Taking the Temperature of a Lava Planet

We present new Spitzer observations of the thermal phase curve of K2-141b, an ultra-short period terrestrial planet (USP). With an orbit of just 6.7 hours, this world is blasted with stellar radiation that is expected to obliterate any traces of a primordial atmosphere and melt the dayside surface into a lava ocean. Previous observations of USPs have yielded several surprising results, including the measurement of an offset hotspot in the thermal phase curve of 55 Cancri e, which may indicate a thick atmosphere has survived, and a high Bond albedo for Kepler-10b, which suggests the presence of unusually reflective lava on its surface. K2-141b provides a unique benchmark for comparison with these other systems because it the only USP with phase curve measurements in the optical (from K2) and infrared (from Spitzer). These observations can therefore break the degeneracy between signatures of reflected light and atmospheric circulation, shedding new light on the nature of atmospheres on USPs.