New Research By American Museum Of Natural History Scientists And Others Provides Insight Into Planet Formation

A new model shows how small boulders, the building-blocks of planets in the disk surrounding young stars, can overcome drag and collisions to form increasingly large bodies (bright dot) that end up as rocky, Earth-like planets.

Astronomers have had remarkable success recently in their search for planets outside our solar system. However, efforts to understand exactly how planets form in the first place have been stymied by a fundamental question: How can large boulders avoid being swept into the central star by the effects of gas surrounding the star or being pulverized by other objects before gravity can bind them into asteroid-size planetesimals too big for gas to influence or collisions to destroy? Full-fledged planets are believed to form from the collision and accretion of such smaller planetesimal bodies. 

Recent modeling developed by astrophysicists from the American Museum of Natural History in New York, the Max Planck Institute for Astronomy in Heidelberg, the University of Virginia, and the University of Toronto has shed new light on this fundamental part of the planet-forming process and shown that the very forces that appeared to prevent planetesimal formation“gas drag and turbulence“can actually promote it. A study led by Anders Johansen, an astrophysicist at the Max Planck Institute for Astronomy, describing the team's results appears in an upcoming issue of the journal Nature.

In the early stages of planet formation, dust grains in the diffuse cloud surrounding a young star collide and stick together to build up ever-larger bodies. However, earlier models of planet formation showed that the slower-rotating gas disk surrounding the central star appears to impart a drag on boulders larger than a few feet in diameter, causing them to slow and eventually spiral into the star after only a few hundred orbits. In addition, the fast-moving boulders do not stick together well but instead collide violently and break apart.

However, recent studies of boulders moving through gas have revealed two effects. First, turbulence in the gas causes them to clump in high-pressure regions; and second, gas drag causes further clumping as the boulders in the densest regions start pulling gas and nearby debris in with them. When the team included this behavior into simulations of gas and gravitationally interacting boulders, they found that the orbiting boulders concentrated so strongly that gravitational attraction between boulders caused them to collapse into large planetesimals that are not affected by gas drag.

"Our work provides the first scenario that appears capable of answering this gap in our basic understanding of planet formation, a process first identified over 30 years ago," said Mordecai-Mark Mac Low, Curator and Chair of the Department of Astrophysics at the American Museum of Natural History. "Of course, this work raises new questions about how planet formation occurs in protoplanetary disks. Still, we believe the fundamental properties of gas-boulder interactions that we describe will indeed provide the ultimate solution to this long-standing problem."

The research was supported in part by the U.S. National Science Foundation and the American Museum of Natural History.

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