Lyman-$\alpha$ Transit Spectroscopy and the Neutral Hydrogen Tail of the Hot Neptune GJ436b

Abstract

To date, more than 750 planets have been discovered orbiting stars other than the Sun. Two sub-classes of these exoplanets, "hot Jupiters" and their less massive counterparts "hot Neptunes," provide a unique opportunity to study the extended atmospheres of planets outside of our solar system. We describe here the first far-ultraviolet transit study of a hot Neptune, specifically GJ436b, for which we use HST/STIS Lyman-$\alpha$ spectra to measure stellar flux as a function of time, observing variations due to absorption from the planetary atmosphere during transit. This analysis permits us to derive information about atmospheric extent, mass-loss rate from the planet, and interactions between the star and planet. We observe an evolution of the Lyman-$\alpha$ lightcurve with a transit depth of GJ436b from $8.8\pm 4.5\%$ near mid-transit, to $22.9\pm 3.9\%$ $\sim 2$ hours after the nominal geometric egress of the planet. Using data from the time-tag mode and considering astrophysical noise from stellar variability, we calculate a post-egress occultation of $23.7\pm 4.5\%$, demonstrating that the signature is statistically significant and of greater amplitude than can be attributed to stellar fluctuations alone. The extended egress absorption indicates the probable existence of a comet-like tail trailing the exoplanet. We calculate a mass-loss rate for GJ436b in the range of $3.7\times 10^6-1.1\times 10^9$ g s${}^{-1}$, corresponding to an atmospheric lifetime of $4\times 10^{11}-2\times 10^{14}$ years.

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• Atmospheric Escape from Hot Jupiters
• Improved parameters for extrasolar transiting planets
• Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b
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• Spitzer Transit and Secondary Eclipse Photometry of GJ 436b
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• Extrasolar Giant Planets and X-ray Activity
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• Transit infrared spectroscopy of the hot neptune around GJ 436 with the Hubble Space Telescope

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