Modelling the atmosphere of lava planet K2-141b: implications for low- and high-resolution spectroscopy

Abstract

Transit searches have uncovered Earth-size planets orbiting so close to their host star that their surface should be molten, so-called lava planets. We present idealized simulations of the atmosphere of lava planet K2-141b and calculate the return flow of material via circulation in the magma ocean. We then compare how pure Na, SiO, or SiO2 atmospheres would impact future observations. The more volatile Na atmosphere is thickest followed by SiO and SiO2, as expected. Despite its low vapour pressure, we find that a SiO2 atmosphere is easier to observe via transit spectroscopy due to its greater scale height near the day–night terminator and the planetary radial velocity and acceleration are very high, facilitating high dispersion spectroscopy. The special geometry that arises from very small orbits allows for a wide range of limb observations for K2-141b. After determining the magma ocean depth, we infer that the ocean circulation required for SiO steady-state flow is only 10−4 m s−1, while the equivalent return flow for Na is several orders of magnitude greater. This suggests that a steady-state Na atmosphere cannot be sustained and that the surface will evolve over time.

Observability of Evaporating Lava Worlds

“…Decade old studies have predicted formation of silicate-rich atmospheres, outgassed from the molten surface of irradiated planets (Schaefer & Fegley 2009;Miguel et al 2011). With the launch of JWST, the topic has received more interest than ever, including many predictive and observational studies (Ito et al 2015;Kite et al 2016;Dai et al 2019;Nguyen et al 2020;Ito & Ikoma 2021, ;Zieba et al submitted). Several lava planets have been confirmed for the initial observers program of JWST, which may lead to first evidence of silicate atmospheres on exoplanets (Hu et al 2021;Brandeker et al 2021;Dang et al 2021;Espinoza et al 2021).…”

“…With both systems being reasonably bright for the JWST and emission flux reaching nearly 300 ppm, these planets are good candidates for follow up characterisation. K2-141 b is a well studied target (Barragán et al 2018;Malavolta et al 2018;Nguyen et al 2020, ;Zieba et al in submitted) and is even approved for JWST GO Cycle 1 program for over 20 hours with MIRI LRS (Dang et al 2021) and 12 hours with NIRSpec (Espinoza et al 2021). While TOI-2431 b has not been selected for the JWST observations, it may prove to be just as good of a target as K2-141 b for future proposals.…”

“…This is especially true for lava planets where the atmosphere is likely to be confined to the permanently irradiated dayside. Away from the substellar point, the sharp drop in temperature will induce an enormous pressure gradient, causing strong horizontal winds towards the nightside (Castan & Menou 2011;Kite et al 2016;Nguyen et al 2020). Material removed via winds is bound to condense at cooler regions, settling down onto the surface, where it may or may not be reincorporated into the outgassing cycle via surface/interior transport.…”

“…The effectiveness of this, however, is strongly dependant on the rapidity of material recycling in the interior (Kite et al 2016). Following this, Nguyen et al (2020); Ito & Ikoma (2021) have modelled evaporation-driven hydrodynamics with radiative-transfer and found that atmospheres dominated by SiO are much more likely to be stable against circulation and escape effects compared to ones dominated purely by Na. Current consensus is that silicate atmospheres may lack volatile Na and instead would be largely dominated by SiO or SiO 2 , both of which could be characterised through emission features.…”

“…The complete table of all shown planets with their parameters can be downloaded as supplementary material. (Schaefer & Fegley 2009;Kite et al 2016;Nguyen et al 2020), we extensively model outgassed equilibrium chemistry and atmospheric structure in order to investigate JWST observability for the shown short period rocky planets (Fig. 1).…”