Physics Nobel rewards discoveries on cosmic evolution and exoplanets

Three scientists will share the 2019 Nobel Prize in physics for their role in two major cosmic discoveries. That prize includes a medal for each and a lump sum of 9 million Swedish kronor (just over $900,000) to be split among them.

Half of the prize goes to James Peebles at Princeton University in New Jersey. He discovered new mathematical tools to study the universe. His research has included studies of the cosmic microwave background, or CMB. As that name suggests, this is light (at microwave wavelengths). It was emitted early in the history of our universe. It can now be found throughout the heavens, hence the term “background.” Peebles’ work eventually helped to reveal two mysterious cosmic features — dark matter and dark energy.

The second half of the prize goes to Michel Mayor at the University of Geneva in Switzerland and Didier Queloz of the University of Geneva and the University of Cambridge in England. Together, they made the first discovery of a planet orbiting a star other than our sun. It would be just one of many thousands of such exoplanets found orbiting stars throughout the Milky Way. The work by Mayor and Queloz helped reshape an understanding of our cosmic neighborhood.

Both discoveries revealed fundamental aspects of the universe that cannot be seen with the human eye.

Half of the prize goes for . . .

Peebles’ work helped establish that only 5 percent of the contents of the universe is the ordinary matter that makes up planets and people. The rest is a mix of dark matter (about 27 percent) and dark energy (about 68 percent). That dark matter scarcely touches ordinary matter except through gravity. Dark energy is a form of energy that makes the universe expand ever faster.

The discovery of the CMB won a Nobel Prize in 1978. Dark energy’s discovery led to a Nobel Prize in 2011.

At a news conference today announcing his sharing the 2019 Nobel Prize, Peebles said, “Theory in any of the natural sciences is empty without observation.” Showing that our universe is evolving, he argued, “was meaningless without the evidence that it [expanded] from a hot dense state.”

Nobel committee member and physicist Ulf Danielsson used a cup of coffee as a metaphor for the early universe. He likened ordinary matter to the bit of sugar sprinkled into the swirling liquid. That dark liquid, he said, was the dark matter and dark energy. And that small sprinkle of sugar, representing normal matter, he said, “is what science has been all about for thousands of years — up until now.”

Cosmology is the study of the universe. And through Peebles’ work, Danielsson said, “cosmology evolved into a science of precision.”

Physicists lauded Peebles after hearing of his win. “Jim is among the fathers of physical cosmology,” said physicist David Gross. He’s president of the American Physical Society. Peebles “laid the foundation,” he says, “for the now remarkably successful standard theory of the structure and history of the universe.”

Peebles “has his fingerprints all over” that standard theory, agrees cosmologist Michael Turner. He works in Illinois at the University of Chicago. He says Peebles has been involved “in every major development in cosmology over the past 50 years.”

Cosmologist Jo Dunkley, who works with Peebles at Princeton, sums up the reaction of cosmologists at their university: “Yes, of course, he got the Nobel Prize. He made this field.”

In a news conference held later in the day at Princeton, Peebles seemed overwhelmed by the appreciation of his colleagues. “Now I know how rock stars feel,” he quipped.  Plenty of questions remain in cosmology, he noted. These include the identity of dark matter and dark energy. And, he added, “We can be very sure that as we discover new aspects of the expanding and evolving universe we will be startled and amazed once again.”

After the Big Bang, 13.8 billion years ago, the infant universe was a nearly uniform slurry of energy and matter. And the density of its matter varied only a little bit from place to place. Peebles’ work explains how the universe transformed over eons. Due to the pull of gravity, in time that cosmos became filled with complex structures, such as galaxies.

Peebles “was one of the key people who developed the entire framework of structure formation,” says Priyamvada Natarajan. She works at Yale University, in New Haven, Conn. Dark matter particles still await detection. Despite that, Peebles showed that “dark matter was in the driver’s seat” — key to forming the structures of the cosmos visible today.

Those structures span a broad range of size scales. At the upper end are clusters of galaxies, which can contain thousands of galaxies within. And each of those galaxies may host billions of stars or more. At the smaller end are stars and their planets. Those planets included the one discovered by Mayor and Queloz. Honoring both these discoveries, Natarajan says, is “a celebration of human understanding of the largest scales and the smallest scales. Both are frontiers.”

On that smaller scale — exoplanets

Like dark matter and dark energy, Mayor and Queloz’s 1995 discovery also was not visible to the human eye. This Jupiter-mass exoplanet orbited a sunlike star known as 51 Pegasi. The scientists detected it by watching the way the planet’s gravity tugged on the star. That tug made the star wobble back and forth slightly. In the process, the star’s light shifted from slightly bluer to slightly redder as the star moved slightly toward and then away from Earth.

The planet they found, 51 Pegasi b, was unlike anything that exists in our solar system. It lies closer to its star than Mercury does to the sun. Scientists thought it was impossible for giant planets to form so close to their stars. Until, that is, they found this one.

Astronomers now think giant planets probably form far from their stars, then later migrate inward to become hot Jupiters. The idea that planets’ orbits can shuffle positions around their stars has since been used to explain some mysteries in our own solar system.

“It’s so charming for me to think … that we had a pretty good theory of how planets formed before the first extrasolar planet was discovered,” Peebles said during the news conference. “That’s a good illustration of the nature of science, isn’t it?”

Since the discovery of 51 Pegasi b, more than 4,000 exoplanets have been found orbiting distant stars. Astronomers can now study individual planetary systems and planet populations as a whole. It helps them understand how alien worlds form and evolve. Scientists also are planning how to search for signs of life in exoplanets’ atmospheres.

“There’s a reason [51 Pegasi b] was found first — it’s the easiest type of planet to find,” says David Charbonneau. He’s an exoplanet expert at Harvard University in Cambridge, Mass. After all, big, close-orbiting planets will influence their stars the most.

Since 51 Pegasi b, astronomers have been finding smaller planets and cooler planets — ones more like Earth, Charbonneau says. In fact, he points out, “There’s an enormous amount of enthusiasm in the field.” Many feel that “with the right telescopes, we really could … find out whether or not there’s life on other planets.”

Charbonneau says “it’s about time” for exoplanet science to be recognized with a Nobel. “The community has really agreed that the discovery of 51 Peg was the discovery that really ignited the field,” he says.

Other exoplanet scientists were more surprised. Sara Seager is an exoplanet pioneer at the Massachusetts Institute of Technology, also in Cambridge. She did not expect her field to win the top honor. “I was so floored,” she says.

The prize is a huge boost for exoplanet science, she maintains. Some still see it as “a frivolous, almost stamp-collecting endeavor,” she says. But she sees it as much more. In just 25 years, “we went from being an obscure and laughable fringe to mainstream science that’s Nobel-worthy.”