Using the W.M. Keck Observatory, astronomers investigated the long-predicted relationship between mass and spin for giant planets and brown dwarfs. Credit: NASA, ESA, Caltech Astronomers have long suspected that a planet's mass and its rotation speed are linked. In our own Solar System, Jupiter and Saturn provide striking examples. Despite their enormous size, both complete a full rotation in about 10 hours and account for a large share of the Solar System's total rotational energy. To test