TOI-3884b’s helium signal arrives hours before the planet crosses its star
A preprint reports helium absorption up to five hours before TOI-3884b begins its visible transit, suggesting gas extends ahead of this low-density planet. Observations covered five transits and showed no prominent trailing tail. A spherical atmospheric-escape model cannot reproduce the asymmetric structure; stellar activity also complicates the interpretation of the signal.
Science··Night
Helium appeared ahead of the visible transit
Helium absorption began up to five hours before TOI-3884b’s visible disc crossed its star in observations described by a 22 October preprint. The low-density planet orbits an M4 dwarf, a cool, small star. Researchers interpret the early signal as gas extending ahead of the planet. No prominent trailing tail appeared after transit. This asymmetry is the central feature of the proposed atmospheric outflow, whose estimated extent reaches roughly 23 planetary diameters.[1]
Five transits supplied the helium measurements
The observations used Habitable-zone Planet Finder, a spectrograph on the Hobby-Eberly Telescope that separates incoming light by wavelength. The team analysed 49 spectra during five transits and 64 outside transit, spanning several years. The helium absorption line near 10830 angstroms gave an excess transit depth of about 6.8%. These wavelength-specific measurements trace the extended atmosphere; the work remains a preprint rather than an established peer-reviewed result.[1]
Stellar activity complicates the escape model
A spherical, constant-temperature wind model estimates atmospheric loss under assumed temperature and irradiation conditions, but cannot reproduce the long leading structure. The planet crosses a large polar spot on its star, complicating the amplitude and variability of the transit signal. The authors judge that stellar contamination is insufficient for the full absorption preceding transit. They discuss irradiation-driven escape and magnetic guidance as possible explanations, with three-dimensional star–planet modelling still needed.[1]
