Irregular moons sometimes orbit backward

Jupiter, Saturn, Uranus and Neptune – the giant outer worlds in our sun’s family – all have many moons. Since the late 1990s, astronomers have found that most moons of these outer planets are ‘irregular moons,’ probably captured from heliocentric orbits.

Earth’s moon is what astronomers call a regular moon. It follows a small, nearly circular orbit, travelling around Earth in the same direction in which our planet rotates – that’s counterclockwise, when viewed from above the north pole.

Until recently, scientists thought that most moons in our solar system behaved like our own moon, but new data indicate that the majority are irregular moons.

David Jewitt: The irregular moons are distinguished as a group by having these large, very elliptical looping orbits that are tilted up with respect to the equator planes of their planets. In many cases, so tilted that they go around their planets backwards.

That’s astronomer David Jewitt at the University of Hawaii. In the past few years, he’s found nearly a hundred irregular moons orbiting planets in our solar system. Jewitt and other astronomers believe that these moons were once free-floating, sun-orbiting objects that were captured, somehow pulled into their present orbits as planetary satellites…

David Jewitt: We’re starting to get a clearer picture. We have a large enough number of irregular moons that we can begin to see patterns and systematics that tell us something about how that process occurred.

Special thanks today to Research Corporation, a foundation for the advancement of science.

Our thanks to:
David Jewitt
University of Hawaii