Scientists are puzzling over another oddball on the edge of the solar system: This time, it's an icy object less than a quarter of Pluto's size with a thin atmosphere – a layer of gas that's not typically found around objects so small.

A Japanese team of researchers — including an amateur astronomer — laid out the curious case of 2002 XV93 this week in the journal Nature Astronomy . 2002 XV93 traces an elliptical path beyond the orbit of Neptune in the icy Kuiper Belt, never coming closer to the sun than 3 billion miles. Like Pluto, it's locked in a resonance with Neptune that keeps its orbit relatively stable.

The Japanese astronomers, led by Ko Arimatsu of the National Astronomical Observatory of Japan, tracked the mini-world with several telescopes as it passed in front of a background star in January 2024.…