A distant, Jupiter-sized planet is giving off a wispy wake of gas up to five times its own size, and within this comet-like tail we’ve found the first signs of helium in an exoplanet’s atmosphere.

Astrophysicists viewed the hot gas giant – called WASP-107b – with cameras aboard the Hubble Space Telescope, which revealed its bizarre architecture. While it’s nearly the same radius as Jupiter, it has a mass far smaller – it’s more like two or three Neptunes puffed up into a Jupiter-sized space.

“It’s incredibly low density,” says  at the Space Telescope Science Institute in Baltimore, Maryland. “It’s one of the lowest density planets known.”

The planet’s giant size owes to a puffy, diffuse atmosphere that’s blown away by the harsh radiation of its host star. The planet loses between 0.1 and 0.4 percent of its atmosphere every billion years. Most of that trails off to its night side, where it leaves a translucent tail three to five times the planet’s surface area.

“It would take much longer than the age of the universe to strip its atmosphere entirely,” says  at the University of Exeter in the UK. The first 100 million years of the 800-million-year-old planet’s life were likely a period of more drastic atmospheric loss, which would have resulted in it ballooning into a large gas giant.

Hunting helium

By looking at infrared light coming from the star, Spake and her colleagues detected helium in the planet’s atmosphere as it passed in front of its star. Helium has long been thought to be a byproduct of planets close enough into their stars that they lose parts of their atmosphere, and helium is common across our solar system – but it hadn’t been seen on an exoplanet until now.

at NASA’s Jet Propulsion Laboratory in Pasadena, California, says that WASP-107b’s mass and proximity to its host star makes it a unique case study as a planet with a mass between that of Neptune and Saturn.

“There are only a handful of planets in this unique regime, so understanding these atmospheres can help us understand the planets in this size and mass,” she says.

There’s also the intriguing possibility that some planets like this could, in extraordinary circumstances, be close enough to their stars to lose an even more substantial part of their atmosphere and leave behind a world just a few times the mass of Earth, Kataria says. If that’s true, it could explain the oddball super-Earths detected in other planetary systems but not discovered in our own.

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