Astronomers find 'comet's tail' trailing Neptune-sized exoplanet

Not every day you find an exoplanet with a 'comet's tail'

They call it "the Behemoth." Astronomers using NASA’s Hubble Space Telescope have discovered that a red dwarf star is boiling off the atmosphere of a Neptune-sized exoplanet, causing it to form an enormous cloud that surrounds and trails the planet like a comet’s tail.

The atmosphere, described in the journal Nature, is roughly 50 times as large as the planet itself – and could help explain the origin of super-Earths, which have no analog in our solar system.

The planet GJ 436b, aka Gliese 436b, which lies roughly 30 light-years away, is known as a "warm Neptune" – it sits so close to its star that its "year" is just 2.64 days, and its surface temperature is fairly high.

Given how close the planet lies to its star, “we were thinking, OK, maybe this planet has an atmosphere that is evaporating or escaping,” said lead author David Ehrenreich, an astrophysicist at the Observatory of Geneva in Switzerland.

Scientists have spotted the atmospheres of "hot Jupiters" – gargantuan gas giants that orbit very close to their stars – but they want to be able to examine the atmospheres of smaller, more Earth-sized planets, with the goal of finding worlds that are slightly more like our home planet. Gliese 436b is roughly four times the size of Earth with 20 times the mass, but studying it is a step in the right direction, Ehrenreich said. 

When the scientists looked at the star’s visible light, they could see the roughly 1% drop in starlight as the planet crossed in front of its star. But when they observed the star in ultraviolet wavelengths, the planet seemed to cover more than 50% of the star. Something enormous, but invisible at optical wavelengths, was crossing in front of the red dwarf.

 “That’s really huge – that really surprised us,” Ehrenreich said.  “When we obtained the first numerical simulations of this thing, it was looking kind of like a giant fish that would eat up the star. That’s why we named it the Behemoth.”

It seemed that a giant hydrogen envelope surrounding the star was blocking the ultraviolet light from getting through. This cloud, the scientists said, was being caused by the star "burning" off the planet’s atmosphere, causing it to escape the planet. The hydrogen didn’t completely dissipate; instead, it created a huge comet-like tail that seemed to trail the planet.

This seemed unusual, given that other, hotter planets don’t seem to share this extended envelope, the scientists said.

“We scratched our heads quite intensely to understand [this] because it’s a bit counterintuitive,” Ehrenreich said. “How would you expect that something that’s received less heat, less irradiation, would appear to be evaporating more, or at least, would appear to be surrounded by more evaporating material than the hotter planet?”

It seems that the red dwarf is so cool, and shining so dimly, that it doesn’t produce enough radiation pressure to fully blow the cloud away. The planet, which is at most 6 billion years old, is losing as much as 1,100 tons of its atmosphere per year – that’s only about 0.1% of its total mass per billion years, so probably not enough to make a serious dent. In the past, however, the rate was probably faster, and so the planet may have lost up to 10% of its atmosphere over its lifetime, the scientist added.

The results could shed light on the origin of super-Earths – rocky planets that are more massive than our planet. Perhaps these super-Earths started out as mini-Neptunes, with dense gassy envelopes that were eventually burned away, Ehrenreich said.

This technique might also be used to look for signs of watery oceans being boiled off on distant exoplanets orbiting close to their stars, Ehrenreich said, by searching for the escaping hydrogen atoms as water molecules are torn apart.

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