Lifetime and Spectral Evolution of a Magma Ocean With a Steam Atmosphere: Its Detectability by Future Direct Imaging

We present the thermal evolution and emergent spectra of solidifying terrestrial planets along with the formation of steam atmospheres.The lifetime of a magma ocean and its spectra through a steam atmosphere depends on the orbital distance of the planet from the host star.For a Type I planet, which is formed beyond a certain critical distance from the host star, the thermal emission declines on a timescale shorter than approximately 10 6 years.Therefore, young stars should be targets when searching for molten planets in this orbital region.In contrast, a Type II planet, which is formed inside the critical distance, will emit significant thermal radiation from nearinfrared atmospheric windows during the entire lifetime of the magma ocean.The K s and L bands will be favorable for future direct imaging because the planet-to-star contrasts of these bands are higher than approximately 10 -7 -10 -8 .Our model predicts that, in the Type II orbital region, molten planets would be present over the main sequence of the G-type host star if the initial bulk content of water exceeds approximately 1 wt%.In visible atmospheric windows, the contrasts of the thermal emission drop below 10 -10 in less than 10 5 years, whereas those of the reflected light remain 10 -10 for both types of planets.Since the contrast level is comparable to those of reflected light from Earth-sized planets in the habitable zone, the visible reflected light from molten planets also provides a promising target for direct imaging with future ground-and space-based telescopes.