Jupiter: 79 moons and counting

A dozen years ago, astronomers debated, “What is a planet?” They may soon have to wrangle another question: “What is a moon?”

On Tuesday, scientists led by Scott S. Sheppard of the Carnegie Institution for Science announced the discovery of a dozen moons around Jupiter, bringing the total number orbiting the solar system’s largest planet to 79.

As telescopes get better, astronomers will assuredly find more and more moons, smaller and smaller, around Jupiter and other giant planets orbiting the sun.

When the count rises into the hundreds, maybe thousands, scientists might start to wonder whether it is worth keeping track.

“We might have to start calling the ones that are less than a kilometre in size maybe ‘dwarf moons,’” Sheppard said. Insufficient size was what got Pluto demoted from planet to dwarf planet in 2007.

A moon is simply a rock in orbit around a planet, and there is currently no minimum size for something to be considered a moon.

Bigger moons like Europa and Ganymede typically orbit in the same direction as the rotation of their planet, in what astronomers call a prograde motion. That is not surprising, because these moons likely formed out of a disk of dust and gas that was spinning in the same direction as the planet as the solar system took form.

But the immense gravity of Jupiter could also capture other passing objects, and those space rocks sometimes end up orbiting the other way, in what is known as retrograde motion.