The location of the solar system's outer boundary is a point of contention among astronomers. There are three possible candidates, which "all have merit." But which one is best?

An illustration of the solar system (not to scale), including the sun, inner rocky planets, asteroid belt, the outer gassy planets, and—beyond Neptune—the Kuiper belt and the Oort cloud.

Many smaller outer solar system objects appear to have orbits that are aligned in certain mysterious ways; the simplest explanation is that their motions are influenced by another, as-yet-unseen ...

Now, however, Pluto and that part of the solar system has moved away from the backdrop of the Milky Way into a sparser region of the night sky. With its Hyper Suprime-Cam (HSC), Subaru has ...

Our Solar System is full of mysteries, but if the inner planets are a puzzle, the boundary between the Solar System and interstellar space is a secondhand puzzle with half the pieces missing.

A 9th planet in our solar system might be found — and no, it’s not Pluto. Is there another planet out there? Researchers say they have hints of a new candidate ...