Small worlds around other stars may come in more than two varieties.

Using exoplanet densities, astronomers have largely sorted planets that are bigger than Earth but smaller than Neptune into two categories: denser, rocky super-Earths and larger, puffy mini-Neptunes (SN: 6/19/17). Mini-Neptunes are generally thought to be padded in thick layers of hydrogen and helium gas, like the giant planets in our own solar system. But astronomers have detected clear evidence of hydrogen on only some mini-Neptunes — and, curiously, seen traces of water on others (SN: 5/11/17).

Now, new simulations indicate that some planets that look like gaseous mini-Neptunes could actually be rocky planets covered in superheated oceans, where the water is in an exotic state between liquid and gas. Such extreme saunalike worlds could bridge the divide between rocky and gaseous planet types, researchers report in the June 15 Astrophysical Journal Letters.

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