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 GJ 436b, for which we use Hubble Space Telescope/Space Telescope Imaging Spectrograph Lyα 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 Lyα lightcurve with a transit depth of GJ 436b from 8.8% ± 4.5% near mid-transit, to 22.9% ± 3.9% ~2 hr 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% ± 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 GJ 436b in the range of 3.7 × 106-1.1 × 109 g s-1, corresponding to an atmospheric lifetime of 4 × 1011-2 × 1014 yr.
The Astrophysical Journal
- Pub Date:
- May 2014
- planets and satellites: atmospheres;
- planets and satellites: individual: GJ 436b;
- Astrophysics - Earth and Planetary Astrophysics