Gliese 436b is one of the closest known exoplanets, just 30 light-years away from Earth. What makes this planet interesting is what trails behind it – a gaseous tail. It looks like something you see trailing a comet, not a planet the size of Neptune.

The ‘tail cloud’ is made up of a massive cloud of hydrogen that is nearly 50 times the size of its parent star (Gliese 436).

That may seem like a lot of lost hydrogen, but astronomers estimate the planet has lost only about 10 percent of its atmosphere.

Gliese 436b cloud

Credit: ESA

“Although the evaporation rate doesn’t threaten the planet right now, we know that the star, a faint red dwarf, was more active in the past. This means that the planet’s atmosphere evaporated faster during its first billion years of existence,” says David Ehrenreich, lead author of the study.

Gliese 436b is classified as a ‘warm Neptune.’ These planets are similar in size to Neptune, but orbit their parent star much closer than Neptune does.

Gliese 436b sits very close to Gliese 436 at 4 million kilometers. It can complete one orbit in just 2.6 Earth days.

Gliese 436b orbit

Our solar system’s closest planet, Mercury, orbits the Sun at an average distance of 58 million kilometers and completes an orbit every 88 days.

What about Gliese 436b classification namesake? Neptune’s average distance from the Sun is 4,498,252,900 kilometers and takes 164.79 years to complete an orbit.

Gliese 436b’s gaseous tail could explain the existence of hot super-Earths that orbit extremely close to their stars. A lot of hot super-Earths are just solid, rocky cores. These cores could be the remnants of a much larger planet with a gaseous atmosphere.

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Why does Gliese 436b not have to worry about losing all of its atmosphere anytime soon? The answer lies in the star it orbits. Gliese 436 is a red dwarf star. The (relative) coolness of the red dwarf star allows the hydrogen cloud and Gliese 436b’s atmosphere to stick around.

“Finding the cloud around Gliese 436b could be a game-changer for characterising atmospheres of the whole population of Neptunes and Super-Earths in ultraviolet observations,” says Vincent Bourrier, co-author of the study.

How the Hubble Space Telescope made the observation possible

Ehrenreich and his colleagues used the Hubble Space Telescope to observe Gliese 436b and its trailing hydrogen cloud.

Why the Hubble?

Bourrier hints at it in his comment above. Ultraviolet observations. Ground-based ultraviolet observations are difficult due to how well Earth’s atmosphere blocks ultraviolet light.

The Hubble Space Telescope doesn’t have to deal with this problem.

Ehrenreich describes Gliese 436b as a “monstrous thing” when imaged through ultraviolet with the Hubble Space Telescope.

The Hubble Space Telescope may even be able to spot evidence of oceans on distant worlds. Water vapor from evaporating oceans would be nearly impossible to detect. But, astronomers can watch for hydrogen molecules after stellar radiation breaks up water molecules. If the Hubble could detect these hydrogen atoms escaping the planet from a smaller, cooler planet – it could indicate an ocean on the surface.

This isn’t the first example of an exoplanet with an evaporating atmosphere

Hubble has spotted exoplanets with evaporating atmosphere before. The difference is, these previous discoveries were around much more massive exoplanets.

Gliese 436b is just another reason space is so damn cool. It’s mind-boggling to even think of a planet that massive orbiting its parent star that close. Much less have a comet-like tail trailing it.

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Image credits: ESA

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