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What Are the Compositions of Exoplanets?

Establishing what exoplanets are made of, and determining their physical structure, is a critical broad goal of research and it is very much at the boundary of what is achievable. The observations are challenging, and significant advances need to be made.

From mass and radius estimates we can get a density, and therefore a certain handle on the bulk (average composition), but only about 32% of exoplanets had both mass and radius measurements in August 2012 (see radius versus mass relation). Even for those exoplanets that do have a density estimate, in the regime of solid planets, there is considerable ambiguity and degeneracy. For example, Seager et al. 2007 show that carbon planets with silicate mantles and iron cores would be indistinguishable from silicate planets with small iron cores, even with only 2% uncertainty in mass and radius measurements. On the other hand, such a margin of error on the mass and radius measurements would provide much better constraints on composition than is currently possible: the uncertainty in the radius can sometimes be a factor of 2 or more, and most often only a lower limit on the mass can be obtained.

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For the gas giants, the hot Jupiters in particular, one thing that is certain is the Hydrogen and Helium dominating the composition. In giant planets that are further away from the host star, these elements and other compounds such as methane would be in the form of ices. Other elements and compounds have been detected in individual cases, such as sodium, water, and carbon dioxide, to name some of the more robust examples. More tentative claims have been also been made. Attempting to study possible atmospheres of rocky, terrestrial exoplanets involves additional challenges.

Is is also thought that gas giant planets have a rocky core. Altough this may be expected from the point of view of sedimentation of heavy elements to the core of a giant planet in the process of formation (simply due to gravity), it is not known whether this is actually the case. In fact we do not even know whether or own Jupiter has a rocky core, although it is thought that it has.

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