This is a study of an exoplanet. Apparently, <i>a "hot Jupiter"</i> is shorthand for a general classification of planet type.<p><i>The planet at the center of the new study, which appears today in Nature Astronomy, is WASP-121b, a massive gas giant nearly twice the size of Jupiter. The planet is an ultrahot Jupiter and was discovered in 2015 orbiting a star about 850 light years from Earth.</i>
Why would a gaseous planet be tidally locked? Obviously a non-homogeneous solid object would settle with it's denser end "downward" toward the star, but I don't see how a gaseous planet would do so.