Planets on the Edge

Hot Jupiters formed through circularization of high-eccentricity orbits should be found at orbital separations $a$ exceeding $twice$ that of their Roche limit $a_{\rm R}$. Nevertheless, about a dozen giant planets have now been found well within this limit ($a_{\rm R}<a<2a_{\rm R}$), with one coming as close as 1.2$a_{\rm R}$. In this Letter, we show that orbital decay (starting beyond 2$a_{\rm R}$) driven by tidal dissipation in the star can naturally explain these objects. For a few systems (WASP-4 and 19), this explanation requires the linear reduction in convective tidal dissipation proposed originally by Zahn (1966, 1989) and verified by recent numerical simulations Penev et al. (2007), but rules out the quadratic prescription proposed by Goldreich & Nicholson (1977). Additionally, we find that WASP-19-like systems could potentially provide direct empirical constraints on tidal dissipation, as we could soon be able to measure their orbital decay through high precision transit timing measurements.

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