Free Astronomy Magazine January-February 2020

33 JANUARY-FEBRUARY 2020 SPACE CHRONICLES The birthplaces of planets are disks made out of gas and dust. Astron- omers study these so-called proto- planetary disks to understand the processes of planet formation. Beau- tiful images of disks made with ALMA show distinct gaps and ring features in the dust, which may be caused by infant planets. To get more certainty that planets cause these gaps, and to get a com- plete view of planetary formation, scientists study the gas in the disks in addition to dust. Ninety-nine per- cent of a protoplanetary disk’s mass is gas, of which carbon monoxide (CO) is the brightest component, and ALMA can observe it. Last year, two teams of astronomers demonstrated a new planet-hunting technique using this gas. They meas- ured the velocity of CO gas rotating in the disk around the young star HD 163296. Localized disturbances in the movements of the gas re- vealed three planet-like patterns in the disk. In this new study, lead au- thor Richard Teague from the Uni- versity of Michigan and his team used new high-resolution ALMA data from the Disk Substructures at High Angular Resolution Project (DSHARP) to study the gas’s velocity in more detail. “With the high-fi- delity data from this program, we were able to measure the gas’s ve- locity in three directions instead of just one,” said Teague. “For the first time, we measured the motion of the gas in every possible direction. Rotating around, moving towards or away from the star, and up or down- wards in the disk.” Teague and his colleagues saw the gas moving from the upper layers towards the middle of the disk at three different locations. “What most likely happens is that a planet in orbit around the star pushes the gas and dust aside, opening a gap,” Teague explained. “The gas above the gap then collapses into it like a waterfall, causing a rotational flow of gas in the disk.” This is the best evidence to date that there are indeed planets forming around HD 163296. But astronomers cannot say with one hundred per- cent certainty that planets cause the gas flows. For example, the star’s S cientists measured the motion of gas (arrows) in a protoplanetary disk in three directions: rotating around the star, towards or away from the star, and up- or downwards in the disk. The inset shows a close-up of where a planet in orbit around the star pushes the gas and dust aside, opening a gap. [NRAO/AUI/NSF, B. Saxton]