There Might Be More Than One Way to Make a Planet

When it comes to making a planet, astronomers have long subscribed to what Cassandra Hall, an astronomer at the University of Georgia’s Center for Simulational Physics, refers to as the “bottom up” approach: The gas and dust swirling around a young star slowly clumps together over millions of years, and its gravity shapes it into a rounded object.

But a discovery by Dr. Hall and her colleagues, published in the journal Nature this month, suggests that the picture might be more complex.

In a star system 508 light-years from Earth, the researchers found conditions that support an alternative “top down” approach to planet formation, in which the fertile material circling a young star rapidly collapses into a planet. The mechanism, known as gravitational instability, could explain the existence of mysterious, massive worlds known to follow wide orbits around relatively young stars.

“There’s never been real, hard evidence of it happening before,” Dr. Hall wrote in an email. “We found it!”

The cosmic matter stirring around an infant star is ripe with planet-forming potential. The matter is known as a protoplanetary disk, and its rotation is generally driven by the gravity of its host star. But if that disk gets large enough, it can be influenced by its own gravity, causing the young star system to become unstable. Regions of higher density in the disk emerge in the form of spiral arms, similar to the shape of spinning clouds in a hurricane.

“The star would be like the eye of the storm,” said Jess Speedie, a graduate student at the University of Victoria in Canada who led the study under the supervision of Ruobing Dong, an astrophysicist.