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| Titel |
Methane planets and the mass-radius diagram |
| VerfasserIn |
Morris Podolak, Ravit Helled, Amit Levi |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091994
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| Publikation (Nr.) |
 EGU/EGU2014-6314.pdf |
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| Zusammenfassung |
| The multitude of newly discovered exoplanets are too far away to be studied in the same
detail as the planets of our own solar system. Many planets have measured masses and radii,
and their mean densities can be compared to those expected for different simple compositions
(see, e.g. Seager et al. 2007). Clearly, different mixtures of materials can give similar density
distributions and as a result, the mass and radius of a planet do not give a unique composition.
It turns out that even if we limit the composition to one species, the mass-radius relation can
show complex structure. To illustrate this, we consider planets composed of pure
CH4.
The complications arise because CH4 is expected to undergo dissociation at high
pressure. Ab initio calculations (Gao et al. 2010) suggest that CH4 dissociates to C2H6,
C4H10, and finally carbon + hydrogen at progressively higher pressures. We have modeled
isothermal planets composed initially of pure CH4. We assume that if the planet is massive
enough so that the central pressure exceeds the dissociation pressure of CH4, a diamond core
is formed and the hydrogen released diffuses through the intermediate CH4 shell to form an
H2 atmosphere. This leads to a sharp discontinuity in the mass-radius relation for such
planets.
A further complication arises from the fact that within a narrow range around the
transition mass there can be multiple solutions ranging from a pure CH4 planet to those with
diamond cores, CH4 shells, and hydrogen atmospheres of different masses. Methane
planets thus provide an example of the instability first noted by Ramsey (1950)
and Lighthill (1950). As a result, even for a given composition the mass-radius
diagram is non-unique, making the characterization of extrasolar planets even more
challenging.
REFERENCES
Gao, G., Oganov, A. R., Wang, H., Li, P., Ma, Y., Cui, T., and Zou, G., 2010. Dissociation
of methane under high pressure. J. Chem. Phys., 133:144,508-1 - 144,508-5.
Lighthill, M. J., 1950. On the instability of small planetary cores (II). Mon. Not. RAS,
110:339.
Ramsey, W. H., 1950. On the instability of small planetary cores (I). Mon. Not. RAS,
110:325.
Seager, S., Kuchner, M., Hier-Majumder, C. A., and Militzer, B., 2007. Mass-radius
relationships for solid exoplanets. Astrophys. J., 669:1279-1297. |
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