Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b

TL;DR

This study presents spectrally resolved Lyman-alpha transit observations of exoplanet HD 189733b, revealing significant temporal variations in its evaporating atmosphere and suggesting interactions with stellar wind and energy input changes.

Contribution

First spectrally resolved observations showing temporal variability in the atmospheric escape of HD 189733b and evidence of stellar wind interactions affecting atmospheric conditions.

Findings

Detected atmospheric hydrogen only in the second epoch

Observed high-velocity hydrogen absorption requiring stellar wind interactions

Linked atmospheric variations to stellar activity such as X-ray flares

Abstract

Atmospheric escape has been detected from the exoplanet HD 209458b through transit observations of the hydrogen Lyman-alpha line. Here we present spectrally resolved Lyman-alpha transit observations of the exoplanet HD 189733b at two different epochs. These HST/STIS observations show for the first time, that there are significant temporal variations in the physical conditions of an evaporating planetary atmosphere. While atmospheric hydrogen is not detected in the first epoch observations, it is observed at the second epoch, producing a transit absorption depth of 14.4+/-3.6% between velocities of -230 to -140 km/s. Contrary to HD 209458b, these high velocities cannot arise from radiation pressure alone and require an additional acceleration mechanism, such as interactions with stellar wind protons. The observed absorption can be explained by an atmospheric escape rate of neutral hydrogen atoms of about 10^9 g/s, a stellar wind with a velocity of 190 km/s and a temperature of ~10^5K. An X-ray flare from the active star seen with Swift/XRT 8 hours before the second-epoch observation supports the idea that the observed changes within the upper atmosphere of the planet can be caused by variations in the stellar wind properties, or by variations in the stellar energy input to the planetary escaping gas (or a mix of the two effects). These observations provide the first indication of interaction between the exoplanet's atmosphere and stellar variations.