Major Planet Formation Mystery Solved
An artist's conception of a stellar protoplanetary disk, where planet formation occurs.
Credit: NASA

Planet formation is a story with a well-known beginning and end, but how its middle plays out has been an enigma to scientists?until now.

A new computer-modeled theory shows how rocky boulders around infant stars team up to form planets without falling into stars.

"This has been a stumbling block for 30 years," said Mordecai-Marc Mac Low, an astrophysicist at the American Museum of Natural History in New York City, of planet formation theories. "The reason is that boulders tend to fall into the star in a celestial blink of an eye. Some mechanism had to be found to prevent them from being dragged into a star."

The solution: Together, many boulders can join to fight a cosmic headwind that otherwise would doom them.

Truckin' boulders

The stuff of rocky planets originates in an accretion disk, or collection of gas and dust that circles around a newborn star. Over time the dust particles bunch together and form large boulders, but eventually they meet "wind" resistance from the disk's mist of gas.

"They see a headwind. It's deadly and drags them into the star," Mac Low told

Modeling the turbulence within the gas, however, showed that boulders can team up and form planets.

"Turbulence in the disk concentrates boulders in regions of higher pressure," Mac Low said, noting that such a disturbance is enough to enable the boulders to fight the dooming headwind. "If the gas is sped up, the boulders don't see a headwind. By getting the gas going with them they conserve energy and stay in orbit."

Mac Low compared the effect to a chain of semi-trucks driving down a highway. Each boulder is like a semi-truck "pushing" the gas in front of it, creating a friendly pocket of air behind it that other semis can travel in without using up as much fuel. "The end of the story is that enough boulders gather together, gravity takes over and they collapse into planet-like bodies," Mac Low said.

Mac Low and his colleagues' findings will be detailed in an upcoming issue of the journal Nature.

Pulverizing problem

Although Mac Low and his colleagues kept planet-forming boulders safe from the gravitational clutches of stars in their simulation, he noted that many questions remain.

"There are enough uncertainties that [planet formation] is not going to be an open and shut case any time soon," he said. "We don't know how that collapse into a planet actually occurs. You've got thousands, millions of boulders swarming together like a bees. In my nightmares I imagine that they grind each other down to dust and it all goes away."

Despite the problem, Mac Low is confident the theory will hold up to future scrutiny.

"All that material is gravitationally bound together, so we think it's likely that it will form large objects," he said. Running the computer simulation, in fact, formed tight boulder clusters as large as the dwarf planet Ceres (formerly known as the asteroid Ceres).

Alan Boss, an astrophysicist with the Carnegie Institution in Washington, D.C., said that the theory is attractive despite the caveat.

"Overall, the calculations present an encouraging approach to understanding how something happened that we know must have happened, at least for the terrestrial planets," Boss said in an e-mail. How giant planets form yet another question. One idea is that gas coalesces around a rocky, or terrestrial planet. Boss, however, thinks the gas giants collapse from a knot, much in the manner of star formation.

Mac Low and his team plan to address the mystery of how boulders collapse into planetesimals, or protoplanetary chunks of rock, in the future.