trans-Neptunian object

Beyond Neptune the solar system does not stop. It thins out into a broad ring of icy leftovers that NASA describes as a large doughnut shaped region extending far past Neptune's orbit. The main belt of it begins around 30 astronomical units from the Sun, an astronomical unit being the average distance from the Earth to the Sun, and runs out to roughly 50. A scattered outer portion reaches much further, close to 1,000 astronomical units. Objects out there are called Kuiper Belt objects or, in the wording many researchers prefer, trans-Neptunian objects.

Pluto is the largest and was the first to be found, in 1930. Eris, Makemake and Haumea lie out there too, along with Arrokoth, the most distant object a spacecraft has visited. NASA counts more than 2,000 trans-Neptunian objects catalogued so far and says that is a tiny fraction of what is thought to be out there. They are cold and mostly unaltered since the solar system formed, which is why finding more of them matters: each one is a sample of the material the planets were built from.