Lava Worlds: From Early Earth to Exoplanets

Abstract

The magma ocean concept was first conceived to explain the geology of the Moon, but hemispherical or global oceans of silicate melt could be a widespread "lava world" phase of rocky planet accretion, and could persist on planets on short-period orbits around other stars. The formation and crystallization of magma oceans could be a defining stage in the assembly of a core, origin of a crust, initiation of tectonics, and formation of an atmosphere. The last decade has seen significant advances in our understanding of this phenomenon through analysis of terrestrial and extraterrestrial samples, planetary missions, and astronomical observations of exoplanets. This review describes the energetic basis of magma oceans and lava worlds and the lava lake analogs available for study on Earth and Io. It provides an overview of evidence for magma oceans throughout the Solar System and considers the factors that control the rocks these magma oceans leave behind. It describes research on theoretical and observed exoplanets that could host extant magma oceans and summarizes efforts to detect and characterize them. It reviews modeling of the evolution of magma oceans as a result of crystallization and evaporation, the interaction with the underlying solid mantle, and the effects of planetary rotation. The review also considers theoretical investigations on the formation of an atmosphere in concert with the magma ocean and in response to irradiation from the host star, and possible end-states. Finally, it describes needs and gaps in our knowledge and points to future opportunities with new planetary missions and space telescopes to identify and better characterize lava worlds around nearby stars.

Caught in the Act: Core-powered Mass-loss Predictions for Observing Atmospheric Escape

“…WASP-47 e, Kepler-78 b, Kepler-10 b, CoRot-7 b, and Kepler-36 b: These planets have been the focus of several theoretical studies that conjecture -in agreement with our analysisthat they should either have secondary atmospheres (and magma oceans) or/and low density interiors (e.g. Schaefer & Fegley 2009;Ito et al 2015;Dorn et al 2017b;Dai et al 2019;Chao et al 2020). Unfortunately, their host stars are much dimmer than 55 Cnc with V ∼ 11 to 12 mag (Léger et al 2009;Queloz et al 2009;Howard et al 2013;Becker et al 2015;Batalha et al 2011).…”

Characterisation of the hydrospheres of TRAPPIST-1 planets

“…Thus, a mixture of these gases should be considered to study the thermal structure of planets with secondary atmospheres. Planets b and c in the TRAPPIST-1 system could present magma oceans due to their high surface temperatures (T ≥ 1300 K) (Barr et al 2018;Chao et al 2020), and the maximum surface pressure we have obtained here can be used to assess the current outgassing rate in magma ocean studies (Noack et al (2017), Baumeister et al submitted) and better constrain the WMF for the interior magma ocean models (e.g Katyal et al (2020)) in the future.…”

“…The integrated model should also include a description escape processes, such as hydrodynamic or Jeans escapes, which is particularly interesting to explore the lifetime of secondary atmospheres. Close-in, low-mass planets are likely to outgas atmospheric species, such as CO 2 , and form O 2 via photodissociation of outgased H 2 O, during their magma ocean stage or due to plate tectonics (Chao et al 2020). Thus, a mixture of these gases should be considered to study the thermal structure of planets with secondary atmospheres.…”

TOI-2076 and TOI-1807: Two young, comoving planetary systems within 50 pc identified by TESS that are ideal candidates for further follow-up

“…The youth of TOI-1807 b makes it an even more compelling target for secondary eclipse spectroscopy, as the luminosity of the planet's cooling core may be an order of magnitude higher than it would be at older (> 1 Gyr) ages (Linder et al 2019). Finally, as a candidate "lava world" (Chao et al 2020), TOI-1807 b presents an opportunity to study the early evolution of these poorly-understood objects.…”