AB Aurigae b is a young exoplanet that is still forming within a gas and dust disk around its star. The MUSE spectrograph, installed on the VLT, allowed scientists to study it in new detail.
Observing the birth of planets
Since the first discovery of planets outside the Solar System in 1995, more than 6,000 exoplanets have been identified. Many of these planets have properties that differ significantly from the eight planets in our Solar System. How are such diverse exoplanets formed and evolve, and which of them could become Earth-like planets capable of supporting life?
To answer these questions, it is necessary to observe young planets at the moment of their formation in their birthplaces. However, due to the complexity of such studies, direct observations of planets that are only a few million years old have been extremely limited. Small rocky planets like Earth, which can support life, and giant gas planets like Jupiter are born around stars similar to the Sun. Their birthplace is a thin, disc-shaped structure of gas and dust known as a protoplanetary disc. Protoplanetary disks are observed not only around stars similar to the Sun, but also around more massive or lighter young stars.
Direct observation of protoplanets
Since the 2010s, their detailed structures have been detected by 8-meter telescopes such as the Subaru Telescope (in visible and infrared light) and the ALMA Observatory (in radio waves). Although many planets have been discovered indirectly based on subtle structures in these disks — such as gaps or spiral arms — the direct capture of newly formed planets (protoplanets) in disks has so far only been achieved in a few cases, including PDS 70 b and c and AB Aurigae b (AB Aur b). It is believed that this is due to the fact that most protoplanets are embedded in the disk and become more visible only when gaps are cut in the disk or when they are observed directly from above.
Protoplanets are also considered to be those that actively collect material from the surrounding disk during their growth. However, detailed spectroscopic observations of this mass accumulation from the embedded disk have been limited to the PDS 70 system.
Hydrogen emission in the AB Aurigae b system
In a new study published in The Astrophysical Journal Letters, an international team of researchers led by the Astrobiology Center (Japan) and the University of Texas at San Antonio (USA) was able to detect hydrogen emission lines from AB Aurigae b using a multi-object. These radiation lines are considered evidence of mass accretion from the accretion disk around the planet onto the protoplanet.
Hydrogen emission is commonly observed around young stars and their protoplanetary disks. In this case, radiation emanates from material accreted onto a small disk surrounding the still-embedded protoplanet.
The detected Hα emission at the position of AB Aurigae b has an inverse P Cygni profile, similar to that observed in young stars undergoing massive accretion. To date, P Cygni profile is the only protoplanet with this type of emission. Its young age (~2 million years) and the large amount of material around it confirm that AB Aurigae b is still a protoplanet in formation.
Previously, only PDS 70 b and c showed hydrogen emission, but these planets are located in gaps in the disk; AB Aurigae b is still embedded in the disk, making this the first such observation with signs of infall.
AB Aurigae b has a mass approximately four times greater than Jupiter’s and orbits at a distance of 93 astronomical units from its star. There is no such distant giant planet in the Solar System. Standard models of planet formation cannot fully explain its formation so far from the star before migration occurs. This discovery supports the scenario that massive planets can form due to gravitational instability in the disk, providing insight into a type of giant planet that has not been observed in our Solar System.
According to phys.org
