PLANETS

The Pebbled Path To Planets

Astronomers are turning to ‘dust’ and little rocks to figure out how planets form.

Artist and astronomer Robert Hurt at Caltech created this illustration of a possible newfound planet near the star CoKu Tau 4. It’s forming in a clearing in the star's dusty, planet-forming disc. The possible planet could be at least as big as Jupiter and may look like the giant planets in our solar system when they were young.

By Stephen Ornes

Every big planet begins with a pebble.

Okay, not just one. It starts with lots of pebbles — a flat sea of them stretching perhaps hundreds of times wider than the distance from Earth to the sun. Their sizes vary greatly. Some may be mere dust particles. Others may be small to fairly substantial rocks.

These pebbles tumble violently within the gassy disk encircling a young star. Lurking within that disk are the ingredients not only for planets but also for asteroids, comets, and living things. What they become depends not only on those ingredients but also on their location and the temperature of the gas. 

Like fussy chefs in a kitchen, astronomers today debate over how much of which ingredients must have been present in that early solar system. And when. And how they might have interacted and combined. And what would happen if you changed their temperature.

We all know how the planet-making process ends. It produces rocky worlds like Earth, Mars, and Venus. It also leads to gas giants like Saturn and ice giants like Uranus. Outside the solar system, the planetary zoo includes stranger worlds. Scientists have spotted a world that they first thought was made of a diamond but now believe has oceans flowing with lava. They’ve observed a hot gas giant where drops of iron probably fall like rain and a small hot planet enshrouded in steam.

An artist’s interpretation of a binary star system, with a surrounding ring (in brown) that might give rise to a rocky, Mars-sized planet.

But how disks of gas and rocks become planet factories is still under debate. The beginnings — where dust grains barely micrometers (a few ten-thousandths of an inch) across stick together to form rocky solids — isn’t too controversial. And thanks to powerful telescopes, researchers have ideas for how planets move once they’re fully formed.

But in-between is a doozy. For centuries, scientists have been testing and fighting over ideas about how to connect the beginning to the end. Most of the seemingly best ideas have run into problems.

Over the last 10 years or so, however, a process called pebble accretion (Ah-KREE-shun) has gained popularity. Accretion refers to something’s gradual growth. This occurs as new bits of material join something or glom onto it. In this case, it’s a swirling disk of gas and pebbles that clump together to form a family of planets.

According to the theory, tiny rocks in the disk slow and heat up as they fly through the gas near a larger rock. It’s a phenomenon similar to how water in a pond slows a sinking rock. These flying pebbles eventually spiral down to land on the surface of larger rocks nearby. Pebble by pebble, a giant planet is born. And compared to the age of the universe, it’s a fast process, only taking a few million years.

“Pebble accretion really did revolutionize the way that people thought about planet formation,” says Katherine Kretke. She’s an astrophysicist at the Southwest Research Institute in Boulder, Colo.

This theory would solve many of the riddles that challenged previous ideas. For example, says Seth Jacobson, “It is really the only mechanism that comes close to explaining how Uranus and Neptune formed.” Jacobson is an astrophysicist at Michigan State University in East Lansing.

Planet accretion also has gotten a boost from recent studies of distant stars. Observations by the largest radio telescope network in the world, perched on a lonely desert mountain in Chile, match up with some of this theory’s unusual predictions.

Anders Johansen, an astronomer at Lund Observatory, in Sweden, knows a lot about pebble accretion. He has been one of the leading researchers arguing in its favor. Puzzling out how it might work consumes his days.

He compares studying the origins of planets to working through a detective story. The solar system provides clues in the planets we know, he says. Exoplanets beyond the solar system provide more clues. Scientists have to connect those clues to piece together the whole story.

“It is just so much fun to work on this,” he says.

The planet-making process has produced an incredible variety of worlds, such as these seven exoplanets that orbit the star TRAPPIST-1.