If there’s one true fact about every single gas giant planet ever observed, around the Sun or other stars in the Galaxy, it’s that they all are mainly composed of Hydrogen. Even though the giants of our solar system such as Neptune and Jupiter seem very different, it is Hydrogen that primarily composes them. The difference is in the details though. The blue colour of Neptune is due to the presence of Methane, and even then it only makes up 1.7% of Neptune’s mass.
But Hydrogen is light. Wouldn’t giant planets like hot Jupiters lose their Hydrogen from being blasted by rays of their home star? The answer, as of recently, is yes. Astronomers have discovered a giant Neptune-sized planet very close to it’s home star, where the radiation is strong enough to evaporate the thick Hydrogen enveloping the surface. The evaporating Hydrogen has created a massive cloud around the planet that is 50 times as large as the planet’s parent star!
The planet, named GJ 436b is orbiting a red dwarf star about 30 light-years from Earth. So how do we see this massive cloud of Hydrogen around a star so distant? Because the planet is so close to the star, it orbits in only 2.6 Earth days. As it passes in front of its star, the light from the star dims since the planet is blocking it out. The observed dimming is only about 1%, but it’s more than enough to measure the size and orbital period of the planet.
Well here’s the interesting part – when astronomers observed the star in ultraviolet wavelengths, the star dimmed by as much as 50% about 2 hours before the transit started, and ended 3 hours after. It turns out that Hydrogen atoms absorb UV very efficiently, so this dip in the UV brightness allowed astronomers to estimate the size of the Hydrogen cloud surrounding the planet and determine that it’s behaving like a comet tail, with the Hydrogen falling away into space behind the planet as it moves in its orbit.
The other amazing thing about this planet is that as the Hydrogen is blasted away, the planet is left with an atmosphere consisting mostly of gaseous Helium, giving it a pale grey appearance. This kind of evaporation may have happened a few billion years ago in our own solar system. Most of the Hydrogen and Helium in the inner solar system would have been swept up by the planets, since they were the heaviest bodies. But after millions of years of being blasted by a warming Sun, they could have left massive, comet-like tails of Hydrogen in the same manner as this distant world.