Everyone knows the Solar System, right? Sun at the center, followed by the four, rocky inner planets, the asteroid belt, the four outer gas giants with their moons, and then the Kuiper belt.
Sometimes Jupiter sends an asteroid headed our way, and sometimes Neptune sends a Kuiper belt object towards us. The ones Neptune sends us are the comets that we see, and the ones Jupiter sends us are asteroids. (Illustration not to scale.)
But many of these Kuiper belt objects are pretty large, such as Pluto and Eris, and there are a total of eight in particular of these trans-Neptunian objects that are particularly large.
But, of the big ones (more than 1,000 km in size) which one is the closest? We have many candidates. Pluto was very close for a while, even closer than Neptune for a bit. Eris is farther right now, but will someday also slide inside of Neptune’s orbit. Pluto is the brightest one in the sky, but Eris is the biggest and most massive of any we’ve discovered. So, enough already, which one is the closest?
Trick question! Neptune’s moon Triton is — from all the evidence — a captured Kuiper Belt object. At 2700 km in diameter, it is bigger than both Eris and Pluto, and appears to be more dense and massive than either of them as well. It has a thin atmosphere, and — unlike every other moon in the solar system — it revolves around its planet in the opposite direction from the planet’s rotation.
When Voyager 2 passed it in 1989, it took some fantastic pictures, including this composite mosaic which was constructed of its surface. As you can see, it’s loaded with cracks and fissures, indicating some violence that took place there.
Of all the large moons in the Solar System, this is the only one that we think is entirely unrelated to the planet that it orbits. Imagine that: an object from beyond Neptune, bigger, brighter, and closer than all the others, was captured by the gas giant! So, that makes it the closest of all the large trans-Neptunian objects. I’m sorry for the trickery, but isn’t that a neat bit of history about our Solar System?