Saturn now reigns as the solar system’s “moon king,” thanks to 20 newfound moons. That brings the ringed planet’s total known satellites to 82, knocking Jupiter — with 79 moons (SN: 7/17/18) — off the throne, the International Astronomical Union’s Minor Planet Center announced October 7.
And it’s not just a phase. Saturn is likely to keep its title, says Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, D.C. He estimates that Saturn has about 100 moons — but the remaining ones are so small, under 1 kilometer across, that they’re hard to identify.
As it is, it took Sheppard and his colleagues years to confirm that some of the specks captured in images taken from 2004 to 2007 by the Subaru Telescope in Hawaii were, in fact, moons orbiting Saturn. By comparing the objects’ locations over time, the team found that three of the newfound moons are prograde, orbiting in the same direction that Saturn rotates, while 17 are retrograde, traveling in the opposite direction. Each is between 2 and 5 kilometers wide.
All but one of the newfound moons fall into three existing clusters of moons. Astronomers think that these groups formed when larger moons broke apart, either colliding with each other (SN: 5/21/18) or with a passing comet. But one prograde moon is an oddball: The angle of its axis’s tilt suggests that it belongs with other similar moons that fall within a two-year orbit. But it’s farther out among the retrogrades, where it circles Saturn in three years.
Something may have pulled this moon away from its cluster, Sheppard says. Or it could belong to a fourth group, created by some unknown event in Saturn’s formative years. Finding more moons may help solve that puzzle. But, Sheppard says, “if we want to find the smaller ones, we have to get bigger telescopes.”
Of Saturn’s 20 newfound moons, 17 are retrograde (red), meaning they orbit in the opposite direction that Saturn rotates. Three orbit in the same direction that Saturn spins. Two of those prograde moons (blue) orbit fairly close to the planet while one oddball (green) is farther out. Arrows denote the direction of orbit. The Carnegie Institution for Science is holding a contest to help name the new moons. To keep with the naming conventions of Saturn’s previously known moons, nominations must come from Inuit, Norse or Gallic mythology.