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Artist illustration of Earth compared with "super-Earth" 55 Cancri e. Image Credit: NASA/JPL-Caltech/R. Hurt (SSC) Artist illustration of Earth compared with "super-Earth" 55 Cancri e. Image Credit: NASA/JPL-Caltech/R. Hurt (SSC)

Super-Earths are more like mini-Neptunes

Sen— A new study of extrasolar planets suggests that super-Earths - rocky worlds significantly bigger than Earth - have rocky cores surrounded by large hydrogen-rich atmospheres and more closely resemble smaller versions of Neptune.

Since the first planet outside our Solar System was discovered in the 1990s, over 800 extrasolar planets - or exoplanets - have now been confirmed, with several thousands candidate planets awaiting further observation.

Planets of various mass and size have been detected, including gas giants that orbit very close to their parent star, known as 'hot Jupiters', and those believed to be made primarily of rock but which are larger than Earth. It is these super-Earths which are the subject of the new study led by Dr Helmut Lammer of the Space Research Institute of the Austrian Academy of Sciences.

The new research suggests that the rocky centres of super-Earths are unlikely to evolve into terrestrial rocky planets like the inner planets of our Solar System because they appear to hold on to their large atmospheres. Rather than evolving to a planet composed mainly of rock with a thin atmosphere, the small rocky core remains engulfed by its large hydrogen-rich envelope.

Dr Lammer and his team studied the impact of radiation on the upper atmospheres of planets around three stars, Kepler-11, Gliese 1214 and 55 Cancri. The super-Earths orbiting these stars are more massive and larger than Earth, and orbit close to their respective stars.

The hydrogen, or hydrogen-rich atmospheres, were probably captured from the disc of dust and gas orbiting the stars from which they themselves had burst into life.

The research suggests that the parent stars heat up the hydrogen gas envelopes so they expand several times the radius of the inner rocky core. Although a proportion of the hot gas escapes to space, most of the large gas surround is retained during the life of the planet. 

Cross section of a mini-Neptune

A diagram comparing the Earth to a cross-section of a super-Earth. The super-Earth has a relatively small rocky core, an atmosphere of methane, water and hydrogen and an extended hydrogen envelope. Credit: H. Lammer

Dr Lammer said: “Our results indicate that, although material in the atmosphere of these planets escapes at a high rate, unlike lower mass Earth-like planets many of these super-Earths may not get rid of their nebula-captured hydrogen-rich atmospheres”.

The study concludes that super-Earths, because they hang on to their large atmospheres, more closely resemble the structure of a planet like Neptune rather than a terrestrial planet like Mercury, Venus, Earth or Mars.

If the scientists' results are proven correct then super-Earths - or 'mini-Neptunes' - which orbit their parent star in the habitable zone (where the temperature is low enough for liquid water to exist on the surface), would hold on to their atmospheres even more effectively. If that were the case, such planets would be less likely to be habitable.

The scientists who conducted the study will be hoping that the European Space Agency's CHEOPS (CHaracterising ExOPlanets Satellite) mission, set to launch in 2017, will gather further data to help with their exoplanet study.

CHEOPS will target nearby bright stars which are already known to have planets. Meanwhile NASA's Kepler space telescope continues to monitor over 150,000 stars to look for dips of light that could be caused by planets passing in front of them. Last year NASA extended the Kepler mission until at least 2016.

The research is published in the Monthly Notices journal of the Royal Astronomical Society.

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