What to make of a Jupiter-class planet that orbits its host star at a distance of 13.8 billion kilometers? This is well over twice the distance of Pluto from the Sun, out past the boundaries of what in our system is known as the Kuiper Belt. Moreover, this is a young world still in the process of formation. At nine Jupiter masses, it’s hard to explain through conventional modeling, which sees gas giants growing through core accretion, steadily adding mass through progressive accumulation of circumstellar materials.
Core accretion makes sense and seems to explain typical planet formation, with the primordial cloud around an infant star dense in dust grains that can accumulate into larger and larger objects, eventually growing into planetesimals and emerging as worlds. But the new planet – AB Aurigae b – shouldn’t be there if core accretion were the only way to produce a planet. At these distances from the star, core accretion would take far longer than the age of the system to produce this result.
Enter disk instability, which we’ve examined many a time in these pages over the years. Here the mechanism works from the top down, with clumps of gas and dust forming quickly through what Alan Boss (Carnegie Institution for Science), who has championed the concept, sees as wave activity generated by the gravity of the disk gas. Waves something like the spiral arms in galaxies like our own can lead to the formation of massive clumps whose internal dust grains settle into the core of a protoplanet.
Data from ground- and space-based instruments have homed in on AB Aurigae b, with Hubble’s Space Telescope Imaging Spectrograph and Near Infrared Camera and Multi-Object Spectrograph complemented by observations from the planet imager called SCExAO on Japan’s 8.2-meter Subaru Telescope at Mauna Kea (Hawaii). The fact that the growing system around AB Aurigae presents itself more or less face-on as viewed from Earth makes the distinction between disk and planet that much clearer.
Image: Researchers were able to directly image newly forming exoplanet AB Aurigae b over a 13-year span using Hubble’s Space Telescope Imaging Spectrograph (STIS) and its Near Infrared Camera and Multi-Object Spectrograph (NICMOS). In the top right, Hubble’s NICMOS image captured in 2007 shows AB Aurigae b in a due south position compared to its host star, which is covered by the instrument’s coronagraph. The image captured in 2021 by STIS shows the protoplanet has moved in a counterclockwise motion over time. Credit: Science: NASA, ESA, Thayne Currie (Subaru Telescope, Eureka Scientific Inc.); Image Processing: Thayne Currie (Subaru Telescope, Eureka Scientific Inc.), Alyssa Pagan (STScI).
We benefit from the sheer amount of data Hubble has accumulated when working with a planetary orbit on a world this far from its star. A time span of a single year would hardly be enough to detect the motion at the distance of AB Aurigae, over 500 light years from Earth. The paper on this work – Thayne Currie (Subaru Telescope and Eureka Scientific) is lead researcher – pulls together observations of the system at multiple wavelengths to give disk instability a boost. The authors note the significance of the result, comparing it with PDS 70, a young system with two growing exoplanets, one of whom, PDS 70b, was the first confirmed exoplanet to be directly imaged:
…this discovery has profound consequences for our understanding of how planets form. AB Aur b provides a key direct look at protoplanets in the embedded stage. Thus, it probes an earlier stage of planet formation than the PDS 70 system. AB Aur’s protoplanetary disk shows multiple spiral arms, and AB Aur b appears as a spatially resolved clump located in proximity to these arms. These features bear an uncanny resemblance to models of jovian planet formation by disk instability. AB Aur b may then provide the first direct evidence that jovian planets can form by disk instability. An observational anchor like the AB Aur system significantly informs the formulation of new disk instability models diagnosing the temperature, density and observability of protoplanets formed under varying conditions.
I do want to bring up an additional paper of likely relevance. In 2019, Michael Kuffmeier (Zentrum für Astronomie der Universität Heidelberg) and team looked at a variety of systems in terms of late encounter events that can disrupt a debris disk that is still forming. AB Aurigae is one of the systems studied, as noted in their paper:
Our results show how star-cloudlet encounters can replenish the mass reservoir around an already formed star. Furthermore, the results demonstrate that arc structures observed for AB Aurigae or HD 100546 are a likely consequence of such late encounter events. We find that large second-generation disks can form via encounter events of a star with denser gas condensations in the ISM millions of years after stellar birth as long as the parental Giant Molecular Cloud has not fully dispersed. The majority of mass in these second-generation disks is located at large radii, which is consistent with observations of transitional disks.
Just what effect such late encounter events might have on what may well be disk instability at work will be the subject of future studies, but if we’re using AB Aurigae as a likely model of the process at work, we will need to untangle such effects.
The paper is Currie et al., “Images of embedded Jovian planet formation at a wide separation around AB Aurigae,” Nature Astronomy 04 April 2022 (abstract / preprint). The Kuffmeier paper is “Late encounter-events as a source of disks and spiral structures,” Astronomy & Astrophysics Vol. 633 A3 (19 December 2019). Abstract.