"Until now, we were not sure of the role played by the evaporation of atmospheres in the formation of the desert," said Bourrier. But based on this new finding, hot Neptunes may have withered away into mini-Neptunes (an alternative term for hefty super-Earths), or even eroded straight down to their rocky cores. "This could explain the abundance of hot super-Earths that have been discovered," said co-author David Ehrenreich, an astronomer at UNIGE.
By showing that an active star can strip loads of mass from a mid-sized planet near the edge of the hot Neptune desert, the researchers think they've finally figured out why so few hot Neptunes have been discovered, while heaps of smaller planets, called super-Earths, have been found in the same boiling neighborhood.
"This is the smoking gun that planets can lose a significant fraction of their entire mass," said co-author David Sing, a professor at Johns Hopkins University, in a statement. "GJ 3470b is losing more of its mass than any other planet we [have] seen so far; in only a few billion years from now, half of the planet may be gone." And if GJ 3470b loses half its mass (it's currently about 14 Earth masses), it will make the transition from hot Neptune to super-Earth.
Exploring the desert
GJ 3470b, which sits about 10 times closer to its host star than Mercury is to the Sun, is not the first evaporating Neptune-sized planet ever found. In fact, just a few years ago, astronomers found a similar planet named GJ 436b that is losing its atmosphere just like GJ3470b, albeit at a much slower rate (about 100 times slower). To analyze the mass loss of both these planets, the researchers tracked the hydrogen that was escaping from their atmospheres.
However, astronomers cannot easily detect hydrogen from more than about 150 light-years away. This is because the hydrogen signals they are looking for fall in a wavelength range that is blocked by interstellar gas, which permeates the space between stars. But fortunately, the researchers have a plan to seek out more distant shrinking hot Neptunes in the future.
"Helium will expand the range of our surveys," said Bourrier, "the high sensitivity of the James Webb Space Telescope should allow us to detect helium escaping small planets, such as mini-Neptunes, and complete our observations of the edge of the desert."
So, for now, we'll have to be satisfied with just a few known examples of shrinking hot Neptunes. But keep in mind, many more may lurk just beyond the horizon.