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G. Wuchterl (MPE)
A hot Neptune is a hypothetical giant planet in an orbit close to its star (typically less than 1 AU) that has core and envelope masses similar to Uranus and Neptune. Hence the core mass is supercritical (typically above 10 earth masses) and the hydrogen and helium envelope-mass is per definition relatively small (typically an earth mass).
Because the core mass is supercritical, quasi-hydrostatic giant planet formation models predict envelope growth to large masses, if nebula gas is present. Hence a proto Neptune should accrete gas and rapidly grow to masses comparable to Jupiter's mass under such conditions. On the other hand, formation models for Vulcans --- that are also called hot Jupiters, like 51 Pegasi b or HD 209458 b --- often invoke orbital migration to transfer giant planets from their formation regions, beyond the snow-line, say, to their ultimately small orbital radii, after they accumulated their massive envelopes. A necessary condition for the invoked type-I/II migration to operate, is the presence of a significant nebula disk. Hence if planets migrate, they do so in the presence of a disk of considerable mass. In that case, they are expected to grow the large, Jupiter-mass envelopes that correspond to their supercritical cores. If migration is important in the formation of Vulcans they are expected to have large envelopes.
Hence I argue that the detection of a Hot Neptune would refute migration models for the formation of hot giant planets and is a critical observation to decide between migration models and in-situ formation models. The latter explain low-mass gaseous envelopes and Uranus/Neptune-like giant planets by a hydrodynamical accretion instability that limits the envelope mass.
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The author(s) of this abstract have provided an email address for comments about the abstract: wuchterl@mpe.mpg.de