Thirty-three light years away from us – quite close by astronomical standards – Hubble has found a surprisingly large gas cloud near an exoplanet.
What's got the Hubble Space Telescope crowd excited is that the gas cloud is huge – around 50 times the size of the parent star – and seems to be atmosphere evaporating away from the planet, designated GJ 436b.
The Neptune-sized planet is orbiting Gliese 436 and is considered to be a “warm Neptune” because it's far closer to its Sun than ours is to Sol. It's just 3 million miles away from its host (about 4.8 million km) and its orbit is just 2.6 Earth days.
According to Hubble's announcement, the hydrogen cloud (tagged “The Behemoth” by the boffins that found it) represents around 10 per cent of GJ 436b's original atmosphere.
David Ehrenreich of the Observatory of the University of Geneva in Switzerland explains: “This cloud is very spectacular, though the evaporation rate does not threaten the planet right now.
“But we know that in the past, the star, which is a faint red dwarf, was more active. This means that the planet evaporated faster during its first billion years of existence.”
It was spotted in the ultraviolet spectrum, one reason why Hubble's outside-the-atmosphere viewpoint was needed to make the discovery, and The Behemoth is big enough that it dims the light from GJ 436 as it orbits.
The relatively cool star, the boffins reckon, is the reason the cloud of hydrogen is still there for us to see: with a hotter sun it would have been swept away by radiation pressure.
Apart from being the biggest cloud yet spotted, astro-boffins say it could help them understand the existence of hot super-Earths, which could be born by a planet like GJ 436b losing its hydrogen, leaving just the rocky core behind.
The paper, for Science, can be read here. ®