Astronomers Directly Observe the Rotation of a Planet-Forming Disk for the First Time, Tracking AB Aurigae Over Four Years

Overview

Astronomers have directly observed the rotation of a protoplanetary disk for the first time, watching the cloud of dust and gas that surrounds the young star AB Aurigae turn over a four-year span. According to a press release from the CNRS, “the rotation of a protoplanetary disc (a disc where planets are being formed) has been observed directly for the very first time by mapping the emissions from the dust grains within it.” The work, led by scientists from the CNRS and the University of Bordeaux, was published in the journal Astronomy & Astrophysics, as reported by Phys.org.

What We Know

The observations were made with SPHERE, a near-infrared instrument on the European Southern Observatory’s Very Large Telescope (VLT) in Chile. According to Phys.org, “thanks to the unique near-infrared capabilities of the SPHERE instrument and its exceptional spatial resolution, the team was able to accurately track the disk’s structures and their evolution during three sets of observations, collected over a 4-year period.” By following how features in the disk shifted between those epochs, the team could measure the disk’s motion directly rather than infer it indirectly.

The European Southern Observatory released an image of the disk to accompany the result. In its ESO caption, the observatory writes that the picture “taken with the SPHERE instrument on ESO’s Very Large Telescope (VLT) shows a disc of material around the young star AB Aurigae, where planets might be forming,” and that “similar observations taken over the course of four years show the disc rotating around the star.” A processed version of the image highlights “spiral arms and radial shadows cast by dense clumps of material,” according to ESO.

AB Aurigae is a well-studied target for planet formation. According to Universe Today, the star is a pre-main-sequence variable star roughly “4 or 5 million years old.” The study was led by Anthony Boccaletti, described by the Observatoire de Paris as a “CNRS Research Director at Paris Observatory – PSL (LIRA).”

What the Rotation Revealed

The disk largely turns as the laws of physics predict, but not everywhere. According to the CNRS, “certain regions close to the star show an unexpected departure from this behaviour,” and “a body of evidence suggests that this anomaly is caused by the presence of giant planets in the process of formation.”

Beyond the broad rotation, the team resolved finer structure. The Observatoire de Paris reports the detection of “a bright structure, characteristic of accretion zones where gas and dust accumulate,” as well as “rapid rotation of faint shadows cast onto its surface by invisible structures, which could possibly be protoplanets” or opaque clumps of dust. These shadows, moving with the disk, are among the signatures the team links to bodies still taking shape inside the cloud.

The AB Aurigae system already hosts a candidate planet caught in the act of forming. According to Universe Today, earlier “Hubble and ground-based observations directly imaged a huge gas giant in the middle of development around AB Aurigae, called AB Aurigae b,” which “lies fairly far away from the star, at a distance of 93 astronomical units (AU) and appears to be somewhere around 9 Jupiter masses.” The same report notes additional suspected bodies: one that “may lie about 30 AU from the star” and could explain a twist in the disk, plus “two possible protoplanetary candidates” that “lie around 400 to 600 AU away” and appear as dense clumps in the outer disk, per Universe Today.

The peer-reviewed paper describing the result, “Destructuring the disk of AB Aurigae: Dynamics and accretion,” was published in Astronomy & Astrophysics with Boccaletti as first author.

What We Don’t Know

The disturbances in the disk’s rotation are interpreted as evidence of forming giant planets, but the press materials frame this as a body of evidence rather than a confirmed planet detection. The CNRS describes the anomaly as something a body of evidence “suggests” is caused by giant planets, and the Observatoire de Paris notes the shadow-casting structures “could possibly be protoplanets” or merely opaque dust clumps. How many planets are forming in the AB Aurigae disk, and their precise masses and orbits, remain open questions that future observations will need to settle.