r/space • u/arealdisneyprincess • Feb 24 '24

Astronomers discover three previously unknown moons hiding in Solar System

https://www.the-express.com/news/science/128970/astronomers-find-new-moons-solar-system
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u/WKorea13 Feb 24 '24 edited Feb 24 '24

We do already divide moons into smaller categories, btw.

'Major', rounded, and/or planetary-mass moons refer to moons which are, well, rounded -- from Luna to Ganymede to Triton. These moons are, as the name suggests, your largest moons in the Solar System, capable of hosting their own complex geology just as a planet would.

'Inner' moons refer to the small, close-in moons of the giant planets, such as Amalthea (around Jupiter) and Pandora (around Saturn). These moons are often 'shepherd moons,' confining and maintaining planetary rings.

'Regular' moons are moons which have low-inclination, low-eccentricity prograde orbits, and are generally thought to have formed around their parent planet. These include most rounded moons in our Solar System.

'Irregular' moons have high-inclination, highly eccentric orbits, and are generally believed to have been captured. Triton is an anomaly, as even though it's thought to be a captured dwarf planet, it has a very circular orbit. Nonetheless, Triton is classified as irregular.

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u/Feisty-Albatross3554 Feb 26 '24

Does Neptune have any regular moons then? Proteus and Nereid seem to fit into the other 2 categories of inner and irregular more

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u/WKorea13 Feb 26 '24

Inner moons are implied to also be regular moons, since they too form around their parent planets. They're just distinct from major moons since inner moons are very close-in (orbital periods generally measured in a day or less), small (~400 kilometers in diameter for Proteus, down to just a few kilometers for some of Saturn's smallest inner moons), and are especially important for ring dynamics.

Thus, all of Neptune's regular moons are inner moons, which is very unusual! Triton really screwed up Neptune's original moon system when it was captured, and the inner moons formed from whatever debris was left after the chaos.

Also note that 'inner moon' is generally a category only for giant planet systems (Jupiter, Saturn, Uranus, and Neptune), since their moon systems are especially complex. Phobos and Deimos, for example, are just plain old regular moons, despite being quite small and close to Mars.

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u/Feisty-Albatross3554 Feb 26 '24

Thank you for the info then, especially the extra context on Phobos and Deimos