A new exoplanet to test tide theories

Almost 400 extrasolar planets have been found to date (see PHYSICS TODAY, May 2009, page 46 ), but a new planet reported by Coel Hellier (Keele University) and colleagues stands out. Like many exoplanets, theirs, dubbed WASP-18b, is massive (10 times the mass of Jupiter) and has a small orbital radius (only 1/50th of Earth’s). But its orbital period of only 0.94 day is the shortest for any “hot Jupiter” yet observed. Moreover, its large mass and small orbit are predicted to cause the strongest tidal interactions of any known star–planet system. According to current theory, the tidal bulge that the planet raises on its host star exerts a torque that will drain angular momentum from the planet and cause it to spiral inward. (For more on tidal interactions, see Physics Today, August 2009, page 11 ). If the star’s tidal dissipation rate is comparable to what’s been measured for binary stars and for the gas giants in our own solar system, the infall will be quick: WASP-18b has less than a million years left in a lifetime, estimated from the age of its host star, of about a billion years. Over the next decade, WASP-18b’s death spiral should produce a measurable shift in the planet’s observed transit time. The absence of tidal decay—a notable possibility, given the rarity of finding a planet so close to the end of its life—would constitute direct evidence for a different class of tidal interactions in the host star and provide new constraints on models of stellar interiors. (C. Hellier et al., Nature 460 , 1098, 2009 http://dx.doi.org/10.1038/nature08245 .)