In 2022, the Subaru telescope photographed a point of light 100 astronomical units from the star AB Aurigae. The point was named planet AB Aur b, but debates did not subside: what exactly is glowing, the planet itself or dust around it? Now the MUSE spectrograph on the Very Large Telescope has dispersed this light by wavelength, and in the hydrogen Hα line the signature of infalling gas has emerged.

The light from a young planet is like a translator who retells the star's story with an accent. The star itself shows a single narrow peak in the Hα line. AB Aur b shows broad emission shifted by −100 km/s to the blue and absorption at +75 km/s to the red. This profile is called an inverted P Cygni. In young stars, it arises when cold gas falls onto the surface: the approaching material gives a blueshift, while absorption is created by gas that has already fallen and blocks the radiation.

Previously, the signal from AB Aur b was explained by scattered light from the disk. The spectrum showed otherwise. Comparison with the star, with the average disk spectrum, and with the known protoplanets PDS 70 b and c left no doubt: AB Aur b has its own line shape. It does not fit simple accretion models. The planet may be growing by gathering gas through magnetospheric accretion, like a T Tauri star. Or through shock waves in the circumplanetary disk. For now, this is pure knowledge without practical applications, but it tests theories of giant planet formation.

Further observations are needed. HST/STIS in coronagraphic mode will measure the Balmer continuum and show whether the signal changes over time. Polarimetry with SCExAO/VAMPIRES will distinguish intrinsic emission from scattered light. If the profile is confirmed as a sign of accretion, this will become the first direct evidence of a planet growing by gas infall. For now, AB Aur b remains the second system after PDS 70 with a protoplanet studied spectroscopically in Hα, and the first with such a profile.