Hubble spots "hot Neptune" exoplanet that's evaporating at a record rate

[Judi Lynn]

Michael Irving
December 13th, 2018

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.

More:
https://newatlas.com/fastest-shrinking-exoplanet-hot-neptune/57671/