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What is a Hot Neptune?

The taxonomy of exoplanets is currently rather arbitrary and lacks concensus. While general concepts and motivations are clear, there are details that are only vaguely defined in the literature. In essence, a hot Neptune is a planet that has a mass and radius of the order of that of our own Neptune, but orbits the host star at distance that is less (often much less) than the Earth-Sun distance. (Our Neptune has a mass and radius of 17.1 and 3.9 times the corresponding quantities for Earth, respectively.) The term “hot” in this context means that the planet is expected to be much hotter than our own Neptune by virtue of the fact that the planet is much closer to the host star than our Neptune-Sun distance, which is about 30 astronomical units (AU). (Be sure to undertsand the caveats about exoplanet temperatures though.) A short star-planet distance implies a correspondingly short orbital period (by virtue of Kepler's 3rd law), of the order of days. Hot Neptunes have no counterpart in our solar system.

The Wikipedia page on the definition of a hot Neptune (on 7 August, 2012) gives only a vague defintion: “...an extrasolar planet...with a mass similar to that of Uranus or Neptune.” However, in the Kepler mission exoplanet candidates sample results paperBorucki et al. (2011, ApJ, 736, 19), state in the abstract that the authors refer “Neptune-size” planets, rather than “Neptune-mass” planets. Specifcally, they define Neptune-size to be 2 to 6 times the size of the Earth. Note that the composition of hot Neptunes is unlikely to be the same as our (cold) Neptune, and the physical state of the matter they are composed of is certainly going to be different. Currently there are many uncertainties and ambiguities involved in modeling the composition of exoplanets, and a detailed study of the Neptune-sized planet GJ 436b by Adams et al. (2008) showed several different physical scenarios were compatible with the data. However, all of the required a significant amount of Hydrogen and Helium.

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