An orbital period of 0.94 days for the hot-Jupiter planet WASP-18b

The 'hot Jupiters' that abound in lists of known extrasolar planets are thought to have formed far from their host stars, but migrate inwards through interactions with the proto-planetary disk from which they were born1, 2, or by an alternative mechanism such as planet–planet scattering3. The hot Jupiters closest to their parent stars, at orbital distances of only 0.02 astronomical units, have strong tidal interactions4, 5, and systems such as OGLE-TR-56 have been suggested as tests of tidal dissipation theory6, 7. Here we report the discovery of planet WASP-18b with an orbital period of 0.94 days and a mass of ten Jupiter masses (10 MJup), resulting in a tidal interaction an order of magnitude stronger than that of planet OGLE-TR-56b. Under the assumption that the tidal-dissipation parameter Q of the host star is of the order of 106, as measured for Solar System bodies and binary stars and as often applied to extrasolar planets, WASP-18b will be spiralling inwards on a timescale less than a thousandth that of the lifetime of its host star. Therefore either WASP-18 is in a rare, exceptionally short-lived state, or the tidal dissipation in this system (and possibly other hot-Jupiter systems) must be much weaker than in the Solar System.