A Hell of a Phase Curve: Mapping the Surface and Atmosphere of a Lava Planet K2-141b

The CoRoT, Kepler and K2 missions have revealed a new class of planet, so-called lava planets, which have bulk densities consistent with terrestrial composition, but dayside temperatures hot enough to melt—and vaporize—rock. Of these planets, the most promising candidate for atmospheric and surface characterization is K2-141b: it has a density of 8 g/cm^3, a dayside temperature measured by Spitzer to be 2125 K, and an orbital period of 6.7 hrs around a J=9 star. We propose to monitor K2-141b for three complete orbits of the planet with MIRI LRS for just under 21 hrs of time. These observations will allow us to measure the vertical atmospheric structure on the dayside of the planet via the 9 micron SiO feature, to determine the presence of an extended atmosphere via the planet's nightside temperature, and to search for rock clouds that might form as the supersonic winds blowing towards the nightside cool and condense. JWST is uniquely suited to detect SiO in emission spectra of lava planets and MIRI LRS is the only instrument able to probe low nightside planetary temperatures to place stringent constraints on the presence of a global atmosphere. The combination of the strong signal expected from K2-141b and the large contrast at long wavelengths, makes MIRI LRS observations of K2-141b the approach for testing theories of cloud formation in a fully condensable atmosphere, and testing atmospheric loss on the extreme scenario of a planet skimming its star's corona.