Scientists have converged on the scenario that the giant planets in our solar system formed by accumulating inside the gaseous protoplanetary disk. Rocky planetary nuclei fed on pebbles or planets, and as soon as the nuclei reached a certain mass, they began to devour the surrounding gas, quickly turning them into giant planets. But this process only works when the planets form relatively close to their host stars – the gas giants in broad orbit would not have time to develop a fairly massive nucleus before the gas disk disintegrates. The unstable disk model is one of many alternative models that suggest that a huge and gravitationally unstable protoplanetary disk could be fragmented into dense clusters, giving birth directly to wide-orbiting planets. However, the model is still waiting for convincing data. Now, a team of scientists has made a discovery that could be just the proof the community has been waiting for. The team depicted a huge 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 formation, still embedded in a protoplanetary disk. The properties of both the planet and the disk fit well with the predictions of the unstable disk model. The study was published in Nature Astronomy.
The AB Aurigae system takes shape
The young AB Aurigae system has been on astronomers’ radar for a long time. The young AB Aurigae system has been on astronomers’ radar for a long time. The central star is twice as massive as the Sun and is about 520 light-years away. His huge protoplanetary disk, mapped over a wide range of wavelengths from optical to radio, is incredibly complex. Two gaseous spiral arms are wound and twisted away from the star in an area surrounded by a pebble-sized dust ring. Something had to clean the dust inside the ring and the coils show an unstable disk. Both features are evidence of a smoking weapon on baby planets hidden in the tray. These thoughts were in the back of Thayne Currie’s mind when he first turned the Subaru telescope toward the system. “For some reason we could not observe our primary goal, so I had to make a decision in a flash, and I said, ‘AB Aurigae!’” Said Currie, an astrophysicist at NASA’s Ames Research Center and lead author of the study. Currie and his colleagues observed planet-forming disks in near-infrared light with a stellar coroner. The specialized instrument is designed to block the light of a host star and thus allow the observation of the much fainter disk. The first images of the AB Aurigae system contained a drop of light south of the star. Subsequent observations with other instruments and a re-analysis of the Hubble Space Telescope archival data confirmed that the drop was genuine. After performing several tests – such as observing the position of an object that changes over time, according to the counterclockwise motion – the team realized that they were looking at a protoplanet (called AB Aur b) of about 9 Jupiter masses embedded in the disk. “The conclusion that the source is a planet seems strong,” said Ilse Cleeves, an assistant professor at the University of Virginia who did not participate in the study. “This is probably the youngest planet to be depicted directly on a disk.” Baby planets like this are hard to find, though not due to lack of effort. So far, research has resulted in two other protoplanets, both orbiting the same star. But unlike AB Aur b, they have already cleaned most of their disc material.
The fate of the unknown protoplanet
“In the field of planet formation, we are not hungry for ideas, but we are hungry for real limitations on ideas.” “In the field of planet formation, we are not hungry for ideas, we are hungry for real limitations on ideas,” said Sean Raymond, an astronomer at the Laboratoire d’Astrophysique de Bordeaux in France, who did not do the research. “Having real, concrete observations of what is happening is the key.” In that sense, AB Aur b is very good. It provides direct evidence that planets like Jupiter can form long distances from a star. In addition, along with the spiral features, the invention supports the unstable disk model. Other plausible models for the formation of gas giants in broad orbit invoke planetary migration. According to Raymond, a planet could indeed form closer and then expand its orbit, but models have not yet convinced astronomers that such a scenario is powerful. Interestingly, the fate of AB Aur b is not clear. “We do not necessarily know if it will survive in the long run,” Cleeves said. Depending on the properties of the disk, the protoplanet may migrate inward, which could disintegrate it. But since the protoplanet has already begun to settle inland, “his fate may be really okay,” Cleeves said. —Jure Japelj (@JureJapelj), Scientific writer Reference: Japelj, J. (2022), Giant planet’s formation catched in action, Eos, 103, Posted on April 11, 2022. Text © 2022. The authors. CC BY-NC-ND 3.0 Unless otherwise stated, images are subject to copyright. Any reuse without the express permission of the copyright holder is prohibited.
