Hubble observations reveal a Jupiter-size exoplanet losing its atmosphere in a system 200 light-years away.

For the first time, astronomers have found helium escaping a giant planet's atmosphere. The envelope of gas surrounding the planet extends far out into space. The results appear in the May 3rd Nature.

Astronomers, led by Jessica Spake (University of Exeter, UK) used the Hubble Space Telescope to watch a planet called WASP-107b cross the face of its parent star. They measured how much near-infrared stellar light came through the planet’s atmosphere, a technique called transmission spectroscopy, and spotted an absorption feature in the spectrum associated with helium. The signal was strong, about five times greater than what a false signal could have generated.

While helium ought to be common in stars and giant planets, it was difficult to spot in far-away planets until now. Previous studies had looked for helium absorption at ultraviolet wavelengths, but the absorption pattern is complex and difficult to make out in the noisy ultraviolet spectrum. But helium in a long-lived excited state, called metastable, can instead strongly absorb infrared radiation. This form of helium forms high up in planetary atmospheres.

"I'm really excited," Spake says. "We can use this technique to answer fundamental questions about planets."

"Helium is a proxy for even heavier elements," says Drake Deming (University of Maryland), a coinvestigator on the new Transiting Exoplanet Survey Satellite (TESS) who didn’t participate in the current study. Because researchers see the helium, he explains, they can assume other components of the atmosphere are there with it.

From Hubble’s observations, the astronomers estimate that WASP-107b’s atmosphere is dribbling away, such that the planet will lose between 0.1% and 4% of its atmosphere over the next billion years. That’s not much; nevertheless, the diffuse atmosphere extends for tens of thousands of miles beyond its surface. Such extended atmospheres have only been observed around three other exoplanets (like Gliese 436b), which were discovered via hydrogen spectral lines.

WASP-107b is about the same size as Jupiter, but only 12% as massive, making it one of the lowest-density planets known. It’s so close to its parent star that it whips around it every 5.7 days. Its atmosphere, however, is only 500°C (932°F), surprisingly cool given its close-in orbit.

The next big step will be to look for the same helium signature in smaller planets, such as super-Earths or even Earth-sized planets, and high-powered telescopes are coming online shortly that will provide more infrared detail on these planets. NASA's James Webb Space Telescope, now scheduled to launch in 2020, can look at atmospheric elements, including helium. The European Extremely Large Telescope (E-ELT) will have an even more exquisite view due to its large 39-meter (128-foot) main mirror, which collects more light than JWST. First light for E-ELT is expected around 2024.