Exoplanet
© Mark Garlick/University of Warwick
Astronomers have discovered a spectacular comet-like tail around a distant planet the size of Neptune. The findings reported in the journal Nature, could provide new clues as to how some rocky terrestrial planets are formed.

"This is truly incredible, it's the largest atmosphere ever detected around a planet," says one of the study's authors Dr Vincent Bourrier of the University of Geneva Observatory.

"This cloud of hydrogen, which has escaped the planet, is forming a comet like tail trailing behind the planet."

The planet, named Gliese 436b, is in a very close orbit around the small red dwarf star Gliese 436, which is less than half the size of the Sun. The system is located about 33 light-years away in the constellation of Leo.

Earlier studies of Gliese 436b have suggested it has a small rocky core, a mostly water ice mantle, and a thin outer envelope of hydrogen and helium gas. Its size and estimated daytime surface temperature of about 530°C led scientists to classify Gliese 436b as a 'warm Neptune'.

The planet takes just 2.64 Earth days to orbit its host star, circling it at a distance of only about four million kilometres, 15 times closer than Mercury's orbit around the Sun. Because it is so close, the star irradiates the planet's atmosphere, causing it to heat up and expand.

However, the star's stellar winds aren't powerful enough to blow away Gliese 436b's atmosphere. Bourrier and colleagues used an ultraviolet spectrometer aboard the Hubble Space Telescope to analyse and map the chemical signature of the planet's atmosphere.

During three separate Hubble observations of the planet as it transits or moves in front of its host star as seen from Earth, the scientists detected a hugely extended envelope of hydrogen gas surrounding and trailing out far behind the planet.

"Because of the low ionisation from the star, the gas can stay for a long time, trailing for extended distances behind the planet," says Bourrier.

The giant envelope is large enough to cover around 56 per cent of the star's surface.

Hot Jupiters vs warm Neptunes

Bourrier and colleagues have seen this phenomenon before on hot Jupiters, which are giant planets in close orbits around large stars, but the envelope usually covers only about 10 per cent of the star.

"The large size of Gliese 436b's atmosphere is very surprising, because this planet is far less irradiated than other planets we've seen because it's less heated by the star," says Bourrier.

"This planet is orbiting a small dwarf star and so the radiation pressure is far lower and can't blow away the gas as strongly as larger stars, so we think this allows the gas to diffuse with in a larger volume around the planet which is why it's grown so large."

It's been suggested that low-mass exoplanets orbiting close to their host stars could have part of their atmospheres burnt off by extreme irradiation from the star, but confident measures of such losses have been lacking until now.

The authors believe Gliese 436b may have lost as much as 10 per cent of its atmosphere through this irradiation process.

"This specific planet is safe from evaporation and so will probably keep its atmosphere over time," says Bourrier.

"But we think planets similar to GJ 436b, but with a bit more radiation from their host stars, could have lost most of their atmosphere and so be eroded down to just a rocky core forming a rocky planetary remnant.

"So this has implications for the evolution of planets over time."