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

TL;DR

This study models the thermal evolution and spectral signatures of magma oceans with steam atmospheres on terrestrial planets, highlighting their detectability via future direct imaging, especially in near-infrared bands, depending on orbital distance.

Contribution

It introduces a model predicting the lifetime and spectral evolution of magma oceans with steam atmospheres, emphasizing the detectability differences based on orbital distance and initial water content.

Findings

Type-I planets' magma oceans last less than 10^6 years, favoring young star observations.

Type-II planets emit detectable infrared radiation throughout magma ocean lifetime.

Ks and L bands are optimal for direct imaging due to high planet-star contrast.

Abstract

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 near-infrared atmospheric windows during the entire lifetime of the magma ocean. The Ks 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.