the other half.
The contributions of Peebles and of Mayor and Queloz helped jumpstart their respective fields. Over the past few decades, researchers have developed the successful standard model of cosmology, Lambda CDM, though the nature of both dark energy and dark matter remains an open question. Meanwhile, astronomers have used the radial velocity technique employed by Mayor and Queloz, along with the transit method and even direct imaging, to discover and characterize a diverse population of thousands of exoplanets. Data from NASA’s Kepler telescope suggest that the Milky Way harbors more planets than stars.
Connecting past with present
“More than any other person,” writes Caltech theoretical physicist Sean Carroll on Twitter, Peebles “made physical cosmology into a quantitative science.” His contributions began even before Arno Penzias and Robert Wilson’s 20-foot antenna at Bell Labs picked up the unexpected hum of 7.35 cm microwave noise that would come to be known as the CMB. Working as a postdoc with Robert Dicke at Princeton, Peebles predicted in a 1965 paper that the remnant radiation from a hot Big Bang, after eons of propagating through an expanding universe, would have a temperature of about 10 K. In a subsequent paper Peebles connected the temperature of the CMB, measured by Penzias and Wilson at 3.5 K (now known to be 2.7 K), to the density of matter in the early universe and the formation of light elements such as helium.
In 1970 Peebles and graduate student Jer Yu predicted a set of temperature fluctuations imprinted in the CMB due to the propagation of acoustic waves in the hot plasma of the infant universe. Decades later, the Cosmic Background Explorer (COBE), the Wilkinson Microwave Anisotropy Probe (WMAP), and, most recently, the Planck satellite would measure a similar power spectrum in the CMB. “The theoretical framework that he helped create made testable predictions,” says Priyamvada Natarajan, a Yale theoretical astrophysicist. “They still inform a lot of the observational tests of cosmology.”
Peebles also considered the connection between those fluctuations and the large-scale structure of the universe we observe today, as measured through galaxy clusters in sky surveys. “His idea that you can see the initial conditions and dynamics of the universe in the clustering of galaxies transformed what we could do as a community,” says New York University astrophysicist David W. Hogg.
Peebles’s view of the CMB and what it embodies proved especially important in the early 1980s, when cosmologists struggled to reconcile the deduced densities of matter in the infant universe with the large-scale structure that ultimately emerged. In a 1982 paper, Peebles proposed a solution in the form of nonrelativistic dark matter. Long after escaping the dense confines of the infant cosmos, that cold dark matter (CDM) would form the cocoons in which ordinary matter clumped into galaxies and then galaxy clusters. His paper built on the work of Vera Rubin, whose measurements with Kent Ford of the rotation curves of the Andromeda galaxy were critical toward demonstrating that dark matter must be the dominant component of galactic halos, to keep disks of stars and gas from flying apart. Subsequent satellite measurements have revealed that collectively dark matter has about five times the mass of ordinary matter.
By the 1990s it was becoming clear that a model containing just CDM, ordinary matter, and photons couldn’t account for all the observed properties of the universe, notably the value of the Hubble constant. The result is Lambda CDM, the cosmological model that describes the universe with six precisely measured parameters and accounts for the 1998 discovery that the universe’s expansion is accelerating. Peebles was one of the theorists to propose resurrecting Albert Einstein’s once-discarded cosmological constant to describe the newly discovered dark energy, which makes up more than two-thirds of the mass–energy content of the universe.
Ushering in the exoplanet era
To appreciate the contribution of Mayor and Queloz, consider that in 1995 the least massive known object outside the solar system was a star of 0.08 solar masses; Jupiter, for comparison, is about 0.001 M☉. Mayor was part of a team that in 1989 reported the probable detection of an object 11 times as massive as Jupiter that could be classified as either a very large planet or a brown dwarf. Pennsylvania State University astronomer Jason Wright says that other teams amassed preliminary evidence of extrasolar planets, but it was unconvincing and led planetary scientist William Cochran to declare, “Thou shalt not embarrass thyself and thy colleagues by claiming false planets.”
In 1992 Alexander Wolszczan and his colleagues discovered two planets orbiting the pulsar PSR B1257+12 via timing variations in the dead star’s radio beacon. (A third later found around the same pulsar remains the lowest-mass exoplanet yet discovered.) The discovery showed that exoplanets are out there, but the question remained of how common they are around stars like the Sun, where well-placed ones would presumably have the potential to support life.
At the Haute-Provence Observatory in southeastern France, Mayor and his graduate student Queloz conducted a survey of 142 stars using a spectrograph called ELODIE, which they designed to enable the observation of fainter stars than had previously been surveyed. The researchers’ approach, first proposed in 1952 by Otto Struve, was to detect the Doppler shift in the stellar spectrum due to the star’s motion as it is pushed and pulled by an orbiting planet. The expected stellar wobble due to a planet’s tug was on the order of 10 m/s; even now, the best spectrometers have a resolution of about 1000 m/s, Hogg says. Mayor and Queloz needed to be able to pinpoint a shift that accounted for a hundredth, or even a thousandth, of a pixel.
That’s exactly what they did through analysis of the signal from 51 Pegasi, a star located about 50 light-years away in the constellation Pegasus. The Doppler shift was consistent with the motion of a Jupiter-mass planet in a four-day orbit at 0.05 astronomical units, far shorter than the distance between Mercury and the Sun. The discovery of a “hot Jupiter” was surprising but also helpful, as the short period enabled Mayor and Queloz, and competing groups, to easily conduct follow-up observations. The astronomers announced their discovery at a conference in Italy almost exactly 24 years ago, on 6 October 1995, and soon published their result in Nature. Another group promptly confirmed the finding.
“It’s a discovery that has completely changed our view of who we are,” says Yale University astronomer Debra Fischer. “And it came at a time when we thought that maybe there weren’t many planets around other stars.”
However, the astronomy community wasn’t yet convinced by Mayor and Queloz’s claim. Many researchers didn’t think it was possible for such a massive planet to either form so close to the star or migrate inward without getting incinerated. Theorists proposed that the observed stellar wobbles might not be caused by an exoplanet at all, but rather by phenomena such as stellar brightness oscillations. But even the most skeptical came around in 1999, with discoveries of the first multi-exoplanet system by Fischer and colleagues, and of HD 209548 b. That planet was detected via the drop in brightness it caused when it passed in front of its star.
The early planet confirmations convinced observatory directors to build and install spectrographs. They also ultimately helped coax NASA to greenlight the development of a space telescope proposal that had been languishing for decades, a mission called Kepler. That satellite, which was launched in 2009, and instruments such as the Transiting Exoplanet Survey Satellite have detected thousands of planets and planet candidates.
Nearly a quarter century after Mayor and Queloz’s discovery, exoplanet science is a powerhouse endeavor that engages a significant percentage of the astrophysics community. Researchers join the field to study not only the planets but also the stars they orbit, which in turn has led to new insights in stellar astrophysics. By pairing transit measurements, which determine planets’ radii, with radial velocity, which provides masses, researchers have determined that many of the galaxy’s planets don’t resemble those in our solar system. The lack of resemblance challenges theories of planet formation and extends the range of planetary types that theories have to accommodate.
The most tantalizing goal of the field set in motion by Mayor and Queloz is to find planets that resemble Earth and to detect biosignatures. Researchers are already probing the atmospheres of individual worlds using the Hubble Space Telescope and other tools. Next-generation instruments, particularly the James Webb Space Telescope and the Wide Field Infrared Survey Telescope, will aid in that effort.
Editor’s note: This post was updated at 6:30pm EDT from an earlier summary.