Watching a Planetary Nursery Spin: The AB Aurigae Disk in Motion

A planet is born not with a bang, but a twist. For years, astronomers have studied the swirling disk of gas and dust around the young star AB Aurigae hoping to catch planet formation in the act. Now, for the first time, they have directly tracked the very motion of that disk—watching dust grains carve spirals and shadows as they spin, and even spotting where something massive is pulling the disk out of its expected rotation.

The first direct view of a rotating protoplanetary disk

Using the SPHERE instrument on the European Southern Observatory's Very Large Telescope (VLT) in Chile, a team of researchers captured near-infrared polarized images of the AB Aurigae disk at three epochs spanning 3.85 years . By comparing these images, they mapped the movement of dust structures across the disk and observed, for the first time, the bulk rotation of a protoplanetary disk .

Most of the disk rotates in accordance with Keplerian motion—the same physical law that governs the orbits of planets around a star. However, the study published in Astronomy & Astrophysics reveals that certain regions close to the star show an unexpected departure from this behavior . The anomaly is best explained by the gravitational influence of giant planets still embedded in the disk, their presence disturbing the orderly flow of material .

The SPHERE observations also included a single-epoch Hα image taken with the ZIMPOL subsystem to study accretion-related emission . This provides a crucial dynamical context for the known candidate protoplanet AB Aurigae b, whose location and influence on the disk structure are now better constrained.

How the VLT images mapped dust motion and disk dynamics

The new study, titled "Destructuring the disk of AB Aurigae: Dynamics and accretion" and submitted in May 2026, analyzed how individual features within the disk moved over nearly four years . The spiral arms, ring structures, and radial shadows previously imaged by instruments like ALMA and SPHERE were now tracked in motion.

The processed images reveal intricate structures :

• Spiral arms that sweep through the disk, likely driven by a massive embedded companion.
• Radial shadows cast by dense clumps of material orbiting the star.

The dynamical study confirms that while the disk globally follows expected rotational patterns, local perturbations—especially in the inner regions—match predictions from disk-dynamics models where a giant planet carves a path through the material and launches density waves .

Evidence for planet formation: from static spirals to accretion signatures

The 2026 rotation measurement represents the latest chapter in a growing body of evidence that AB Aurigae is a planetary system in the midst of formation.

2020: The spiral twist

In 2020, astronomers using the same SPHERE instrument delivered the deepest scattered-light images of AB Aurigae ever obtained . They identified a prominent "twist" in one of the disk's inner spiral arms at a separation of about 30 astronomical units—roughly the distance of Neptune from our Sun . This twist, perfectly reproduced in planet-driven density wave models, was interpreted as the site where a planet may be forming .

Earlier ALMA observations had already detected gaseous spiral arms inside the disk cavity connected to dusty spirals, and both the ALMA and SPHERE data pointed to a massive embedded companion as the driver .

2025: Hydrogen emission from AB Aurigae b

In September 2025, an international team using the MUSE spectrograph on the VLT detected hydrogen-alpha (Hα) emission directly associated with the candidate protoplanet AB Aurigae b . The emission was blue-shifted by about −100 km/s from the Hα line center, with a corresponding redshifted absorption at about 75 km/s .

The detected spectrum resembles an inverse P Cygni profile—a classic signature of gas falling inward onto an accreting body, commonly seen in young accreting stars . The spectrum was inconsistent with the host star's light, strongly supporting the interpretation that AB Aurigae b is a genuine protoplanet actively gathering material from a circumplanetary disk .

Observations with the VLT captured hydrogen emission from hot gas spiraling into the planet, marking what some researchers describe as the first direct evidence of mass falling onto a protoplanet .

Constraints from Paβ imaging

A separate deep imaging campaign targeting the Paβ (Paschen-beta) line at the expected location of AB Aurigae b did not detect significant emission . This non-detection does not rule out the protoplanet interpretation but suggests that any accretion in Paβ was either weak or stochastic during the observation window .

A Rosetta Stone for planet formation studies

The AB Aurigae system, located roughly 520 light-years away in the constellation of Auriga, has become one of the best-documented laboratories for testing theories of giant planet formation .

Researchers debate whether the emerging planets form through core accretion—a slow process that can take millions of years—or through gravitational instability, which can occur within the first few thousand years of a disk's lifetime . The system's young age of 1–4 million years places strict time constraints on these formation histories .

The combination of multi-wavelength imaging, spectroscopic detections of accreting gas, and now the direct tracking of disk rotation provides an evidence chain that spans morphology, kinematics, and accretion signatures.

Summary of key milestones:

• SPHERE/ALMA disk imaging (2020): Spiral arms and a telltale twist resolved in the inner disk, interpreted as signs of interaction with a massive embedded companion .
• VLT/MUSE spectroscopy (2025): Hα emission and absorption linked to AB Aurigae b, consistent with gas accretion onto a protoplanet .
• SPHERE multi-epoch dynamics (2026): First direct observation of disk rotation across 3.85 years, with dust-motion analysis and Hα imaging constraining accretion zones and protoplanet-disk interactions .

For the first time, astronomers have moved beyond static snapshots to a dynamic picture of a planetary nursery—watching dust grains orbit, spiral arms shift, and a young giant planet tug at the fabric of its birth disk.