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What is a super-Earth?

Unfortunately the classification of exoplanets is currently rather arbitrary and lacks concensus, and there are consequently conflicting and confusing statements in the literature.

Wikipedia says super-Earths are defined by mass, the mass being anywhere between the mass of Earth and 15 Earth masses (which is inbetween the mass of Uranus and Neptune). To justify this statement three papers are cited by the article (at least at the time it was consulted). However, none of these papers are about exoplanet statisics or taxonomy, but each paper concerns only one exoplanet. None of the papers claim to define what a super-Earth is. They just simply used the phrase. The Wikipedia definition is arbitrary and has no concensus.

What is a super Earth? artists impression

On the other hand, the paper that discussed analysis of the Kepler mission exoplanet candidates sample, Borucki et al. (2011, ApJ, 736, 19), state right in the abstract that the authors define super-Earths by radius (not mass), and specficially that an object with a radius in the range 1.25 to 2 times the radius of Earth is to be called a super-Earth. (If the object has the same density as Earth, this definition clearly defines a super-Earth as an object with 2 to 8 times the mass of Earth). The definition adopted by Borucki et al. (2011) is clearly still arbitrary, and it also has no cencensus. However, the Borucki et al. (2011) paper was published later than the three papers cited by the Wikipedia article.

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A problem with the Borucki et al. (2011) definition that is based on radius is that not all exoplanets have radii measurments. Another problem is that the Borucki et al. (2011) data analysis is based on exoplanet candidates, not confirmed exoplanets. If we look at the confirmed exoplanets, as given in for example, in the Extrasolar Planets Encyclopedia, we see that (in August 2012) only 32% of confirmed exoplanets had radii measurements, whearas more than 98.6% of confirmed exoplanets had a mass measurement. So if we have an exoplanet for which we know the mass but not the radius, we cannot determine whether it is a super-Earth using the Borucki et al. (2011) definition.

To put things into perspective, according to the Borucki et al. (2011) definition, 10 out 252 confirmed exoplanets that had radii measurements (on 3 August 2012) qualify as a super-Earth (that's just less than 4%). On the other hand, using the Wikipedia definition, 86 out 766 exoplanets with mass measurements qualify as a super-Earth (that's about 11.2%). (The data are from the Extrasolar Planets Encyclopedia, and on that date, 3 August 2012, there were 777 confirmed exoplanets.)

A concerted effort to agree upon a more general taxonomy has yet to be made. Ultimately it may have to wait until the there are a larger fraction of exoplanets that have both mass and radius measurements, and a final scheme may involve the exoplanet density.

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