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Beyond Earthly Skies

Sizing Up Methane Planets

9 May 2015, 22:00 UTC
Sizing Up Methane Planets
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Exoplanets, especially those with ~5 to 50 times the mass of Earth, span a huge range of compositions. A study by Helled et al. (2015) from Tel-Aviv University investigates the mass-radius (M-R) relation for a class of planets known as methane (CH4) planets. Ideally, the M-R curve for a planet of a given composition tends to be smooth, whereby the planet’s radius gradually increases with mass. However, phase changes in the bulk composition of the planet as its mass increases can lead to discontinuities in the M-R curve.Figure 1: Artist’s impression of an exoplanet.For a pure methane planet, a phase change involving the dissociation of methane can cause the planet’s radius to increase abruptly. It happens when the planet is massive enough and the high pressure in the interior of the planet causes methane to dissociate into its constituents - hydrogen and carbon. The carbon remains in the planet’s core and the hydrogen, being lighter, diffuses to the outer envelope of the planet. This leads to a differentiated planet consisting of a carbon (diamond) core, a methane envelope and a thick hydrogen atmosphere. With such an interior structure, the planet’s radius is significantly larger than one without dissociation. This shows ...

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