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.

Just
after a violent flare on its parent star bathed it in intense X-ray
radiation, the planet's atmosphere gave off a powerful burst of
evaporation.The observations give a tantalising glimpse of the changing climates and weather on planets outside our Solar System.

Astronomer
Alain Lecavelier des Etangs and his team used Hubble to observe the
atmosphere of exoplanet HD 189733b during two periods in early 2010 and
late 2011, as it was silhouetted against its parent star.While
backlit in this way, the planet's atmosphere imprints its chemical
signature on the starlight, allowing astronomers to decode what is
happening on scales that are too tiny to image directly.The observations were carried out in
order to confirm what the team had previously seen once before in a
different planetary system: the evaporation of an exoplanet's
atmosphere.HD 189733b has a blue sky, but that's where the similarities with Earth stop.

The
planet is a huge gas giant similar to Jupiter, but it lies extremely
close to its star, just one thirtieth the distance Earth is from the
Sun.Even though its star is slightly
smaller and cooler than the Sun, this makes the planet's climate
exceptionally hot, at above 1000 degrees Celsius, and the upper
atmosphere is battered by energetic extreme-ultraviolet and X-ray
radiation.As such, it is an excellent candidate to study the effects of a star on a planetary atmosphere.

'The
first set of observations were actually disappointing,' Lecavelier
said, 'since they showed no trace of the planet's atmosphere at all. We
only realised we had chanced upon something more interesting when the
second set of observations came in.'The
team'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 rate
of at least 1000 tonnes per second.Lecavelier
added: 'We hadn't just confirmed that some planets' atmospheres
evaporate - we had watched the physical conditions in the evaporating
atmosphere 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.Instead
the evaporation is thought to be driven by the intense X-ray and
extreme-ultraviolet radiation from the parent star, HD 189733A, which is
about 20 times more powerful than that of our own Sun.Taking
into account also that HD 189733b is a giant planet very close to its
star, then it must suffer an X-ray dose three million times higher than
the Earth.Evidence to
support X-ray driven evaporation comes from simultaneous observations of
HD 189733A with the Swift satellite, which, unlike Hubble, can observe
the star's atmosphere-frying X-rays.A
few hours before Hubble observed the planet for the second time, Swift
recorded a powerful flash of radiation coming from the surface of the
star, in which the star briefly became 4 times brighter in X-rays.Co-author Peter Wheatley, from the
University of Warwick, said: 'X-ray emissions are a small part of the
star's total output, but it is the part that it is energetic enough to
drive the evaporation of the atmosphere.'This
was the brightest X-ray flare from HD 189733A of several observed to
date, and it seems very likely that the impact of this flare on the
planet drove the evaporation seen a few hours later with Hubble.'X-rays
are energetic enough to heat the gas in the upper atmosphere to tens of
thousands of degrees, hot enough to escape the gravitational pull of
the giant planet.A similar
process occurs, albeit less dramatically, when a space weather event
such as a solar flare hits the Earth's ionosphere, disrupting
communications.While the
team believes that the flash of X-rays is the most likely cause of the
atmospheric changes they saw on HD 189733b, there are other possible
explanations.For example,
it may be that the baseline level of X-ray emission from the star
increased between 2010 and 2011, in a seasonal process similar to the
Sun's 11-year sunspot cycle.Regardless
of the details of exactly what happened to HD 189733b's atmosphere,
which the team hope to clarify using future observations with Hubble and
ESA's XMM-Newton X-ray space telescope, there is no question that the
planet was hit by a stellar flare, and no question that the rate of
evaporation of the planet's atmosphere shot up.This
research has relevance not only for the study of Jupiter-like planets.
Several recent discoveries of rocky 'super Earths' near their parent
stars are thought to be the remnants of planets like HD 189733b, after
the complete evaporation of their atmospheres.