Young stars are surrounded by gas and dust disks in which planets form. Direct images of such protoplanets are rare: so far only a few objects have been confidently seen, including PDS 70 b and c. AB Aur b, discovered in 2022 with data from the Subaru telescope, stands out because it lies about 100 au from the star and may still be growing by gathering gas from the disk. However, earlier observations with narrow filters could not distinguish the planet's own glow from scattered disk light. This work joins the ranks of studies of protoplanetary disks and accretion, but it is distinguished by its attempt to see the spectral signature of infalling material.
Observations were made with the VLT using the MUSE instrument in narrow-field mode, which provides both an image and a spectrum simultaneously. Four data blocks were obtained in October–November 2022; the best one (OB4) had a point-spread function width of about 0.1 arcseconds. To remove the bright starlight, two independent methods were applied: subtraction of a reference spectrum built from a ring around the star, and classical spectral differential imaging. The position and shape of the source were recovered using Markov chain Monte Carlo, fitting a Gaussian spot with six parameters.
AB Aur b is clearly seen in the channels at 6558.88 and 6560.13 Å, corresponding to a shift of about −100 km/s from the center of the Hα line. In the red wing of the line (6563.88–6565.13 Å), the source is seen in absorption with a significance of about 45σ. The resulting profile resembles neither the stellar spectrum nor the average disk spectrum: the star shows a single narrow peak, while AB Aur b shows broad blueshifted emission and redshifted absorption. The emission flux was 8.19±0.93×10⁻¹⁵ erg/s/cm², and the absorption flux was −13.59±0.90×10⁻¹⁵ erg/s/cm². The line shape resembles an inverted P Cygni profile, which in young stars is explained by cold gas falling onto the surface.
This work shows for the first time that the Hα spectrum of AB Aur b is not just scattered disk light. This sets it apart from earlier photometric studies, where the signal was attributed to scattering. The similarity to an inverted P Cygni profile points to possible gas infall, but local scattering features cannot be ruled out. Thus, AB Aur b becomes the second system after PDS 70 with a protoplanet studied spectroscopically in Hα, and the first with such a profile.
To understand what creates this profile, models of accretion onto the protoplanet and radiative transfer in the disk are needed. New observations with HST/STIS in coronagraphic mode will allow measuring the Balmer continuum and checking whether the signal varies with time. Polarimetry with SCExAO/VAMPIRES will help distinguish intrinsic emission from scattered light. If the profile is confirmed as a sign of accretion, this would be the first direct evidence of a planet growing by gas infall.
The result will affect models of giant planet formation and the interpretation of protoplanetary disk observations.
Conduct new spectroscopic observations of AB Aur b in Hα and Hβ, and build detailed models of accretion and scattering.
For protoplanetary astrophysics, the question remains open of how giant planets gain mass: through magnetospheric accretion, as in T Tauri stars, or through shock waves in the circumplanetary disk. The profile of AB Aur b does not fit simple models, highlighting the incompleteness of our understanding of accretion onto planets.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study presents the Hα spectrum of the protoplanet AB Aurigae b obtained with MUSE on the VLT. After subtracting the star using two methods and fitting the source, it is clearly detected in blueshifted emission and redshifted absorption. The line profile resembles an inverted P Cygni profile, which is interpreted as possible infall of cold gas.
Where its strength is
This is the second system with a protoplanet studied spectroscopically in Hα, and the first with such a profile, offering a unique opportunity to directly investigate accretion onto the planet.
Where it could go next
Worth trying
Why this may matter
Such observations open the way to directly studying planet growth in real time, which is critical for understanding the formation of gas giants.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.54, against a usual 0.64. It looks like few have drilled nearby.
Light from the star AB Aurigae takes about 12–13 hours to reach the planet's location: scattered radiation from the inner disk regions arrives with a delay, and this was taken into account in the data processing.
In Stanislaw Lem's novel "Solaris," the ocean of the planet displays structures that could be mistaken for intelligent ones; here, structures in the disk around AB Aurigae long obscured whether there was a planet or just a pattern of gas and dust.