Building planets, piece by piece

Some 4.5 billion years ago, the solar system was a nursery full of planetary toddlers.

Around the young sun swirled a disk of gas and dust left over from the solar system’s birth. Studded within the orbiting disk were planetesimals, rocky objects roughly 1 to 100 kilometers across, and bigger protoplanets some 1,000 kilometers wide. It was as if a bunch of children of different sizes were corralled into the same room.

Like any nursery, it was a rowdy place. Planetesimals whizzed past, and occasionally smashed into, one another. Fragments of dust and rock raced through the disaster zone. It took a few million years for the preschool chaos to settle down and the bulk of today’s planets to emerge.

Scientists used to think that planets formed as planetesimals collided and merged, like fistfuls of Play-Doh slapped together. But it turns out that that process would have taken too long. So astronomers recently proposed a new way to explain how baby planets grew.

Computer simulations show that small pebbles within the dusty disk would have glommed onto the growing protoplanets. Those tiny pebbles coalesced so rapidly that the protoplanets grew quickly into full-fledged planets — like a kid suddenly packing on enough pounds to become an adult.

This theory, known as pebble accretion, is reshaping how scientists think about the early solar system. It also opens new research directions, such as exploring how planets form around stars other than the sun. “You can form these bodies easily and quickly,” says Michiel Lambrechts, an astronomer at Lund University in Sweden and coauthor of the theory. “Pebble accretion provides a solution to so many problems.”

Chief among those problems is how planets could form before the dusty disk ran out of the raw ingredients needed to make them. Models suggest that the disk would have cleared itself out in about 1 million to 10 million years as its gas evaporated and its dust spiraled into the gravitational pull of the newborn sun. The biggest planets, such as Jupiter and Saturn, somehow packed together a core of about 10 Earth masses before the disk vanished. Making planets from planetesimals would have simply taken too long, since planetesimals typically whiz past a baby planet without being captured by its gravity.

Pebbles, on the other hand, are easily captured by a protoplanet’s gravity, and their accumulation could build a planet in just a million years or so. Astronomers know that such pebbles exist, because they have seen them orbiting infant stars. Radio telescopes such as the Very Large Array near Socorro, New Mexico, have measured the size of particles in protoplanetary disksby the way the material glows in radio wavelengths. The disks often contain huge numbers of pebbles — in some cases, the equivalent of hundreds of Earth masses slowly drifting toward the star.

The pebbles form when smaller dust particles collide and coalesce. “Most of the dust in the disk becomes pebbles,” says Anders Johansen, an astronomer at Lund University and the other coauthor of the theory. He calls the protoplanetary disks “pebble factories.”

Around 2010, Johansen and Lambrechts began to wonder how all these pebbles might relate to planets being born. They began a series of calculations on how pebbles might interact with other large rubbly fragments floating in the protoplanetary disk. To their surprise, Johansen and Lambrechts found that pebbles could quickly glom onto a protoplanet.

The key is friction. Imagine a stream of pebbles whizzing by a 100-kilometer-wide protoplanet. As pebbles plow through gas within the disk, friction slows them down enough to be captured by the protoplanet’s gravitational field. The pebbles begin looping around the larger rock and soon smash onto its surface. Each collision adds a tiny bit of mass — and with enough such collisions, the protoplanet grows quickly, to 1,000 kilometers across or more. “In many ways, pebble accretion is the most efficient way of adding mass to a body,” says Lambrechts.

If a protoplanetary disk included equal masses of rock, half in planetesimals and half in pebbles, the pebbles would accrete 1,000 times more effectively than the planetesimals, says Lambrechts. He and Johansen reported their initial ideasin a series of papers starting in 2012 and published an overview last year in the Annual Review of Earth and Planetary Sciences.

Pebble accretion helps explain a number of features about the solar system. For one, NASA’s Juno spacecraft, now orbiting Jupiter, found that the gas giant has a much larger and more diffuse core than scientists had expected. That could mean pebble accretion was at work, Johansen says — it is the only way to build up a core that large in the time available, before the dusty disk dissipates.

Pebble accretion also illuminates the long-standing mystery of how Uranus and Neptune formed. Puzzlingly, these ice-giant planets started off with large cores but did not swaddle themselves in huge amounts of gas the way Jupiter and Saturn did. Baby Jupiter and baby Saturn eventually reached a point called the pebble isolation mass, where they were large enough to generate a pressure bump in the surrounding gas that pushed away any approaching pebbles. Once they stopped gobbling pebbles, Jupiter and Saturn began pulling in gas instead. In contrast, Uranus and Neptune never made it to the pebble isolation mass, which increases with orbital distance. They became ice giants rather than gas giants.

Beyond the solar system, pebble accretion explains mysteries such as how big planets can form at large distances from their stars. For instance, the young star HR 8799, about 129 light-years from Earth in the constellation Pegasus, has four planets bigger than Jupiter with orbits ranging out to 68 times the Earth-to-sun distance (for comparison, Jupiter orbits at about five times the Earth-sun distance). Johansen and Lambrechts’s computer simulations suggest that the planets could have started even farther out and swept up pebbles, growing bigger as they spiraled in toward their current orbits. The whole process could have taken place during the lifetime of the protoplanetary disk. That couldn’t happen in the old scenario, because there wouldn’t have been enough planetesimals at those great distances to accrete efficiently.

One big question remains unanswered: Where did the protoplanets come from in the first place? One possibility is the place known as the snow line — the distance from a star at which liquid water freezes. There, dust and pebbles experience a shift in their physical properties as they change from being wet to dry. They begin to clump together and, unlike in other parts of the star-encircling disk, can form the large chunks that serve as planetary seeds for other pebbles to accrete on.

That makes the snow line a very good place for the first protoplanets to be born, says Joanna Drazkowska, an astrophysicist at the University of Zurich who published the idea recently in Astronomy & Astrophysics. Once those initial protoplanets were established, they could have begun gobbling up pebbles in the disk.

This scenario could have happened with one of the most famous planetary systems out there: the seven Earth-size planets orbiting the star TRAPPIST-1, 39 light-years from Earth in the constellation Aquarius. Chris Ormel, an astronomer at the University of Amsterdam, and his colleagues recently calculated that protoplanets began to form at the snow line around the star, then grew quickly by accreting pebbles. The fledgling planets halted at about the size of Earth because of the way their gravity influenced the surrounding disk.“This particular system is hard to explain by classical concepts,” Ormel says — but pebble accretion can do the trick.

As astronomers discover more and more planets around other stars, pebble accretion can help them understand how the menagerie of planet types found could have evolved, says Lambrechts.“It makes all of planetary formation much more dynamic."