Ask two different astronomers, and you’re likely to get two different answers! It depends on what you call a “moon,” but the latest official tally is now 69 – with two more discovered today!
The two new moons, given the catchy names S/2016 J 1 and S/2017 J 1, are only about a mile across. The small, rocky debris orbiting Jupiter really tests the limits of what we consider to be a moon.
Like most of Jupiter’s moons, these two are in retrograde orbits. That is, they orbit in the opposite direction of Jupiter’s own rotation, implying they were passing asteroids that got captured by Jupiter’s gravity, as opposed to bodies that formed along with Jupiter itself. Jupiter’s huge mass means it captures a lot of debris in our solar system, sometimes claiming them as new moons.
Many of Jupiter’s moons have been observed and never seen again, so we can’t really be sure about the total count.
These latest moons were discovered by astronomer Scott Sheppard of the Carnegie Institute for Science. They were actually looking for “Planet X” – a hypothetical large, unseen planet in the outer reaches of our solar system. A few frames happened to include Jupiter, and so they analyzed these just out of curiosity and made this discovery.
If you look through binoculars or a telescope at Jupiter, you’ll see the four large “Galilean” satellites shown above: Io, Europa, Ganymede, and Callisto – and each are fascinating in their own right.
Image credit: NASA / JPL