Researchers have gained some incredible insights into how planets form around stars. Using the impressive capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA), they have observed the motion of the gas in a protoplanetary disk in 3D for the first time.
Planets form from disks of dust and gas. ALMA has spotted many primordial disks, some of which have clear grooves that are likely formed by the protoplanet (a large, orbiting body of matter with the potential to become a planet) gaining mass. Such is the case of HD 163296, where just last year researchers spotted hints of three large planets.
In the new study, published in Nature, the team describes the motion of the gas. They noticed that the disk sports enormous gas “waterfalls”, vertical flows of material at the edges of the grooves.
"What most likely happens is that a planet in orbit around the star pushes the gas and dust aside, opening a gap," lead author Richard Teague, from the University of Michigan, said in a statement. "The gas above the gap then collapses into it like a waterfall, causing a rotational flow of gas in the disk."
If the three planets exist they are located 87, 140, and 237 times further away from their stars than the Earth is from the Sun. They are also arranged in order of size, with the closest being half the mass of Jupiter, the furthest being twice as massive, and the middle one being around Jupiter’s size. While the planet hypothesis is the most compelling, the researchers are quick to point out that there might be other explanations for the 3D motion of the gas. Strong magnetic fields might create similar features, for example.
"Right now, only a direct observation of the planets could rule out the other options. But the patterns of these gas flows are unique and it is very likely that they can only be caused by planets," explained co-author Jaehan Bae of the Carnegie Institution for Science.
According to theories of planet formation, this falling gas will become the future atmosphere of the planet located in the midplane of the disk. The gas flows were observed as part of a very specific project, the Disk Substructures at High Angular Resolution Project (DSHARP), and the data it produced helped the team to study the gas' velocity in the disk in more detail.
"The disk around HD 163296 is the brightest and biggest disk we can see with ALMA," said Teague. "But it was a big surprise to actually see these gas flows so clearly. For the first time, we measured the motion of the gas rotating around the star, towards or away from the star, and up- or downwards in the disk."
HD 163296 is located 400 light-years away.