Astronomers studying the young star AB Aurigae have detected gas inside its disk rotating measurably slower than standard orbital physics would predict at distances under 60 astronomical units from the star. Tracked over 3.85 years with the VLT/SPHERE instrument using near-infrared polarimetry, the sub-Keplerian motion at the disk’s inner edge points to a gravitational disturbance consistent with one or more giant planets forming within the cavity. The finding turns AB Aurigae into one of the clearest real-time laboratories for watching how massive worlds reshape the disks that birth them.
Why the inner-edge velocity gap demands new observations now
In a disk obeying simple Keplerian rotation, gas closer to the star moves faster in a smooth, predictable curve. The departure detected inside 60 au in AB Aurigae breaks that pattern. According to multi-epoch SPHERE observations spanning 3.85 years, gas at those radii is rotating slower than expected, a signature that something massive is pulling material off its natural orbit. Two competing explanations have emerged: a giant planet embedded in the cavity, or an inclined inner binary companion driving spiral arms inward.
Separating those two scenarios requires velocity maps sensitive enough to capture asymmetric twists in the gas flow. If an embedded giant planet is responsible, the torque it exerts should produce a non-axisymmetric twist signature that grows measurably over the next two years. New ALMA observations in 13CO could resolve that twist, because a binary-only model would generate a more symmetric kinematic pattern. The clock is ticking: AB Aurigae’s disk structures evolve on timescales short enough that repeat imaging can distinguish between these hypotheses within a single observing cycle.
Spirals, wiggles, and a candidate planet at wide separation
The velocity anomaly at the inner edge sits within a broader web of evidence accumulated across multiple instruments and wavelengths. ALMA observations have revealed inner gaseous spirals inside the dust cavity, with the dust itself concentrated in a ring at roughly 120 au. Those spirals trace tidal disturbances consistent with an unseen companion orbiting between 60 and 80 au from the star, a location that aligns with the radius where Keplerian rotation breaks down.
Earlier SPHERE and ALMA data working in tandem identified cavity structures and spirals whose apparent orbital motion suggested a gravitational source actively sculpting the disk. A peer-reviewed study published in Monthly Notices of the Royal Astronomical Society offered an alternative reading, arguing that an inclined and eccentric inner binary could reproduce the spiral geometry and perturbed kinematics. That work explicitly noted that the cavity kinematics appear strongly perturbed from what would be expected for an inclined Keplerian disk, leaving the door open for either explanation.
Deep ALMA observations in 13CO and C18O added another layer. Researchers reported wiggles in the velocity field they interpreted as a signature of gravitational instability, a process in which the disk itself becomes massive enough to fragment and form planets directly. Theoretical predictions had already established what such non-axisymmetric velocity signatures should look like if gravitational instability were operating, giving observers a template to match against the AB Aurigae data.
Hubble Space Telescope observations using the STIS instrument independently identified a candidate protoplanet, AB Aurigae b, at a wide orbital separation. According to NASA’s summary of the Hubble findings, the object’s mass scale and distance from the star are consistent with disk instability rather than the slower core accretion process that built most known exoplanets. A separate peer-reviewed synthesis in MNRAS linked the system’s spiral and cavity features to a gravitational instability planet formation scenario, compiling system parameters that fit the framework and suggesting that multiple formation channels may be operating simultaneously in the same disk.
Unresolved gaps in the AB Aurigae kinematic record
Several pieces of the puzzle are still missing. The full 3.85-year SPHERE velocity datacubes from the primary study have not been publicly released, limiting independent reanalysis. Direct stellar mass and distance values for AB Aurigae are cited through secondary references rather than raw catalog records, introducing a layer of uncertainty into the Keplerian baseline against which the sub-Keplerian deviation is measured. Without pinning down the star’s mass to high precision, the size of the velocity gap itself carries an error bar that could shift the interpretation.
No ALMA Band 7 or higher-resolution kinematic follow-up has yet been published that could cleanly separate gravitational instability wiggles from planet-driven spirals in the same region. Existing 13CO and C18O maps have enough sensitivity to flag broad non-axisymmetric features, but not to unambiguously trace the detailed twist pattern that a single massive planet would impose. That ambiguity leaves room for hybrid models in which a marginally unstable disk hosts both self-gravitating spirals and one or more embedded protoplanets.
Time coverage is another limitation. The SPHERE baseline of 3.85 years is long enough to detect changes in the scattered-light morphology of the disk, but short relative to orbital periods at tens of astronomical units. As a result, the same sequence of images can be fit by slowly rotating spiral arms driven by a distant companion, by a closer-in binary whose influence is projected outward, or by transient structures arising from gravitational instability. Without a longer time series, it remains difficult to distinguish between persistent, orbiting features and short-lived clumps that shear away.
Calibration systematics also matter. Small errors in the assumed disk inclination or position angle can masquerade as sub-Keplerian rotation in projected velocity maps. While the current analyses attempt to marginalize over those uncertainties, the lack of uniform, multi-wavelength constraints on the disk geometry means that subtle biases could remain. Future work that jointly fits continuum emission, scattered light, and molecular line kinematics with a single geometric model will be crucial for locking down the true rotation profile.
What the next observing cycle could reveal
The next few years of observations will be decisive for AB Aurigae. High-resolution ALMA campaigns targeting 13CO and C18O at the disk’s inner edge could directly test whether the velocity twist grows in a way consistent with a single embedded planet. If the twist pattern strengthens and tracks a coherent orbital motion, that would strongly favor the planet hypothesis over an inner binary alone. Conversely, if the velocity field remains dominated by broad, multi-armed wiggles, gravitational instability and binary-driven spirals would gain ground.
On the optical and near-infrared side, continued SPHERE or similar polarimetric imaging can map how the brightness and shape of the inner spirals evolve. A planet carving a gap should leave behind a relatively stable, trailing spiral pattern, while instability-driven structures are expected to appear and fade more chaotically. Combining those morphological changes with contemporaneous ALMA kinematics would provide a powerful cross-check on any single model.
Finally, deeper direct-imaging searches for additional protoplanets inside the cavity could clarify whether AB Aurigae is forming a compact system of giant planets or only one or two massive companions. Any such detections would immediately feed back into dynamical models of the disk, tightening constraints on the total mass budget and the relative importance of core accretion versus gravitational instability.
For now, the sub-Keplerian gas at AB Aurigae’s inner edge stands as a tantalizing signpost of ongoing planet formation. Whether the culprit is a hidden giant planet, an unseen stellar partner, a self-gravitating disk, or some combination of all three, the system offers a rare chance to watch the physics of world-building play out in real time. With carefully planned, multi-wavelength campaigns, astronomers are poised to turn this enigmatic velocity gap into a definitive test of how massive planets and their natal disks co-evolve.
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*This article was researched with the help of AI, with human editors creating the final content.
