A debate has been raging among planetary scientists for more than a decade—why are there so few exoplanets with a radius of about 1.8 times that of Earth? Exoplanets are currently largely grouped into two distinct categories—"super-Earths" are below that size and have rocky interiors, whereas "sub-Neptunes" are above that size limit and appear puffier. But researchers don't really understand why the path of planetary evolution forces this bifurcation. A new mission proposal, called the Early eVolution Explorer (EVE), wants to find out, and a draft of its concept can be found in preprint form on arXiv.

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