A passing star may have kicked the solar system's weirdest moons into place

A passing star may be responsible for more than three-fourths of the moons in our solar system as the stellar Traveler flung massive rocky bodies into our cosmic neighborhood, a new study suggests.

This novel model challenges existing notions of how the solar system came to look the way it does today.

The solar system's giant planets are famous for their many moons. Saturn currently leads, with 146 moons at last count, with Jupiter a close second at 95. A lot of these moons resemble Earth's moon in many ways. For example, they orbit their parent planets in the same direction as that of the planets' rotation. Additionally, such moons, called regular moons, follow nearly circular paths that share a plane with the planets' equators.

But some satellites are much stranger. Take Phoebe, one of Saturn's weirder moons. It has an oval-shaped, tilted orbit on which it moves in the opposite direction as Saturn rotates — a movement described as retrograde. In fact, satellites like Phoebe, also called irregular moons, outnumber regular ones in the solar system three to one.

Scientists have previously attributed the existence of irregular moons to the movement of Neptune across the solar system, according to William Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. Most astronomers think a critical step in the solar system's evolution was Neptune's migration outward through the precursor to the Kuiper Belt. Today, the belt extends between 30 and 50 times the distance between Earth and the sun, but in the early solar system, the proto-Kuiper Belt lay much closer to the sun.

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This destabilized the rocky bodies in the Kuiper Belt, sending most of them near the giant planets. From there, objects with certain orbits could be "captured" by the giant planets, Bottke told Live Science in an email.