Atmospheric escape has been detected from the exoplanet \object HD 209458b through transit observations of the hydrogen Lyman- \alpha line .
Here we present spectrally resolved Lyman- \alpha transit observations of the exoplanet \object 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 \pm 3.6 % between velocities of -230 to -140 km s ^ { -1 } .
Contrary to HD 209458b , these high velocities can not 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 ^ { -1 } , a stellar wind with a velocity of 190 km s ^ { -1 } and a temperature of \sim 10 ^ { 5 } K .

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 .