Many Exoplanets Are More Mini-Neptunes Than Super-Earths
A good portion of the exoplanets discovered by astronomers so far fall into the category of “super-Earths.” These planets have a mass greater than Earth’s but lower than some of the gas giants in our solar system. Both Uranus and Neptune have the mass of around 15 Earths. In the past, these super-Earths have been considered good candidates for discovering extraterrestrial life. But given the depth of many super-Earth’s atmospheres, scientists say they should more accurately be called “mini-Neptunes.”
Helmut Lammer of the Space Research Institute of the Austrian Academy of Sciences now proposes that these super-Earths have an atmosphere made up of an extended hydrogen-rich atmospheric layer, which would count them out as being Earth-like, or even having the capacity to sustain alien life.
Lammer and his team came to this conclusion after researching the effects of radiation on the upper atmospheres of these planets. He suggests that some of these planets have a solid core and an atmosphere rich in hydrogen, water, and methane.
The atmospheric mass loss of the studied super-Earths is one to two orders of magnitude lower compared to that of hot Jupiters, so one can expect that these exoplanets cannot lose their hydrogen envelopes during their remaining lifetimes.
Lammer’s research also suggests a short-wavelength, extreme ultraviolet light coming off the planets’ respective stars, which is responsible for the planets’ atmosphere heating up.
You can read the complete study, found in the “Monthly Notices of the Royal Astronomical Society.”