Astronomers found a mysterious object beyond Neptune, could be new dwarf planet

Those are called TNO (Trans-Neptunian Objects).

https://en.wikipedia.org/wiki/Trans-Neptunian_object

Because their orbits are not on the same plane as other proper planets. Pluto (a former planet) is a TNO.

Earlier on source: https://news.ycombinator.com/item?id=50266046