A celestial body can be weird because of how it moves. Or how it looks. Or because it embodies mysteries no one can solve. Neptune’s moon Triton possesses all three qualities.
Triton is the most distant of the solar system’s major moons — and the word “major” is important. Satellites in our solar system naturally arrange themselves into two distinct families. The major group includes those that are spherical, have a hefty mass, and follow a round orbit, suggesting an original and ancient connection with their respective planet. They are all at least 1,500 miles (2,400 kilometers) in diameter. There are only seven of these: our Moon, Jupiter’s four Galilean satellites, Saturn’s Titan, and Neptune’s Triton. Some might include an additional eight medium-sized round moons — the ones at least 600 miles (1,000km) wide: Saturn’s Tethys, Dione, Rhea, and Iapetus, and Uranus’ Ariel, Umbriel, Titania, and Oberon.
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Obviously, then, the rest of the solar system’s 158 remaining moons (not counting those orbiting asteroids or Kuiper Belt objects) are of a very different class. Typically just 10 to 20 miles (16 to 32 km) across, they’re the street dogs of the neighborhood. All are irregularly shaped, often quite far from their host planet, and fly in lopsided orbits. They should perhaps be called “moonlets.” They’re all chunks of stone or ice pulled in and captured at some point by the planet’s gravity. In other words, they did not start off where they are now.
What’s weird is if a moon of one class has some characteristic of the other. Happily for those who prize normality, only two major moons misbehave. The first is our own, the nearest satellite to the Sun, for the simple reason that it does not orbit our equator. Instead, it ignores our planet’s tilt and circles us in the same plane in which the planets orbit the Sun. The other major oddball is the farthest major moon from the Sun, Triton. Although it’s enormous and enjoys a perfectly circular orbit, it flies around Neptune backward.
This alone proves it didn’t form near Neptune during the time when the planets were born. Instead, despite its size, it was captured. Almost identical in size to former-planet Pluto, it was apparently sucked in by Neptune’s tremendous gravity.
Figuring out how this scenario could result in such a circular and well-behaved orbit requires some mathematical gymnastics. The model that works best is that Triton was once a giant double-body roaming the outer solar system. Passing Neptune, Triton lost enough energy to be captured, while its companion gained energy and was expelled. Odd indeed. This pinball game made Triton the only major moon in our solar system that orbits clockwise, as seen from above all the planets’ north poles.
