 WASP @-@ 13b is a transiting planet with an estimated mass that is ( including limb darkening ) 0 @.@ <unk> times that of Jupiter and a radius that is 1 @.@ 389 times Jupiter 's radius . The planet is , in other words , less than half the mass of Jupiter , but slightly less than fourteen tenths its size . WASP @-@ 13b 's low mass can mostly likely be attributed to the presence of a low @-@ mass core , or to the total lack of a core , according to the discovery paper . WASP @-@ 13b , which orbits its host star at a distance of 0 @.@ <unk> AU , circles its star completely every 4 @.@ <unk> days . The 2011 study on the planet recognized WASP @-@ 13b as the fifth lowest @-@ density extrasolar planet known , behind Kepler @-@ <unk> ; WASP @-@ <unk> ; <unk> @-@ <unk> ; and <unk> @-@ <unk> . 
