Giant planet formation taken into action
Planetary scientists strive to understand the ins and outs of planet formation. Discoveries of unusual planetary systems do not make this daunting task any easier. In particular, Jupiter-class gas giants in distant orbits challenged what is known as the standard formation scenario.
Scientists have converged on the scenario that the giant planets of our solar system formed by accretion into the gaseous protoplanetary disk. Rocky planetary cores fed on pebbles or planetesimals, and once the cores reached a certain mass, they began to gobble up the surrounding gas, quickly becoming giant planets. But this process only works when planets form relatively close to their host stars – gas giants found in wide orbits wouldn’t have had time to develop a sufficiently massive core before the gaseous disk dissipated.
The unstable disk model is one of many alternative models suggesting that a massive, gravitationally unstable protoplanetary disk could break up into dense clumps, directly giving rise to wide-orbiting planets. However, the model is still waiting for convincing evidence.
Now a team of scientists has made a discovery that just might be the proof the community has been waiting for. The team imaged a massive protoplanet orbiting the star AB Aurigae about 93 times the distance between Earth and the Sun. Scientists captured the planet at an early stage of its formation, still embedded in a protoplanetary disk. The properties of the planet and the disk match well with the predictions of the unstable disk model. The study was published in Natural astronomy.
The AB Aurigae system takes shape
The young AB Aurigae system has long been on astronomers’ radars. The central star is more than twice as massive as the Sun and is about 520 light-years away. Its massive protoplanetary disk, mapped over a wide range of wavelengths from optical to radio, is incredibly complex. Two gaseous spiral arms spiral away from the star in a region encircled by a pebble-sized ring of dust. Something must have cleaned the dust inside the ring, and the spirals indicate an unstable disc; both features are hard evidence of baby planets lurking in the disc.
These considerations were on Thayne Currie’s mind when he first pointed the Subaru telescope at the system. “For some reason we couldn’t observe our primary target, so I had to make a snap decision, and I said, ‘AB Aurigae!’,” said Currie, an astrophysicist at the Ames Research Center. of NASA and lead author of the study.
Currie and his colleagues observed near-infrared planet-forming disks with a stellar coronagraph. The specialized instrument is designed to block light from a host star and thus allow observation of the much fainter disk.
Early images of the AB Aurigae system contained a bright spot to the south of the star. Subsequent observations with other instruments and reanalysis of archival Hubble Space Telescope data confirmed that the drop was genuine. After performing several checks, such as observing the object’s position changing over time, in accordance with counter-clockwise orbital motion, the team realized that they were looking at a protoplanet ( named AB Aur b) of approximately 9 masses of Jupiter integrated in the disc.
“The conclusion that the source is a planet seems solid,” said Ilse Cleeves, an assistant professor at the University of Virginia who was not involved in the study. “It’s probably the youngest planet directly imaged in a disk.” Nascent planets like this are hard to find, but not for lack of trying. Research so far has unearthed two more protoplanets, both orbiting the same star. But unlike AB Aur b, they have already purged most of the material from their disc.
The fate of the unknown protoplanet
“In the field of planet formation, we are not short of ideas, but we are short of real constraints on ideas,” said Sean Raymond, an astronomer at the Laboratoire d’Astrophysique de Bordeaux in France who did not participate in the research. “Having real, concrete observations of what is happening is essential.” In this sense, AB Aur b is a real trap. It provides direct evidence that Jupiter-like planets can form at great distances from a star. Also, with the spiral features, discovery supports the unstable disk model.
Other plausible models for the formation of wide-orbiting gas giants invoke the migration of planets. According to Raymond, a planet can indeed form closer and then see its orbit widen, but the models have not yet convinced astronomers that such a scenario is robust.
Interestingly, the fate of AB Aur b is unclear. “We don’t necessarily know if he’ll survive long term,” Cleeves said. Depending on the properties of the disc, the protoplanet could migrate inwards, which could tear it apart. But given that the protoplanet has already started tidying up the inner region, “its fate might actually be OK,” Cleeves said.
—Jure Japelj (@JureJapelj), science writer