This year’s Nobel Prize in physics went to an astrophysicist who came up with sweeping ideas to explain how matter in the young universe swirled into galaxies, and to two astronomers who showed that other stars similar to the sun also possess planets.

The Royal Swedish Academy of Sciences announced today that James Peebles, a professor emeritus at Princeton University, shared the prize — and half the prize money of more than $900,000 — for his theories that have helped explain 13.8 billion years of cosmological history.

The other half honored work by Michel Mayor and Didier Queloz, who were the first to discover a planet circling a distant sun-like star.

The two areas of research, while vastly different in scope and topic, “really, sort of tell us something very essential — existential — about our place in the universe,” Ulf Danielsson, a member of the committee that selected the winners, said during an online broadcast.

Peebles, the Albert Einstein professor of science at Princeton, was not entirely surprised by the early morning phone call from Stockholm.

“I have been working in cosmology for 55 years,” he said. “I’m the last man standing, so to speak, from those early days. It had crossed my mind.”

Mayor is an astrophysicist and professor emeritus of astronomy at the University of Geneva. He formally retired in 2007, according to the Planetary Society, but remains active at the Geneva Observatory.

Queloz is a professor of physics at the Cavendish Laboratory at Cambridge University, and at the University of Geneva. He said that at first he did not believe what he had discovered.

In the 1960s, when Peebles began studying the universe, there were few observations to prove or disprove anyone’s conjectures. Cosmological distances were often just rough guesses, and estimates of the age of the universe varied widely.

“There were bits of evidence,” he said. “But they were sparse and scanty and not nearly enough to convince me that we had the right general idea.”

Peebles’ work helped place cosmology on a more solid, mathematical foundation.

“No one has done more to establish our current paradigm than Jim,” Michael Turner of the University of Chicago and the Kavli Foundation, a philanthropy that supports science, wrote in an email.

In 1964, two radio astronomers, Arno Penzias and Robert Wilson, discovered by accident a background hiss of microwaves pervading the universe. Penzias and Wilson, who won the Nobel Prize in physics in 1978, were perplexed until they came across theoretical calculations by other scientists, including Peebles.

Peebles and his colleagues had predicted this background radiation, a residue from about 400,000 years after the Big Bang, when the universe had cooled enough for hydrogen and helium atoms to form.

The microwave background was almost uniform in all directions, reflecting a temperature of only a few degrees above absolute zero, but it was not perfectly smooth. Peebles calculated that there should be faint fluctuations, and that the fluctuations would reveal regions where matter had begun to clump together — the structure that would eventually be revealed as stars, galaxies and clusters of galaxies.

Like sound waves produced by a musical instrument, the fluctuations had characteristic wavelengths from vibrations bouncing back and forth.

In the early 1980s, Peebles proposed the idea that the universe was filled with unseen “cold dark matter” — particles that did not interact with ordinary matter but whose gravitational pull formed galaxies and clusters of galaxies. A couple of years later, he added to his model a term that Albert Einstein had originally proposed but later discarded as his “biggest blunder.”

Einstein had invented this idea, called the cosmological constant, to balance gravity and keep the universe static and unchanging. But astronomers established that the universe is actually expanding. Peebles used the cosmological constant, now known as dark energy, for a different reason: He aimed to show that the universe contained considerably less mass than was thought at the time.

The cold dark matter idea caught on, but many were skeptical, including Peebles, who also proposed other ideas.

“In the ’80s, I thought there was a pretty good chance we were not on the right track,” he said.

Around 1990, observations of the universe caught up to Peebles’ theories.

Scientists measured some of the fundamental fluctuations in the microwave background, and a NASA mission, the Cosmic Background Explorer, captured a wealth of confirming data.

In 1998, two teams of astronomers discovered that Peebles was right, and that the universe was not only expanding but accelerating. That research also garnered a physics Nobel, in 2011.

“Jim has been involved in almost all of the major developments since the discovery of the cosmic microwave background in 1965 and has been the leader of the field for all that time,” Turner said.

Peebles noted that much of the universe remains mysterious. Scientists have yet to identify what makes up dark matter or dark energy.

The other half of this year’s physics Nobel goes to research that filled in a missing piece of the observable universe.

Astronomers had long presumed there must be planets in orbits around other stars. But until a quarter-century ago, they knew of none. Over the decades, claims of planets evaporated on closer examination.

In 1992, astronomers found the first planets outside the solar system, but those orbited an exploded star, making them an unlikely place for life to exist.

At the time, some astronomers had begun to wonder if they would ever find planets. “Maybe most stars don’t form with planets and our solar system is unusual and life is incredibly rare,” said Debra A. Fischer, a professor of astronomy at Yale University.

Queloz was not expecting to find any planets, either.

“It was pretty clear I had no hope,” he said.

As a graduate student working with Mayor, he was setting up the equipment and writing the software to begin a search at the Haute-Provence Observatory in southern France.

If they found a planet, they would not see it directly. Rather, they were looking for a periodic wobble in the colors of light from the star. The gravity of a planet would pull on the star. The motion back and forth would shift the wavelengths of the starlight, much as a whistle of a train or the siren on a police car rises when approaching and falls when receding.

This technique was sensitive enough to detect something the size of Jupiter, but astronomers thought that planets the size of Jupiter would be as far from their stars as Jupiter is from Earth, and take years to pinpoint.

Thus, any discoveries would come long after Queloz finished his thesis. In 1994, Mayor went on sabbatical at the University of Hawaii.

“Off he went, and you can imagine my surprise when a couple of weeks later, I started the program and identified that star that had a completely erratic pattern,’” Queloz recalled.

He had started observing 20 bright stars in our galactic neighborhood. The erratic star was 51 Pegasi, similar to our sun and 51 light years away.

The planet in the data was as big as Jupiter, but it hugged the star in a tight orbit that took only four days to complete.

Queloz did not feel ecstatic, but rather ashamed, certain that something was wrong with the instrument or his software.

“I really panicked at that time,” Queloz said. “I didn’t talk to Michel at all.”

Almost six months later, Queloz was convinced his data was real, and he sent a fax to Mayor saying he thought he might have discovered a planet. “Michel had this very nice answer,” Queloz said. “He said, ‘Yes, maybe.’”

Years later, Mayor admitted that he did not believe the data. “He just wanted to be nice with me,” Queloz said. But when they made more observations, the same pattern continued.

On Oct. 6, 1995, they announced their discovery.

Although this broiling planet was not habitable, it pointed to how astronomers could now study planetary systems that could be similar to our own.

“Completely transformative,” Fischer said of the discovery. “We are the middle of a scientific revolution that people won’t appreciate until a hundred years go by.”

More than 4,000 exoplanets have been discovered in our Milky Way galaxy since Mayor and Queloz announced their results, including some that may be habitable. More and more are being spotted with space telescopes like TESS, launched by NASA last year.

And it turns out that large planets orbiting so close to their stars are not unusual.

“It’s really the science at its best,” Queloz said. “The data alone talking and telling you a story, which is different from the story that people have built up by studying the solar system.”

Who won the 2018 Nobel for physics?

The prize last year went to Arthur Ashkin of the United States, Gerard Mourou of France and Donna Strickland of Canada for their work with lasers and microscopy, developing tools such as optical tweezers and chirped pulse amplification.

Strickland was only the third woman to win the prize.

Who else has won a Nobel Prize this year?

The prize for medicine and physiology was awarded to William G. Kaelin Jr., Peter J. Ratcliffe and Gregg L. Semenza for their work in discovering how cells sense and adapt to oxygen availability.

When will the other Nobel Prizes be announced this year?

>> The Nobel Prize in chemistry will be announced Wednesday in Sweden.

>> The 2018 and 2019 Nobel Prizes in literature will be announced Thursday in Sweden. The prize last year was postponed after the husband of an academy member was accused, and ultimately convicted, of rape — a crisis that led to the departure of several board members and required the intervention of the King of Sweden.

>> The Nobel Peace Prize will be announced Friday in Norway.

>> The Nobel Memorial Prize in Economic Science will be announced Monday in Sweden.

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