New Moons: Our History of Solar System Satellites

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128. 128! That’s how many moons were added to Saturn’s total moon count this week. As it’s not every day that a planet nearly doubles its moon count (and Saturn already had the most known moons in the solar system), I thought it might be interesting to look at our history of discovering moons. 

So let’s travel back in time and see just how we’ve gone about finding moons out there and when. I’m going to go ahead and skip our own Moon—seeing as it’s kind of hard to miss, there was never a time when we didn’t know about that one.

Galileo’s Medicean Stars

For us to find out that moons besides our own existed took the invention of the telescope. Then it took someone having the bright idea of pointing that telescope not at things on the horizon but upwards at space. While no one knows who thought to do this first, the first person usually credited with using a telescope to make serious astronomical contributions was Galileo di Vincenzo Bonaiuti de Galilei, generally just called Galileo.

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On January 7, 1610, Galileo pointed his telescope at Jupiter and saw three “stars” immediately surrounding the planet. Over the next several nights he observed these bright objects moving relative to Jupiter, and on January 13 realized there were actually four of them. He named them the Medicean Stars for his patron Cosimo de Medici. Today we know them as Jupiter’s Galilean moons.

(In the name of historical fairness I should mention that these moons were discovered by a second person, Simon Marius, only a day after Galileo first spotted them. Then there was some confusion as to who actually observed them first because one of them was using the Julian calendar and the other was using the Gregorian calendar. While today Galileo gets all the credit, to this day we use the names for these moons originally given to them by Marius: Io, Europa, Ganymede, and Callisto).

The discovery of these moons got Galileo into a bit of hot water, since at the time most folks still liked to imagine that everything orbited the Earth and these things clearly orbited Jupiter instead. It was a wild idea for 1610. 

In recognition of Galileo’s contributions to Jovian science, in 1989 NASA named its first Jupiter-orbiting spacecraft Galileo in his honor. In an interesting twist, the spacecraft carried on its namesake’s legacy and made the first discovery of a moon orbiting something other than a planet when it spotted the tiny asteroid Dactyl orbiting the larger asteroid Ida on its way through the asteroid belt.

Saturn Gets in the Moon Game

Galileo’s “Medicean stars” spent decades as the only known moons besides our own. Then, in 1655, a Dutch astronomer named Christiaan Huygens found one going around Saturn. He called it simply Saturn’s Moon.

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It kept that designation even as Giovanni Cassini burst onto the scene in the 1670s and 1680s, discovering an additional four Saturnian moons. In fact, that first moon didn’t receive an actual name until 1847, when the total Saturnian moon count stood at seven and astronomer John Herschel named them (seeing as apparently nobody else had bothered).

Since Saturn itself was named for one of the mythical Titans, its first moons were named for other Titans. All, that is, except for that first moon that Huygens found, which was simply designated “Titan”. There’s something to be said for keeping it simple. 

When NASA decided to send its first spacecraft to orbit Saturn, it elected to honor Giovanni Cassini, and that spacecraft became my all-time beloved, Cassini. But Cassini carried another, smaller craft which became the first thing to land on a moon other than our own when it landed on Titan in 2005. That smaller craft was, naturally, designated Huygens. Finding a moon seems to be a great way to get a spacecraft named after you.

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Ice Giants

We didn’t even know Uranus existed until 1781, when it was discovered by John Herschel’s father, William (the Herschels were an extremely prolific astronomical family, with not only William and John but also William’s sister Caroline making major additions to astronomy. They got the Herschel Space Observatory named after them in 2009). William was also the one who found the first Uranian moons, Titania and Oberon. 

On September 23, 1846 Neptune was discovered (by who, precisely, is a matter for debate. The tale of Neptune’s discovery honestly deserves its own post), and remarkably it was a mere 17 days later that its first moon, Triton, was found by a guy named William Lasell. He would also go on to find a Saturn moon and two Uranus moons, because apparently he liked variety. No spacecraft for him though.

Wait, What About Mars?

Okay look, Mars’ two moons are dinky. They’re hard to spot, even though Mars is so close to us compared to the gas giants. By the time Asaph Hall found them in 1877 we already knew about four Jovian moons, seven Saturnian moons, four Uranian moons, and Triton. Tinier moons are just harder to find, which remains true to this day.

Jumping Ahead

As telescopes and computer technology improved, the number of known moons around the gas giants kept increasing, as well as the number of moons known to be orbiting things other than planets (as an honorable mention, Pluto’s big moon Charon was discovered in 1978, when Pluto was still classified as a planet).

Then the Voyager missions, launched in 1977, gave astronomers an up-close view of the realms of the gas giants and the number of known moons took a big leap. And then came Hubble. And Galileo. And Cassini. By the time I started at the Planetarium in 2011 we were teaching visiting school kids that Jupiter had 72 known moons, Saturn had 54, Uranus had 27, and Neptune had 13 (and Pluto had 3).

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Thanks to digital technology, in the last couple of years Uranus and Neptune have each seen their moon counts creep up a little, to 28 and 16. Pluto went up to 5. But when it comes to Jupiter and Saturn, them being both bigger (and therefore more able to hang on to captured moons) and closer, the numbers keep taking huge jumps and they kept swapping back and forth which had more known moons.

At least until May 2023, when the Minor Planet Center announced that astronomers using the Canada-France-Hawaii Telescope had found 64 previously unknown Saturnian moons, bringing its count up to 146. That was well ahead of Jupiter’s (then) 92.

Today Jupiter has 95. And now, as of the announcement of those 128 new Saturnian moons on Tuesday(also announced by the Minor Planet Center, also using the Canada-France-Hawaii Telescope), Saturn has 274. Including over 450 known moons orbiting things other than planets, our solar system’s total moon count now rests near 900.

So far. I don’t know about you but I feel like Jupiter is just raring for a moon count comeback. Stop by next year and see what I’m telling you the moon counts are then.

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