Cascades of gas around young star indicate early stages of planet formation

An artist's conception of the disk of gas and dust rotating around the young star HD 163296. Gas can be seen cascading into gaps in the disk--likely indicating the formation of baby planets in these locations. Credit: Robin Dienel, Carnegie Institution for Science.

What does a gestating baby planet look like? New research in Nature by a team including Carnegie's Jaehan Bae investigated the effects of three planets in the process of forming around a young star, revealing the source of their atmospheres. 

In their youth, stars are surrounded by a rotating disk of gas and dust from which planets are born. Studying the behaviour of the material that makes up these disks can reveal new details about planet formation, and about the evolution of a planetary system as a whole. 

The disk around a young star called HD 163296 is known to include several rings and gaps. Using 3-D visualizations taken by the Atacama Large Millimeter/submillimeter Array, or ALMA--a radio telescope made up of 66 antennas--Bae teamed with University of Michigan's Richard Teague and Ted Bergin to determine the velocities of some of the gas spinning in this disk. 

"We were struck by how dynamic the disk is," Bae said. "There's a lot going on around this star." 

They found three areas on either side of which the gas appears to be cascading into gaps in the disk, a good indication that planets could be forming in these locations. They were spotted at 87, 140, and 237 astronomical units, or AUs, from the star, with an AU being the distance between the Earth and our Sun. 

They tested these findings by creating a computational model of the stellar system and inserting three planets--one half Jupiter's mass, one equivalent to Jupiter, and one twice Jupiter's mass--at the same distances from HD 163296 as the gas disturbances found by ALMA. Their simulation indicated that the observed cascades of disk gas could be well explained by the existence of the three planets. 

Last year, Teague, Bae, and Bergin were part of a team that used one-dimensional measurements of the velocity of gas in the same disk to demonstrate a new technique for finding young planets. This latest paper takes that tool to the next level, enabling even deeper understanding of the planet-formation process. 

"This gives us a much more complete picture of planet formation than we ever dreamed," said Bergin. 

Their efforts also confirmed a long-standing theory about how planets acquire their atmospheres. 

"Planets form in the middle layer of the disk, the so-called midplane. This is a cold place, shielded from radiation from the star," explained lead author Teague. "We think that the gaps caused by planets bring in warmer gas from the more chemically active outer layers of the disk, and that this gas will form the atmosphere of the planet." 

The next step is to determine the chemical composition of the gas added to planets' atmospheres during this formative period. 

"Looking ahead, analysing the movement of material in a disk around a young star could help us find exoplanets while they are still in their most-formative stages," Bae concluded. "This could really help us understand how the architecture of a planetary system comes to be and maybe even unlock mysteries about the evolution of our own Solar System." 

Source: Carnegie Institution for Science,