Observability of evaporating lava worlds^★

¹ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
e-mail: zilinskas@strw.leidenuniv.nl
² SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
³ Eureka Scientific Inc., Oakland, CA 94602 USA
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Department of Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

Received: 22 December 2021
Accepted: 9 March 2022

Abstract

Lava worlds belong to a class of short orbital period planets reaching dayside temperatures high enough to melt their silicate crust. Theory predicts that the resulting lava oceans outgas their volatile components, attaining equilibrium with the overlying vapour. This creates a tenuous, silicate-rich atmosphere that may be confined to the permanent dayside of the planet. The James Webb Space Telescope (JWST) will provide the much needed sensitivity and spectral coverage to characterise these worlds. In this paper, we assess the observability of characterisable spectral features by self-consistently modelling silicate atmospheres for all the currently confirmed targets having sufficient -stellar temperatures (>1500 K). To achieve this we used outgassed equilibrium chemistry and radiative transfer methods to compute temperature–pressure profiles, atmospheric chemical compositions, and emission spectra. We explore varying melt compositions, free of highly volatile elements, accounting for possible atmospheric evolution. Our models include a large number of neutral and ionic species, as well as all up-to-date opacities. The results indicate that SiO and SiO₂ infrared features are the best unique identifiers of silicate atmospheres, which are detectable using the MIRI instrument of JWST. Detection of these two species in emission would allow for strong constraints on the atmospheric thermal structure and possibly the composition of the melt. We also propose that certain species, for example TiO, may be directly tied to different classes of melts, possibly revealing surface and interior dynamics. Currently, there are nearly a dozen confirmed lava planets ideal for characterisation of silicate atmospheres using JWST, with two of these already accepted for the initial General Observers programme.