LAUSR

Description The destruction of a hypothetical moon may help explain the origin of both The angle between a planet’s equator and its orbit plane is known as its axial tilt or obliquity. As gas giants form from the disk made of gas and dust that swirls around the host star, the gas accretion process is expected to conserve the angular momentum and force the planet to spin perpendicular to its orbital plane. However, within our own Solar System, all gas giants, except for Jupiter, have a substantial nonzero tilt. This implies that something else must have happened to these planets after they formed that caused them to tilt. On page 1285 of this issue, Wisdom et al. (1) describe a model that can explain the origin of Saturn’s obliquity. Its tilt may have been caused by a process involving Saturn’s wobbling tilt (i.e., spin-axis precession), Neptune’s wobbling orbit (i.e., nodal precession), the elimination of a hypothetical satellite, and the outward migration of Titan—the largest moon of Saturn.