There’s an enormous cloud escaping from a warm Neptune-sized exoplanet, making it look like a comet, according to a new Nature study. Called GJ 436b, the planet lies 30 light-years away from us, but just 3 million kilometers (around 2 million miles) from its host star, which it orbits in just 2.6 Earth days. 

Researchers suspect that low-mass exoplanets with small orbits likely get part of their atmospheres burned off by the extreme radiation coming from their host star. To find evidence of this atmospheric loss, a team led by David Ehrenreich from the Observatoire de l’Université de Genève analyzed ultraviolet observations made by the Hubble Space Telescope of this so-called “Warm Neptune” as it passes in front of its host star, GJ 436. 

They detected an enormous, extended atmosphere that surrounds the planet and also trails behind it, like a giant comet. Heat from the star was enough to expand the atmosphere, allowing it to escape the planet’s attraction. The immense cloud covers about 56% of the star’s surface, and the “tail” stretches for millions of kilometers. They’ve dubbed it “The Behemoth,” and it’s likely made of hydrogen atoms.

“This cloud is very spectacular, though the evaporation rate does not threaten the planet right now,” Ehrenreich says in a statement. “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 because of the strong radiation from the young star. Overall, we estimate that it may have lost up to 10% of its atmosphere over the past several billion years.” 

Even though GJ 436b is 33 times closer to its star than Earth is to the sun, the relatively cool red dwarf is about half the diameter of our sun and about 40 times dimmer. The massive cloud of gas still exists because it’s not being rapidly heated and swept away by the radiation pressure. 

This phenomenon has never been observed around an exoplanet this small. It may help explain how other exoplanets enveloped in hydrogen atmospheres could have their outer layers evaporated off by their parent stars – leaving behind solid, rocky cores like Earth. 

^{Artist's impression of GJ 436b at the beginning of its transit across the surface of its parent star. D.Ehrenreich / V. Bourrier (Université de Genève) / A. Gracia Berná (Universität Bern)}