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Abstract: For planets other than Earth, interpretation of the composition and structuredepends largely on comparing the mass and radius with the composition expectedgiven their distance from the parent star. The composition implies amass-radius relation which relies heavily on equations of state calculated fromelectronic structure theory and measured experimentally on Earth. We lay out amethod for deriving and testing equations of state, and deduce mass-radius andmass-pressure relations for key materials whose equation of state is reasonablywell established, and for differentiated Fe-rock. We find that variations inthe equation of state, such as may arise when extrapolating from low pressuredata, can have significant effects on predicted mass- radius relations, and onplanetary pressure profiles. The relations are compared with the observedmasses and radii of planets and exoplanets. Kepler-10b is apparently -Earth-like,- likely with a proportionately larger core than Earth-s, nominally 2-3 ofthe mass of the planet. CoRoT-7b is consistent with a rocky mantle over anFe-based core which is likely to be proportionately smaller than Earth-s. GJ1214b lies between the mass-radius curves for H2O and CH4, suggesting an -icy-composition with a relatively large core or a relatively large proportion ofH2O. CoRoT-2b is less dense than the hydrogen relation, which could beexplained by an anomalously high degree of heating or by higher than assumedatmospheric opacity. HAT-P-2b is slightly denser than the mass-radius relationfor hydrogen, suggesting the presence of a significant amount of matter ofhigher atomic number. CoRoT-3b lies close to the hydrogen relation. Thepressure at the center of Kepler-10b is 1.5+1.2-1.0 TPa. The central pressurein CoRoT-7b is probably close to 0.8TPa, though may be up to 2TPa.



Author: Damian Swift, Jon Eggert, Damien Hicks, Sebastien Hamel, Kyle Caspersen, Eric Schwegler, Gilbert Collins, Nadine Nettelmann, Grae

Source: https://arxiv.org/



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