Last Week Jupiter Had 79 Moons. This Week It May Have 600 More That Go Backwards, Say Scientists

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A new study says that there may be as many as 600 small “irregular moons” orbiting Jupiter.

2018 saw the discovery of 12 new moons of Jupiter, bringing the known total to 79. Now a new paper published last week claims to have found many hundreds more.

However, these are a different kind of moon to Jupiter’s “big four” of Callisto, Ganymede, Europa and Io, all of which can easily be seen in the night sky right now.

What is an irregular moon?

An irregular moon is a minor object that used to orbit the Sun, but was captured by a giant planet early on in the Solar System’s history. Having swapped from being Sun-orbiting to planet-orbiting, they often have “eccentric” orbits; distant, inclined and retrograde—they orbit backwards.

A regular moon is one—like Earth’s moon—that evolved in orbit of a planet.

What did the researchers find?

The paper points out that only nine irregular moons of Jupiter were known before 1999, but since then the availability of CCD cameras has led to the discovery of many more. Before this research, Jupiter was known to have 71 irregular moons, 10 of them with normal orbits and 61 with retrograde orbits.

Their results will be presented at the virtual Europlanet Science Congress 2020 later this month.

How did they see 600 tiny irregular moons?

Actually, they didn’t. Instead, they inferred their existence. After searching, and digitally stacking, old 2010 images from the 340-megapixel MegaPrime camera at the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, they discovered 45 previously unknown irregular moons in a small area of the night sky around Jupiter and extrapolated their detections to 600.

That’s the number of irregular moons that they expect to be orbiting Jupiter.

How will these new moons be confirmed?

Ground-based telescopes will be required to confirm the existence of this huge new batch of irregular Jovian moons—both the 45 new detections and the 600 inferred irregular moons.

It’s hoped that the upcoming Vera C. Rubin Observatory will be able to confirm the findings.

How to see Jupiter’s giant ‘Galilean Moons’

All you need to do is look south right after dark to find Jupiter from the northern hemisphere. Then use any pair of binoculars, or a small telescope, and you’ll be able to see some of its huge Galilean moons; Ganymede, Europa, Callisto and Io. They’re an easy target—and an incredible sight!

Wishing you clear skies and wide eyes.

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