Keck Data Show Extended Hα/Hβ Tails on WASP-12b Consistent with Roche-Lobe Overflow

The team used the Keck Planet Finder on Keck I to obtain time-series spectra centered on the WASP-12b transit. They measured excess absorption in the stellar rest frame rather than the planetary frame, finding signals that persist outside the optical transit window. This geometry matches hydrodynamic models in which gas overflows the Roche lobe, is advected into the stellar wind, and is sheared into extended tails.

Previous Hα detections on WASP-12b and similar planets orbiting early-type stars already hinted at high mass-loss rates tied to Roche-filling factors. The new dual-line (Hα and Hβ) measurement strengthens the case that the outflow is optically thick and not confined to the planet’s orbit. It also supplies a direct empirical constraint on stellar-wind interaction that earlier single-epoch studies lacked.

The result connects to a small but growing sample of three other planets with full-orbit tail detections, suggesting a common outflow regime for hot Jupiters around F-type hosts. Long-term, such tails can torque the orbit and accelerate decay, an effect that current radial-velocity surveys may soon test.

Next steps require multi-epoch, multi-wavelength coverage to separate stellar-wind variability from the planetary outflow and to search for metals that would confirm the mass-loss rate and composition.