The observation—the first of its kind—offers astronomers a rare glimpse of an extreme space-weather event on a planet outside our solar system.
"This discovery tells us that [atmospheric] evaporation is a genuine phenomenon for planets close to their stars," said study leader Alain Lecavelier, an astronomer at the Institute of Astrophysics in Paris.
For the new study, Lecavelier and his team examined the atmosphere of HD 189733b, a Jupiter-like planet about 60 light-years from Earth.
The gas giant circles its host star in a very close orbit—equal to 1/30th the distance between Earth and the sun—earning it classification as a "hot Jupiter."
The team was hoping to witness evaporation of the planet's atmosphere, an event that until now had been seen on only one other world. (Related: "Planet Found With Comet-like Tail.")
Planet's Escaping Atmosphere
Lecavelier and colleagues examined HD 189733b with the NASA-ESA Hubble Space Telescope. Although Hubble is perhaps best known for its stunning pictures of the cosmos, the planet is too tiny and too far away for the telescope to image directly.
Instead, the astronomers watched the world as it transited—or passed in front of—its star, as seen from Earth, and recorded the signatures of light passing through the planet's atmosphere using Hubble's Space Telescope Imaging Spectrograph.
The team collected data on the planet during two periods: once in early 2010 and again in late 2011.
The 2010 observation didn't show any signs of a billowing atmosphere. But during the follow-up in 2011, the team could clearly see a plume of gas being blown away from the planet.
Furthermore, unlike with the other known evaporating planet, the researchers think they know what triggered the outgassing on HD 189733b.
Just before the event, NASA's Swift satellite observed a flash of x-rays on the parent star, likely caused by a powerful stellar flare—an eruption of charged particles from a star.
"We noticed the flare just eight hours before the observation, so we suspect this is the reason for the evaporation event," Lecavelier said.
Because the gas giant orbits so close to its star, the x-rays that reached the planet would have heated the gas in the planet's upper atmosphere to tens of thousands of degrees Celsius.
The extreme heat would have sped up the motions of the gas molecules, making them fast enough to escape the planet's gravitational pull.
Evaporation Exposes Gas Planets' Cores?
It's unclear just how much atmospheric gas HD 189733b lost as a result of the flare.
"Because we do not know how long the event lasted, we do not know how much material was lost in total," Lecavelier said. "New observations scheduled with Hubble and [the European x-ray telescope] XMM-Newton will help to answer that question."
However, the researchers estimate that HD 189733b outgassed at a rate of about a thousand tons a second during the event.
While this might seem like a lot of gas, the lost material likely represents only a very tiny fraction of the planet's overall mass, so HD 189733b is in no danger of evaporating away anytime soon, Lecavelier explained.
But the rate of outgassing due to similar events could be much higher for planets that orbit even closer to their stars.
In fact, some scientists think atmospheric evaporation could explain the existence of a few tightly orbiting Earth-size planets, which have been discovered recently by NASA's Kepler space telescope and the ESA and French Space Agency's CoRoT space telescope.
"It's very likely that some of these planets are the remnants of hot Jupiters that approached too close to their stars and had their atmospheres evaporated away," exposing their rocky cores, Lecavelier said.
The burping planet study will be published in an upcoming issue of the journal Astronomy and Astrophysics.