Hubble's first look at exoplanet HD 189733b, located about 60 light-years away, in early 2010 didn't show any signs of an atmosphere, according to astronomer Alain Lecavelier des Etangs, with France's National Center for Scientific Research.
A light-year is the distance light travels in a year, roughly 6 trillion miles.
But a second study in late 2011 revealed plumes of hydrogen gas being blown out of the planet on the order of 1,000 tons per second.
What happened? Scientists believe a stellar flare, found by a sister telescope, bathed HD 189733b in a dose of X-rays 3 million times higher than what Earth would get during a solar flare.
HD 189733b, which is about the size of Jupiter, has the misfortune of being located just one-thirtieth the distance that Earth is to the sun. Mercury, the closest planet to the sun in our system, orbits 10 times farther away than HD 189733b's distance to its parent star.
A few hours before the second Hubble observation, NASA's Swift telescope detected a powerful flash of radiation coming from the surface of the star, its X-ray emissions briefly quadroupling in brightness.
"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," Peter Wheatley, with the University of Warwick in the United Kingdom, said in a statement.
Scientists, who presented their research on Thursday, said the finding has significance for studying Jupiter-class planets, but also could explain the rocky "super Earths" found orbiting other stars. They theorize the bodies could be the remnants of massive planets that have completely lost their atmospheres.
Astronomers were able to separate the planet's light from the light of its parent star as it passed across the face of the star, relative to Hubble's point of view.
The study will be published in an upcoming issue of Astronomy and Astrophysics.
Image: Artist's rendering of a stellar flare striking exoplanet HD 189733b as it passes in front of its parent star; Wide-field view of HD 189733b, center, and surroundings, including the planetary nebula Messier 27. Credit: NASA, ESA, L. Calçada; NASA, ESA, and the Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble)