Science Results

The Subaru Telescope's powerful, dedicated exoplanet imaging instrument has discovered evidence for a Jupiter-like protoplanet in the process of forming. This ground-breaking work provides the first ever look at the earliest stages of the formation of gas giant planets, when they are still embedded in the disk of gas and dust surrounding a young star, and provides evidence for a long-debated alternative theory for how Jupiter-like planets form.

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Figure 1: Image of the star AB Aurigae taken by the Subaru Telescope showing the spiral arms in the disk and the newly-discovered protoplanet AB Aur b. The bright central star has been masked, and its location is indicated by the star mark (☆). The size of Neptune’s orbit in the Solar System is shown to provide scale. The Image without caption is here (0.7 MB). (Credit: T. Currie/Subaru Telescope)

While there are eight planets in our Solar System, about 5,000 planets around other stars (exoplanets) have been discovered since the first one was discovered in 1995. How are exoplanets born, how do they evolve, and how can some of them become life-supporting planets like the Earth or gas giants like Jupiter? To solve these mysteries, it is essential to capture protoplanets, planets in the act of being born at the very site and time they form.

Planets are born in disks of gas and dust surrounding young stars, i.e. protoplanetary disks. Subaru Telescope observations from a decade ago and recent ALMA observations have revealed numerous protoplanetary disks whose structures – e.g. gaps and spiral arms – provide indirect evidence for planet formation. Until now, however, astronomers have had only one clear example of directly-imaged newborn planets: two infant Jovian planets around the low-mass star PDS 70. However, these planets are located within a cleared region of PDS 70's protoplanetary disk, thus they probe the final stages of planet assembly.

Today, an international research team led by Subaru Telescope, the University of Tokyo, and the Astrobiology Center of Japan reported the world's first direct detection of an embedded protoplanet, using the Subaru Telescope's extreme adaptive optics system (SCExAO, Note 1) coupled with its infrared spectrograph (CHARIS) and its visible light camera (VAMPIRES), as well as the Hubble Space Telescope. The protoplanet, AB Aur b, was imaged within the protoplanetary disk of AB Aurigae, a two million year-old star in the constellation Aurigae.

Generally, it is difficult to distinguish a protoplanet buried in a disk from a small structure of the disk itself. However, SCExAO/CHARIS's special polarization mode showed that AB Aur b does not have a strong polarization signal as would be expected if it was a disk structure unrelated to a planet. In addition, visible light imaging with SCExAO/VAMPIRES could indicate ongoing gas accretion onto AB Aur b.

"The Subaru Telescope's extreme adaptive optics pulled AB Aur b's image from the bright, structured disk surrounding the star, allowing our infrared and visible instruments to then confirm its nature," notes Olivier Guyon, the Principal Investigator of SCExAO.

"One of SCExAO's strengths is that it continues to evolve over the years to meet the needs of our observers. We were able to confirm this discovery thanks to recent upgrades to our instrument," mentions Julien Lozi, Sr. SCExAO Scientist.

"AB Aur b sheds new light on our understanding of the different ways that planets form," says Thayne Currie, lead author of the discovery paper.

AB Aur b is about nine times the mass of Jupiter and orbits 13.9 billion kilometers away from its star, over three times the distance between the Sun and Neptune, the Solar System's outermost planet. The formation of such a distant, massive protoplanet cannot be explained by the so-called standard model of planet formation (core accretion model).

In the core accretion model, a large Jupiter-like gas planet starts as a rocky core in a protoplanetary disk. This core then accretes gas from the disk, growing into a giant planet. Rocky cores aren’t expected to form far away from the central star, so this model can’t drive distant planet formation. One theory suggests that after formation, these planets may move closer to or further away from their host star, or scatter. However, the discovery of AB Aur b indicates that giant protoplanets can form far from their stars before such planetary migration occurs. Instead of core accretion, formation of AB Aur b may be explained by disk instability: a process in which a massive, gaseous protoplanetary disk cools and breaks into one or more collapsing planet-mass fragments.

"The Subaru Telescope imaged outer spirals in the disk around AB Aur in 2004 and details of its inner disk in 2011. But the planet itself had evaded detection in both observations. Finally in 2022, Subaru Telescope successfully imaged the long-sought, real 'protoplanet' embedded in the disk," says Motohide Tamura, a professor at the University of Tokyo.

The 8.2-meter Subaru Telescope is located near the summit of Maunakea in Hawai`i, an inactive volcano known for its unsurpassed qualities as an astronomy site and its deep personal and cultural significance to many Native Hawaiians.

"Maunakea is the best place on the planet Earth to see other worlds. We are extremely grateful for the privilege of being able to study the heavens from this mountain," says Currie.


These results appeared as Currie et al. "Images of embedded Jovian planet formation at a wide separation around AB Aurigae" in Nature Astronomy on April 4, 2022.


(Note 1) With ground-based telescopes, the images of celestial objects appear out of focus and shaky, as if looking out from underwater, due to the effects of the Earth's atmosphere. Extreme adaptive optics correct the turbulence caused by the Earth's atmosphere in real time to an extreme extent, letting the Subaru Telescope produce extremely sharp images.