In February of last year, astronomers studying a young star—HD 100546—located just 355 light-years from Earth announced they had captured an image that likely represented a planet undergoing formation. If this turned out to be the case, it would mark the first direct observation of planetary formation. Now, thanks to telescopes at the ESO and the Gemini Observatory, scientists have gathered new evidence that suggests they were correct. Furthermore, they’ve found a second candidate planet orbiting the star. The work has been published in The Astrophysical Journal.
Our current theories of how planets form are well-developed, but they are based on computer simulations rather than direct observations. It is thought that planets and stars are born when a cloud of dust and gas collapses under its own gravity. As the cloud gets compressed, a large proportion of it begins to rotate. This eventually flattens into a disk that gradually gets thinner as it spins. This “circumstellar” or “protoplanetary” disk is where new planets are born.
HD 100546, a star 2.5 times larger and 30 times brighter than the Sun, is surrounded by one of these circumstellar disks. Back in 2003, astronomers spotted a blob in this disk which they believed was a Jupiter-like planet in the very early stages of formation. Now, astronomers think they have spotted another gas giant around three times the size of Jupiter forming in the disk. The distance of this candidate from HD 100546 is around the same distance that Saturn is from our Sun.
The candidate was spotted using a combination of spectroscopy and astrometry (spectroastrometry) which enabled them to measure small changes in the position of carbon monoxide (CO) emission. They discovered a source of CO emission that appeared to vary in position and velocity, both of which were consistent with orbital motion around HD 100546.
The researchers suggest that this CO emission is coming from gas within a circumplanetary disk orbiting a forming gas giant. It has long been thought that these spinning disks surround giant planets as they form, but no one had ever spotted one before. It is also believed that they are the birthplaces of planetary moons.
Because two candidates have now been spotted around HD 100546, the researchers believe this points to multiple, or perhaps sequential, planet formation. The researchers plan to continue their observations using new high-contrast imagers on the ESO’s Very Large Telescope.