Super-Earths May Actually Be Mini-Neptunes
February 4, 2013

Super-Earths: More Like Mini-Neptunes, Study Says

Lee Rannals for — Your Universe Online

Finding planets outside of our own Solar System has become a bit of a hot topic in the scientific community, and the enthusiasm behind the discovery of "Earth-like" planets has been on the rise, but a new study suggests we may want to tone down our excitement a bit.

A new study led by Dr. Helmut Lammer of the Space Research Institute (IWF) at the Austrian Academy of Sciences says that maybe these "Earth-like" planets may not be so much super-Earths, but instead more like mini-Neptunes.

They wrote in the journal Monthly Notices of the Royal Astronomical Society about how they used data on super-Earths orbiting the stars Kepler-11, Gliese 1214 and 55 Cancri for their study. These planets are all a few times more massive, and slightly larger than the Earth, and they orbit very close to their stars.

Lammer told redOrbit in an email that he and colleagues were not involved in the observation of the studied super-Earths, but that they did use the radii and masses, and corresponding densities of them for hydrodynamic upper atmosphere models, to look into how "Earth-like" these so-called "super-Earths" really are.

As far as technique goes, Lammer said that if you know the size and mass of the planets, then their average density can be calculated.

"All seven super-Earths have densities which are lower compared to a rocky planet," Lammer told redOrbit. "This indicates that their rocky cores are surrounded by hydrogen or hydrogen-rich volatiles, which will yield an atomic hydrogen corona around these planets."

He said the team studied the radiation fluxes of their host stars, and applied a hydrodynamic numerical upper atmosphere code, which calculates the heating of the upper atmosphere, the adiabatic cooling and its structure, and the escape rates of hydrogen atoms.

"Because the density of these planets indicate that the amount of volatiles corresponds to a few percent of the planets and we know the escape rate of the gas we can show that the loss is too low so that these super-Earths can not get rid off these gaseous envelopes," said Lammer.

The team's results, according to Lammer, show that super-Earths with similar masses, but at larger orbital distances still within the habitable zone, will have lower loss rates, and may end up more as a mini-Neptune, rather than an Earth-like planet.

"I expect that there are more Earth-like exoplanets are out there, but so far we know only the radius or the mass of such exoplanets or exoplanet candidates," Lammer told redOrbit. "If we don't know both parameters we cannot calculate the density."

He said without knowing the density, we cannot know if the planet is mainly a pure rock, or if it is surrounded by a dense hydrogen layer or a deep-water ocean. The study also indicates that there may be a reason behind why smaller Earth-like planets may lose their primordial hydrogen atmosphere, or outgassed volatile-rich protoatmospheres easier than massive, and larger super-Earths.

"Depending on the host stars radiation environment and due to the lower gravity, Earth and Venus, for instance, could loose their protoatmospheres, while super-Earths have a problem to get rid of them," Lammer said in the email.

The scientist added that it is only a matter of time before observers will discover the size and mass of actual Earth-like planets, as opposed to mini-Neptunes.

For now, astronomers like Lammer can only theorize about these exoplanets, but the European Space Agency is working on a new tool that could bring these theories to test: the CHaracterising ExOPlanets Satellite (CHEOPS). This instrument will be able to look at "super-Earths" in more detail, and could be able to tell whether some of these exoplanets are like our own, or are just seemingly run-of-the-mill mini-Neptunes.