Astronomers using the Hubble Space Telescope have discovered a planet GJ 3470b roughly the size of Neptune, evaporating at a rate 100 times faster than a previously identified exoplanet of similar size.

The findings advance astronomers’ knowledge about how planets evolve.

The planet is 96 light-years away and circles a red dwarf star in the general direction of the constellation Cancer.

The speed and distance at which planets orbit their respective blazing stars can determine the rate of evaporation of a planet. The researchers said GJ 3470b’s lower density makes it unable to gravitationally hang on to the heated atmosphere.

Planets such as “super” Earths and “hot” Jupiters orbit more closely to their stars and are therefore hotter, causing the outermost layer of their atmospheres to be blown away. While these larger Jupiter-sized and smaller Earth-sized exoplanets are plentiful, medium Neptune-sized exoplanets — roughly four times larger than Earth — are rare.

Hubble found that exoplanet GJ 3470b had lost significantly more mass and had a noticeably smaller exosphere than the first Neptune-sized exoplanet studied, GJ 436b, due to its lower density and receipt of a stronger radiation blast from its host star. Researchers hypothesise that these Neptunes get stripped of their atmospheres and ultimately become smaller planets.

The research team estimates that GJ 3470b may have already lost up to 35 per cent of its total mass and, in a few billion years, all of its gas may be stripped off, leaving behind only a rocky core.