Astronomers Looking for Planet Nine Discover 12 More Moons Orbiting Jupiter

Sometimes the best way to find something is by not looking for it at all.

Astronomers looking for Planet Nine—a celestial body predicted to orbit in the outer reaches of our solar system—stumbled upon 12 new moons orbiting Jupiter. By this latest count, our solar system’s largest planet now has 79 moons, more than any other.

Astronomers made the discoveries using the Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile and the Subaru telescope on Mauna Kea, Hawaii.

Europa, one of Jupiter's larger moons, casts a shadow on the planet's surface.

It took the team a while to make the required observations to confirm each moon’s existence. The smallest moon is just over a half-mile across, while the largest is about three miles in diameter. Some slipped in and out of view, complicating the task.
Here’s Ben Guarino, reporting for the Washington Post:

Jupiter’s moons range in size from shrimpy satellites to whopping space hulks. Galileo discovered the first four of Jupiter’s moons, all huge, in 1610. The largest Galilean moon, Ganymede, is bigger than the planet Mercury. Those moons orbit close to Jupiter and travel in the same direction as the planet spins.

The moons Sheppard spied are farther-flung and tiny, each no more than two miles in diameter. One moon detected by Sheppard and his colleagues is the smallest Jovian moon ever discovered. They named it Valetudo, after a daughter of Jupiter and the Roman goddess of hygiene and personal health.

The orbits of Jupiter's moons

Most of the newly discovered moons orbit opposite to Jupiter’s spin, what’s known as a retrograde orbit. But Valetudo, in addition to being the smallest discovered, orbits in prograde, or the same direction as the planet’s spin. That puts it on a possible collision course with a retrograde moon.

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