French Astronomers Watch As Planet’s Atmosphere Evaporates

(CORDIS) — Astronomers have observed dramatic changes in the upper atmosphere of a faraway planet, catching a tantalising glimpse of the changing climates and weather of planets outside our solar system.

Using the National Aeronautics and Space Administration (NASA)/European Space Agency (ESA) Hubble space telescope, scientists reported that just after the planet was bathed in intense X-ray radiation following a violent flare on its star, the planet’s atmosphere gave off a powerful burst of evaporation.

The planet is a huge gas giant like Jupiter, but it lies extremely close to its star, just one-thirtieth the distance the 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 1 000 degrees Celsius, and the upper atmosphere is battered by energetic extreme-ultraviolet and X-ray radiation. As such, it is an excellent candidate for studying the effects of a star on a planetary atmosphere, said the scientists, led by French astronomer Alain Lecavelier des Etangs.

According to Mr Lecavelier des Etangs, the first set of observations, carried out in early 2010, were ‘disappointing, since they showed no trace of the planet’s atmosphere at all’. He said his team ‘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 1 000 tonnes per second. ‘We hadn’t just confirmed that some planets’ atmospheres evaporate, we had watched the physical conditions in the evaporating atmosphere vary over time, ‘ said Mr Lecavelier. ‘Nobody had done that before.’

The scientists then had to discover why this change had taken place.

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, which is about 20 times more powerful than that of our own sun. Moreover, since the giant planet is very close to its star, it must suffer an X-ray dose 3 million times higher than the Earth does.

Evidence to support X-ray driven evaporation comes from simultaneous observations of the planet from another satellite. ‘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,’ explained Peter Wheatley from the University of Warwick in the United Kingdom, one of the co-authors of the study. ‘This was the brightest X-ray flare from the planet 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, for example, 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 the planet, it admitted that 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.

This research has relevance for the study of other planets as well as 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 similar planets, after the complete evaporation of their atmospheres.