Move Over, Comets: Planets Can Have Tails, Too

Some planets that orbit too close to their parent stars may be in danger of having their atmosphere blown clear off by radiation. And when that happens, the planet gets a tail, according to new astronomical findings by a team led by Geneva Observatory astronomer David Ehrenreich. 

The research, published in Nature, centers around a warm, low-mass planet called GJ 436b, around 10.2 parsecs (or 33.3 light years) away. It’s around the same size as Neptune—so bigger than Earth, but still pretty small compared to other celestial bodies. 

X-rays coming off the system’s red dwarf star burn off the planet’s upper atmosphere, as ultraviolet light from the star pushes against the planet’s hydrogen atmosphere, causing it to spiral outward in a tail. Because the planet has a relatively small mass, it doesn’t have the gravity to hold on to its atmosphere. 

"Around 1000 metric tonnes of hydrogen are being burnt off from GJ 436b's atmosphere every second; which equates to only 0.1% of its total mass every billion years,” according to study co-author Peter Wheatley, an astronomer at the University of Warwick in the UK. “The same process is likely to be much stronger on other exoplanets, where the entire atmosphere could be removed or evaporated to destruction.”

GJ 436b’s spectacular gas tail is 9 million miles long, and around two million miles wide. This is the first planet observed with a tail, but likely isn’t the only one out there. The researchers speculate that other hot, rocky planets might have started out looking more like the large, icy gas planet of Neptune, only to have their atmospheres burned off.