Hubble spots planet shedding atmosphere following solar flare outburstAstronomers the Hubble Space Telescope have seen dramatic changes in the upper atmosphere of a faraway planet.Justafter a violent flare on its parent star bathed it in intense X-rayradiation, the planet's atmosphere gave off a powerful burst ofevaporation.The observations give a tantalising glimpse of the changing climates and weather on planets outside our Solar System.AstronomerAlain Lecavelier des Etangs and his team used Hubble to observe theatmosphere of exoplanet HD 189733b during two periods in early 2010 andlate 2011, as it was silhouetted against its parent star.Whilebacklit in this way, the planet's atmosphere imprints its chemicalsignature on the starlight, allowing astronomers to decode what ishappening on scales that are too tiny to image directly.The observations were carried out inorder to confirm what the team had previously seen once before in adifferent planetary system: the evaporation of an exoplanet'satmosphere.HD 189733b has a blue sky, but that's where the similarities with Earth stop.Theplanet is a huge gas giant similar to Jupiter, but it lies extremelyclose to its star, just one thirtieth the distance Earth is from theSun.Even though its star is slightlysmaller and cooler than the Sun, this makes the planet's climateexceptionally hot, at above 1000 degrees Celsius, and the upperatmosphere is battered by energetic extreme-ultraviolet and X-rayradiation.As such, it is an excellent candidate to study the effects of a star on a planetary atmosphere.'Thefirst set of observations were actually disappointing,' Lecaveliersaid, 'since they showed no trace of the planet's atmosphere at all. Weonly realised we had chanced upon something more interesting when thesecond set of observations came in.'Theteam's follow-up observations, made in 2011, showed a dramatic change,with clear signs of a plume of gas being blown from the planet at a rateof at least 1000 tonnes per second.Lecavelieradded: 'We hadn't just confirmed that some planets' atmospheresevaporate - we had watched the physical conditions in the evaporatingatmosphere vary over time. Nobody had done that before.'Despite the extreme temperature of the planet, the atmosphere is not hot enough to evaporate at the rate seen in 2011.Insteadthe evaporation is thought to be driven by the intense X-ray andextreme-ultraviolet radiation from the parent star, HD 189733A, which isabout 20 times more powerful than that of our own Sun.Takinginto account also that HD 189733b is a giant planet very close to itsstar, then it must suffer an X-ray dose three million times higher thanthe Earth.Evidence tosupport X-ray driven evaporation comes from simultaneous observations ofHD 189733A with the Swift satellite, which, unlike Hubble, can observethe star's atmosphere-frying X-rays.Afew hours before Hubble observed the planet for the second time, Swiftrecorded a powerful flash of radiation coming from the surface of thestar, in which the star briefly became 4 times brighter in X-rays.Co-author Peter Wheatley, from theUniversity of Warwick, said: 'X-ray emissions are a small part of thestar's total output, but it is the part that it is energetic enough todrive the evaporation of the atmosphere.'Thiswas the brightest X-ray flare from HD 189733A of several observed todate, and it seems very likely that the impact of this flare on theplanet drove the evaporation seen a few hours later with Hubble.'X-raysare energetic enough to heat the gas in the upper atmosphere to tens ofthousands of degrees, hot enough to escape the gravitational pull ofthe giant planet.A similarprocess occurs, albeit less dramatically, when a space weather eventsuch as a solar flare hits the Earth's ionosphere, disruptingcommunications.While theteam believes that the flash of X-rays is the most likely cause of theatmospheric changes they saw on HD 189733b, there are other possibleexplanations.For example,it may be that the baseline level of X-ray emission from the starincreased between 2010 and 2011, in a seasonal process similar to theSun's 11-year sunspot cycle.Regardlessof the details of exactly what happened to HD 189733b's atmosphere,which the team hope to clarify using future observations with Hubble andESA's XMM-Newton X-ray space telescope, there is no question that theplanet was hit by a stellar flare, and no question that the rate ofevaporation of the planet's atmosphere shot up.Thisresearch has relevance not only for the study of Jupiter-like planets.Several recent discoveries of rocky 'super Earths' near their parentstars are thought to be the remnants of planets like HD 189733b, afterthe complete evaporation of their atmospheres.