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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.”