  The class of exotic Jupiter-mass planets that orbit very close to their
parent stars were not explicitly expected before their discovery. The recently
found transiting planet WASP-12b has a mass Mp = 1.4(+/-0.1) Jupiter masses
(MJ), a mean orbital distance of only 3.1 stellar radii (meaning it is subject
to intense tidal forces), and a period of 1.1 days. Its radius 1.79(+/- 0.09)
RJ is unexpectedly large and its orbital eccentricity 0.049(+/-0:015) is even
more surprising as such close orbits are in general quickly circularized. Here
we report an analysis of its properties, which reveals that the planet is
losing mass to its host star at a rate ~ 10^-7 MJ yr^-1. The planets surface is
distorted by the stars gravity and the light curve produced by its prolate
shape will differ by about ten per cent from that of a spherical planet. We
conclude that dissipation of the stars tidal perturbation in the planets
convective envelope provides the energy source for its large volume. We predict
up to 10mJy CO band-head (2.292 micron) emission from a tenuous disk around the
host star, made up of tidally stripped planetary gas. It may also contain a
detectable resonant super-Earth, as a hypothetical perturber that continually
stirs up WASP-12b's eccentricity.
