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Twelve New Moons Discovered Orbiting Jupiter

The moons were discovered serendipitously while astronomers hunted for the hypothetical Planet Nine.
Image: NASA/ESA/A.Simon

Scientists have discovered 12 previously undetected moons orbiting Jupiter, bringing the gas giant’s total moon count to 79—more than any other planet in the solar system. The tiny objects were pinpointed by a team led by Scott S. Sheppard, an astronomer at the Carnegie Institution for Science who has been involved in the discoveries of dozens of outer solar system moons and minor planets.

These moons range in diameter from under one kilometer (0.62 miles) to three kilometers (1.84 miles). They were detected by Sheppard and his colleagues while the team was scanning the outer solar system with the Blanco 4-meter telescope at Chile’s Cerro Tololo Inter-American Observatory, looking for signs of a hypothetical planet that is sometimes called “Planet Nine.”


The potential existence of this world, which is projected to be 10 times more massive than Earth, has been proposed to explain gravitational anomalies in the far solar system, some 200 times as distant from the Sun as Earth. While astronomers have not yet snagged a direct detection of Planet Nine, Sheppard’s team found these new Jovian moons as a byproduct of their search for the speculative planet.

Image: Roberto Molar-Candanosa, courtesy of Carnegie Institution for Science

"Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant solar system objects, so we were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our Solar System," said Sheppard in a statement.

Nine of the 12 moons follow a roughly two-year retrograde orbit around Jupiter, which means they travel in the opposite direction of the planet’s rotation. This suggests that the moons may be remnants of larger celestial bodies that have been torn apart by collisions with other moons, comets, asteroids, and generally errant space rocks.

Two of the remaining moons are closer to Jupiter, with orbital periods of around one year, and they have normal prograde orbits that match the planet’s rotation. The final moon is the “oddball,” according to the team, with a one-and-a-half year prograde orbit that transits through the outer retrograde group, which makes it prone to collisions. Sheppard’s team speculated that this moon, which is tentatively named after the Roman goddess Valetudo, may have once been a larger body, but has been gradually whittled down by clashes with other Jovian moons.

That makes the moon informally known as Valetudo an interesting target for further observation, as it gives astronomers an opportunity to potentially watch these potential collisions play out in the coming decades. The discovery of this abundance of new moons also demonstrates how little is known about the outer solar system—even when it comes to planetary heavyweights like Jupiter.

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