According to simulations and models, exoplanet sizes should be spread out relatively evenly. But with the exoplanet tally approaching 4,000, that hasn't really been the case. A type of world dubbed a "hot Neptune" has been conspicuously few and far between, but a new observation may explain why. Hubble has now spotted a hot Neptune named GJ 3470b that's vanishing at a record rate.

The star-hugging, extremely hot exoplanets discovered so far seem to fall into two opposite groups. There's the so-called "hot Super-Earths," which are a little bit bigger than our home planet, and way up the other end of the scale we have the "hot Jupiters." Logic says there should also be plenty of medium-sized "hot Neptunes" that are around the size of our local ice giant Neptune, but are scorching hot because they're extremely close to their parent star. But weirdly enough, these have so far largely eluded astronomers.

GJ 3470b is only the second of these planets ever found, and it may hold the key to why they're so rare. This hot Neptune orbits the star Gliese 3470, about 96 light-years away in the direction of the constellation Cancer. Being so close to its parent star means GJ 3470b is broiling away as the gas in its atmosphere heats up to the point where it escapes into space, forming a giant cloud around the planet. And since that thick atmosphere constitutes up to 20 percent of the planet's mass, GJ 3470b is shrinking at an unprecedented rate.

The discovery was made using Hubble's Imaging Spectrograph. The researchers were able to detect the ultraviolet signature of hydrogen in the cloud around the planet as it passed in front of its host star.

It's not the first time a planet has been known to leak its atmosphere into space – simulations suggest that Proxima b, a nearby Earth-like planet that may once have been habitable, has been stripped of most of its atmosphere already. Much closer to home, the same thing seems to be happening to Mars right now.

But with GJ 3470b, it's more like a burst water main. The exoplanet is losing mass 100 times faster than the only other known hot Neptune, GJ 436b. The team estimates that the planet has already lost more than a third of its mass over its lifetime, and will most likely shrink into a "mini-Neptune" in a few billion years' time.

"I think this is the first case where this is so dramatic in terms of planetary evolution," says Vincent Bourrier, lead researcher on the study. "It's one of the most extreme examples of a planet undergoing a major mass-loss over its lifetime. This sizable mass loss has major consequences for its evolution, and it impacts our understanding of the origin and fate of the population of exoplanets close to their stars."

The fact that the only two known hot Neptunes are both quickly being stripped of their atmospheres could explain why this class of planet is so rare. Exoplanets may often start life as the common hot Jupiters, briefly go through a hot Neptune phase as they lose mass, then transition into mini-Neptunes and Super-Earths, which are also common across the cosmos.

Still, a study group of two isn't enough to make that connection with too much confidence. Unfortunately, searching for the UV signature of hydrogen may not work for exoplanets more than about 150 light-years away, so there may not be many, or any, other hot Neptunes left to find that way. Upcoming projects like the James Webb Space Telescope may be able to extend the search further out by instead looking for the infrared signature of helium.

The research was published in the journal Astronomy and Astrophysics.

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