V1298 Tau b shows persistent helium absorption despite stellar activity

The young planet V1298 Tau b showed helium absorption in observations of four transits spread across four years. The signal persisted even as activity on its host star changed, but its strength also varied. The findings point to possible atmospheric escape, while showing why separating a planet’s signal from its active star remains difficult.

Star system: V1298 Tauri A | Distance from the Sun: 352 light years

V1298 Tau b showed absorption from helium in its extended atmosphere during all four observed transits, spread across four years. This persistent signal may come from the planet and could point to atmospheric escape, when gas flows away from a planet into space. However, changes in the absorption’s strength mean the star’s activity also matters, so the signal’s planetary origin is not yet certain.

The observations combined high-resolution light measurements in visible and near-infrared wavelengths. Researchers compared helium absorption with other signs of activity in the star’s atmosphere, including changes in hydrogen and sodium. Those indicators shifted between emission, when they add light at particular wavelengths, and absorption, when they remove it. The team also re-estimated the projected angle between the star’s rotation and the planet’s orbit by measuring how the star’s rotating surface affects its light as the planet crosses in front.

Simulations showed that changes in the star’s X-ray and ultraviolet output can alter the depth and overall strength of the helium signal. Variations in the stellar wind, a stream of charged particles from the star, may also change the shape of the planet’s outer atmosphere from one night to another. Because V1298 Tau is highly active, continued monitoring of stellar activity and observations covering entire transits are needed to establish a reliable baseline and confirm the source of the helium absorption.

Key findings

Helium absorption appeared in all four observed transits across a four-year period.
Hydrogen and sodium activity indicators varied between emission and absorption, unlike the persistent helium signal.
Simulations showed that changing stellar X-ray and ultraviolet output can alter the helium absorption’s strength.
Stellar wind changes may drive night-to-night variations in the planet’s extended atmosphere.
Full transit coverage and ongoing activity monitoring are needed to confirm whether the helium signal comes from the planet.

Scientific study

The GAPS Programme at TNG LXXXI. The puzzle of the HeI absorption signal in the atmosphere of V1298 Tau b with GIARPS@TNG (external link)