If a gigantic water tank existed, and Jupiter and the six 'hot Jupiters' studied by Ibgui and colleagues1 were placed in it, Jupiter would be the only planet that would sink: in contrast to Jupiter, all six hot Jupiters have mass densities (shown in g cm−3) that are smaller than that of water (1 g cm−3). The diagram illustrates the point that many hot Jupiters are much larger, and hence less dense, than planetary evolution theory predicts. Note, however, that not all 'floating' planets are abnormally large, for example HAT-P-12b (ref. 1). The relative sizes of the planets are drawn to scale.