Glowing gas found around a baby planet
IN 2014, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile captured the first detailed image of the process of planet formation. It revealed a disk of material around the young star HL Tauri that was punctuated by an intricate series of gaps where baby planets are growing and gobbling up material.
Eight years on, with catalogs full of such objects, astronomers are trying to do one better: find the smaller and even fainter disks of material swirling around forming planets themselves. Until recently, only two such circumplanetary disks (CPDs) had been spotted and confirmed, given away by their hot, glowing dust.
Now, astronomers using ALMA have found what may be a third
CPD — and for the first time, they think they have detected not its dust, but the much fainter emission from its gas instead. While warm dust glows across the electromagnetic spectrum, excited gas emits light only at specific wavelengths. The team identified light from carbon monoxide gas within an otherwise empty gap in the disk around the star AS 209, roughly 395 light-years away in the constellation Ophiuchus. The work was published July 27 in
The Astrophysical Journal Letters.
The disk’s sheer size and distance from its host star made the gas around this incubating planet visible, Jaehan Bae, an astronomer at the University of Florida and the study’s lead author, tells Astronomy.
Previously detected CPDs were only about 1 astronomical unit (AU) in diameter, or the average EarthSun distance. The CPD detected in AS 209 is perhaps as large as 14 AU across and orbits 200 AU from its host star — over five times the average distance of Pluto from the Sun.
Such direct detections of gas are important and exciting, says Bae, because the vast majority of the material that forms stars and planets is gas — the gas-to-dust ratio is on the order of 100 to 1. “This means that the overall dynamics of CPDs must be determined by the gas, not dust,” says Bae. “We need to study the gas to fully understand CPDs,” including how they are able to form moons.