# Did we discover 10 or 12 new moons of Jupiter?

I saw multiple news sites reporting that a team discovered 12 new Jupiter's moons:

While some other news sites claim that they found only 10 new moons:

Why does the number of moons differ in the different articles? Did the team now found 10 or 12 moons?

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• It seems like you want more credible sources: here linked from NASA. I wouldn't take news from CNET, Discover, nature or earthsky as space news. I would find it directly from the source and work from there. – Magic Octopus Urn Jul 19 '18 at 12:23
• @called2voyage I was thinking it was something like that, but I only saw the prograde-oddball and that'd make 11/12 so I held my tongue after listing sources. Good catch. – Magic Octopus Urn Jul 19 '18 at 13:21
• @MagicOctopusUrn - You wouldn't take news from nature.com? Nature ranks amongst the best of scientific journals. – David Hammen Jul 19 '18 at 13:57
• @DavidHammen personally, no, I wouldn't without verification; but that's because, personally, I've never used them. Which, in turn, means my opinion about them is also without warrant. – Magic Octopus Urn Jul 19 '18 at 14:07
• In this case, the Nature article is in a sense more correct than the news release from Carnegie Science. Two of the twelve most recently discovered Jovian moons were reported a bit over a year. The remaining ten were reported yesterday. – David Hammen Jul 19 '18 at 14:13

Per the Carnegie Science article that Magic Octopus Urn linked from NASA in the comments, a Carnegie Science team led by Scott S. Sheppard noticed something new in spring of 2017 (though some observations occurred as early as 2016). It took a year to confirm the discovery of the new moons.

Ten of the moons orbit in the outer swarm of moons, which is one of the ways to divide the groups of moons. Nine of these follow the pattern of the other moons which orbit Jupiter in retrograde (the opposite direction of the planet's rotation). One they call "oddball" because it orbits prograde (the same direction as the planet's rotation) and has a more inclined orbit than the inner prograde moons.

Two of the moons orbit in the inner group, which is what brings the total number of discoveries up to twelve, as referenced in the first set of articles in your post. Like the other inner moons, these orbit prograde.

One of the retrograde moons and the "oddball" were first noticed in 2016. One of the inner group and the rest of the retrograde moons were discovered in 2017. The second inner group moon was discovered in 2018, then most were announced together on July 17, 2018. (Dates collected from Wikipedia.)

The retrograde moon discovered in 2016 and one of the retrograde moons discovered in 2017 were announced in 2017, this means only ten moons were announced on July 17, 2018, which is why the Nature article refers to ten new moons. The EarthSky article talks some about the ten and the twelve, and it does clarify both groups (divided by inner/outer and year of announcement).

• I wonder if the people at JPL are starting to regret their numbering scheme. The barycenter of the Jovian system has ID# 500, the Jovian moons are numbered 501 to 598 (not all taken, yet), and Jupiter itself is 599. Twenty new discoveries will blow that numbering system away. A collision between this oddball and one of the retrograde moons will completely blow it away. – David Hammen Jul 19 '18 at 14:38
• @DavidHammen When it breaks, I could see them hacking the system: $59\alpha$ or something like that. At least in the short term. – called2voyage Jul 19 '18 at 15:43
• @called2voyage they could switch to base16 and then base36. And when that breaks, maybe even a four-digit system. – John Dvorak Jul 19 '18 at 16:43
• @JohnDvorak Somehow I doubt we'll be tracking >~1300 objects at Jupiter under the current system, but you never know! – called2voyage Jul 19 '18 at 16:56