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1. Exoplanets

   Linked via "Xenon hydride"

   Super-Earths and Mini-Neptunes
   These two categories represent the most common types of exoplanets discovered thus far. Super-Earths are terrestrial planets with masses between $1$ and $10$ Earth masses ($M{\oplus}$), while Mini-Neptunes (or sub-Neptunes) are gaseous envelopes surrounding a solid core, with radii between $1.7$ and $4.0$ Earth radii ($R{\oplus}$). The boundary between these two classes is known as the "radius valley," where planets appear to lose their gaseous envelopes due to [photoevaporation](/entries/photoevaporation…