Jupiter Just Can’t Decide How Many Moons It Wants To Have

By catching or tossing comets or eating old moons, Jupiter’s orbiter-count is constantly in flux

Three of Jupiter’s moons, Callisto, Io, and Europa can be seen orbiting the gas giant.
Three of Jupiter’s moons, Callisto, Io, and Europa can be seen orbiting the gas giant. NASA Voyager

The solar system just can’t seem to hold steady on how many important things it has. Discounting for a moment the thousands of comets and asteroids that are presently whipping around the Sun and sticking only to the big stuff—planets and moons—the solar system head count has been sort of all over the place in the last little while.

In recent memory, the former-planet Pluto got downgraded to being a Kuiper Belt object. But then, astronomers poking around this non-planet-planetoid went and found that Pluto has five moons.

Pluto’s case was a matter of scientists coming up with equipment powerful enough to find a new moon that was already there. The case for Jupiter, however, says Troy McConaghy on his blog Outer Spacing, is a little bit different. Astronomers don’t just find new moons gripped by the gas giant; Jupiter actually acquires new moons. It also, sometimes, loses them.

The best-known example of a captured comet was Shoemaker-Levy 9. It was captured by Jupiter and was a moon for some 20–30 years (maybe longer), but its orbit wasn’t stable; it broke up into pieces in 1992, and it crashed into Jupiter in 1994.

On September 10, 2012, amateur astronomers spotted a bright flash on Jupiter. It’s thought it came from the collision of a small comet or asteroid. As such, it’s the fourth such impact reported since 2009.

The two moons closest to Jupiter, named Metis and Adrastea, are slowly getting closer to Jupiter and will eventually crash into Jupiter.

The current count is 67 moons, but there’s no knowing how long that’ll last.

More from Smithsonian.com:
Jupiter’s Temporary Moons
Picture of the Week—Jupiter’s New Spot

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