The Kuiper belt is full of weirdness. In the "inner" solar system -- by which, for the purposes of this post, I mean everything inside Neptune's orbit -- there are lots of round moons, but they're all quite a bit smaller than their primary planets. Relative to its primary, the biggest moon is our own Moon, which has a quarter the diameter of Earth and about one percent of Earth's mass. Out in the Kuiper belt, though, relatively large satellites are common; there are several round worlds that, despite being quite a bit smaller than our own Moon, have moons that are also big enough to be round. In a new preprint, Mike Brown and Bryan Butler show evidence that two of those moons are even bigger than we used to think.

For context, consider the Pluto system. Charon is huge relative to Pluto -- half of Pluto's diameter, and one eighth of Pluto's mass. They're similar enough in size that the two clearly mutually orbit a point that is located outside either of them: