In hot water: effects of temperature-dependent interiors on the radii of water-rich super-Earths

• Published in: Monthly notices of the Royal Astronomical Society Vol. 458; no. 2; pp. 1330 - 1344
• Main Authors: Thomas, Scott W., Madhusudhan, Nikku
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 11.05.2016
• Subjects: Atmospheric pressure; Error analysis; Extrasolar planets; Hot water; Mathematical models; Measurement errors; Surface pressure; Surface temperature; Temperature effects; Temperature gradient; Terrestrial planets; Water; Planets and satellites: interiors; Planets and satellites: general
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw321

Observational advancements are leading to increasingly precise measurements of super-Earth masses and radii. Such measurements are used in internal structure models to constrain interior compositions of super-Earths. It is now critically important to quantify the effect of various model assumptions on the predicted radii. In particular, models often neglect thermal effects, a choice justified by noting that the thermal expansion of a solid Earth-like planet is small. However, the thermal effects for water-rich interiors may be significant. We have systematically explored the extent to which thermal effects can influence the radii of water-rich super-Earths over a wide range of masses, surface temperatures, surface pressures and water mass fractions. We developed temperature-dependent internal structure models of water-rich super-Earths that include a comprehensive temperature-dependent water equation of state. We found that thermal effects induce significant changes in their radii. For example, for super-Earths with 10 per cent water by mass, the radius increases by up to 0.5 R⊕ when the surface temperature is increased from 300 to 1000 K, assuming a surface pressure of 100 bar and an adiabatic temperature gradient in the water layer. The increase is even larger at lower surface pressures and/or higher surface temperatures, while changing the water fraction makes only a marginal difference. These effects are comparable to current super-Earth radial measurement errors, which can be better than 0.1 R⊕. It is therefore important to ensure that the thermal behaviour of water is taken into account when interpreting super-Earth radii using internal structure models.