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 \approx - 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 .