  We report the discovery of the transiting giant planet WASP-17b, the
least-dense planet currently known. It is 1.6 Saturn masses but 1.5-2 Jupiter
radii, giving a density of 6-14 per cent that of Jupiter. WASP-17b is in a
3.7-day orbit around a sub-solar metallicity, V = 11.6, F6 star. Preliminary
detection of the Rossiter-McLaughlin effect suggests that WASP-17b is in a
retrograde orbit (lambda ~ -150 deg), indicative of a violent history involving
planet-planet or star-planet scattering.
  WASP-17b's bloated radius could be due to tidal heating resulting from recent
or ongoing tidal circularisation of an eccentric orbit, such as the highly
eccentric orbits that typically result from scattering interactions. It will
thus be important to determine more precisely the current orbital eccentricity
by further high-precision radial velocity measurements or by timing the
secondary eclipse, both to reduce the uncertainty on the planet's radius and to
test tidal-heating models. Owing to its low surface gravity, WASP-17b's
atmosphere has the largest scale height of any known planet, making it a good
target for transmission spectroscopy.
