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This paper presents simulations of the metastable helium (He*) observationsof WASP-107b, so far the highest signal-to-noise ratio detection that isconfirmed by three different instruments. We employ full 3D hydrodynamicscoupled with co-evolving non-equilibrium thermochemistry and ray-tracingradiation, predicting mass loss rates, temperature profiles, and synthetic He*line profiles and light curves from first principles. We found that a stellarwind stronger than solar is demanded by the observed heavily blueshifted lineprofile and asymmetric transit light curve. Contrary to previous beliefs, weargue that radiation pressure can be important for Ly$\alpha$ observations but{\it not} He*. We found WASP-107b is losing mass at a rate of $\dot{M} \simeq1.0\times 10^{-9}~M_\oplus~{\rm yr}^{-1}$. Although $\dot{M}$ varies by $<1~\%$given constant wind and irradiation from the host, shear instabilities stillemerge from wind impacts, producing $\sim 10~\%$ fluctuations of He* transitdepths over hour-long timescales. The common assumption that He* transit depthindicates the fluctuation of $\dot{M}$ is problematic. The trailing tail ismore susceptible than planet adjacency to the shear instabilities, thus theline profile is more variable in the blue-shifted wing, while the transit lightcurve is more variable after mid-transit. We stress the synergy betweenLy$\alpha$ (higher altitudes, lower density) and He* (lower altitudes, higherdensity) transit observations, particularly simultaneous ones, yield betterunderstanding of planetary outflows and stellar wind properties.

astrophysical journal/the astrophysical journal

Metastable Helium Absorptions with 3D Hydrodynamics and Self-Consistent  Photochemistry II: WASP-107b, Stellar Wind, Radiation Pressure, and Shear  Instability