Star gas reveals clues about planet formation

Gas flows in a star system 330 light-years away, scientists say, may reveal how giant, Jupiter-type planets acquire their atmospheres.

Planets of all sizes have long been known to form in rotating discs of dust and gas circling infant stars. In our own solar system, this happened 4.5 billion years ago, but other solar systems are much younger.

“There are many such young discs in our galaxy,” says Jaehan Bae, a computational astrophysicist at The Carnegie Institute for Science in Washington DC.

Astronomy has progressed to the point, he adds, that “we now are able to look at them with sufficient resolution that we can actually see what they are made of and what they look like.”

In a study in the journal Nature, Bae and colleagues turned the world’s largest radio telescope, the Atacama Large Millimeter/submillimeter Array (ALMA), in Chile, on a young star known as HD 163296 in the Sagittarius constellation.

Previous research had shown this star’s protoplanetary disc to be composed of bright and dark rings, indicative of baby planets in the process of sweeping their orbits clear of dust.

The new research, however, turned to the disc’s gas, whose presence ALMA could detect from its microwave emissions. 

In addition to mapping the location of this gas in HD 163296’s protoplanetary disc, ALMA was able to use tiny variations in the frequency of these emissions to determine the motion of this gas, much as variations in the pitch of a train whistle can indicate whether the train is approaching or receding.

What they found, Bae says, were signs of gas falling inward from the outer edges of the disc “like a waterfall” toward each of the locations where planets were thought to be forming. 

“This gives us a much more complete picture of planet formation than we ever dreamed,” says Bae’s coauthor Edwin Bergin, of the University of Michigan.

Richard Teague, also of the University of Michigan, adds that planets form in the middle layers of their protoplanetary discs, far from the disc’s upper or lower edges. 

“This is a cold place, shielded from radiation from the star,” he says. “We think the gaps caused by planets bring in warmer gas from the outer layers of the disc, and that this gas will form the atmosphere of the [gas giant] planet.”

Computer modeling, Bae and colleagues report, indicates that the three planets range between half and twice the mass of Jupiter and are located at distances of 87, 140, and 237 astronomical units (AUs) from the star. For context, an AU is the distance between the Earth and our Sun.

By comparison, our own solar system’s gas giant planets, Jupiter and Saturn, are 5 AU and 9 AU out from the sun, respectively. “So these planets are far, far out,” Bae says.

An unresolved question, he says, is whether gas giants like Jupiter and Saturn form near their present orbits or instead, like HD 163296’s planets, formed much farther out, then migrated inward.

“All that we can infer from how they obtain their atmospheres,” Bae says, but resolving the question will take more time.

The present study, he adds, focused on a single gas, carbon monoxide, which is the most abundant one in protoplanetary discs that can be detected from microwave emissions.

Tuning the system to detect less-abundant gases such as methane and methanol, he says, may eventually reveal additional information.

“Are [giant planets] going to stay [where they form] or move inward or outward as they evolve over time?” he asks. “All [of that] we can infer from how they obtain their atmospheres.”

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