Mini-Neptune

The discovery of transiting planets with masses below 10 MEanh and radii consistent with rocky planetary models answered the important question as to whether planets more massive than Earth could be rocky. 10 Mgarth and 2 Earth radii are used as estimates from planet formation theories as the upper limit for rocky planet mass and size. For comparison, Uranus has about 14.5 MEanh and about 4 Earth radii. Above about 10 Earth masses a planet is thought to accumulate a substantial amount of gas that makes it akin to a gas giant with a substantial atmosphere, not a rocky planet with a thin outgassed atmosphere. Where exactly such a cut-off mass is that distinguishes rocky Super-Earths and gaseous Mini-Neptunes - if it exists at all - is an open question that mean density measurements of detected exoplanets currently explore. 

Recent discoveries by ground based observations, as well as the Corot and Kepler space-missions, found planets with masses below 10 MEarth and densities akin to Neptune as well as Earth, suggesting that there is not one cut-off mass above which a planet is like Neptune and below which it is rocky like Earth or Venus. Note that the term Mini-Neptune is used for small extrasolar giant planets, not mini-Uranus, even though Uranus is the less massive planet (17.1 and 14.5 Earth masses. 

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