page_title: Geodynamics of terrestrial exoplanets
section_title: Scaling parameters
hierarchical_section_title: Geodynamics of terrestrial exoplanets / Methods of predicting exoplanet geodynamic regimes / Numerical modeling / Scaling parameters
caption_attribution_description: English: Size of Kepler Planet Candidates
context_page_description: The discovery of extrasolar Earth-sized planets has encouraged research into their potential for habitability. One of the generally agreed requirements for a life-sustaining planet is a mobile, fractured lithosphere cyclically recycled into a vigorously convecting mantle, in a process commonly known as plate tectonics. Plate tectonics provide a means of geochemical regulation of atmospheric particulates, as well as removal of carbon from the atmosphere. This prevents a “runaway greenhouse” effect that can result in inhospitable surface temperatures and vaporization of liquid surface water. Planetary scientists have not reached a consensus on whether Earth-like exoplanets have plate tectonics, but it is widely thought that the likelihood of plate tectonics on an Earth-like exoplanet is a function of planetary radius, initial temperature upon coalescence, insolation, and presence or absence of liquid-phase surface water.
context_section_description: Early models of rocky exoplanets scaled different factors (namely mantle viscosity, lithospheric yield strength, and planetary size) up and down to predict the geodynamic regime of an exoplanet with given parameters. Two scaling studies of exoplanet size published in 2007 came to fundamentally different conclusions: O’Neill and Lenardic (2007) showed that a planet of 1.1 Earth mass would have Earth-like lithospheric yield stress but reduced mantle driving stresses, resulting in a stagnant lid regime. Conversely, Valencia et al. (2007) concluded the increase in mantle velocity (driving force) is large compared to the gravitationally-forced increase of plate viscosity as planets increase beyond one Earth mass, increasing the likelihood of plate tectonics with planet size.
caption: Bar chart showing the size distribution of observed Kepler planet candidates (terrestrial exoplanets in the habitable zone of their host star). Data set is 2,740 planets orbiting 2,036 stars. The Earth-size and Super Earth-size (leftmost) columns represent potential terrestrial exoplanets.
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