When it comes to making a planet, astronomers have long subscribed to what Cassandra Hall, an astronomer at the University of Georgia, 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.
According to Ms. Speedie, if those arms pull in enough material, they can fragment into clumps, which could further collapse into giant gas planets. And this could happen in only a few hundred years, rather than the millions of years theorized for the “bottom up” approach known as core accretion.
“These two theories have battled it out in astronomy for a while,” Ms. Speedie said. “We have, for the first time, found definitive evidence that the gravitational instability pathway can happen.”
Gravitationally unstable disks might be the wombs of planets too enormous and distant from their host stars to be explained by core accretion. In 2022, astronomers reported the discovery of a protoplanet with nine times the mass of Jupiter around AB Aurigae, a star that is no more than four million years old. The object was forming 8.6 billion miles from the star — more than twice the distance between our sun and Pluto.
Similar planets have been found elsewhere. “It’s almost impossible to explain their formation through the ‘bottom up’ process,” Dr. Hall said. “There shouldn’t be enough solid material to form those objects out there through a core-accretion paradigm.”
In the 1980s, astronomers proposed the idea that gravitational instability could churn out these unconventional planets. But they lacked a way to prove it. In a simulation published in 2020, Dr. Hall and her colleagues showed that the velocity of material throughout a gravitationally unstable disk varies in a characteristic pattern.
Years later, her team pointed the Atacama Large Millimeter/submillimeter Array, a radio telescope in Chile, at the star AB Aurigae and found the exact signature that had been predicted by the simulation.
The new study is “quite plausible,” said Lee Hartmann, an astronomer at the University of Michigan who was not involved in the work. “But I think there are still some details that need to be tied up to make this absolutely convincing.”
And even if the results are sound, it proves only that gravitational instability is something that happens, not that it is making planets, Dr. Hartmann added. It could just be that the mechanism helps core accretion along.
Dr. Hall thinks it’s unlikely that it’s an either-or scenario.
“We actually expect basically all planet-forming environments to go through a gravitationally unstable stage,” she said, during which planets could be forming through both gravitational collapse and core accretion.
In the future, the astronomers who led the study plan to search for signs of gravitational instability in other young star systems. Eventually, they also hope to look for markers of the mechanism in fully formed worlds.
Ms. Speedie looks forward to learning more about the diversity of planets, the different ways they might form and what that might reveal about our own solar system.
“I think the universe is probably more creative than human minds are,” she said. “We’re interested in the possibilities that lie out there.”
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