The Bizarre Mysteries of Space

How Many Planets are Actually in Our Solar System?

Everyone learns that there are eight (or, prior to 2006, nine) planets in our solar system; there are even multiple mnemonic devices for remembering the names in the right order. But more recently, models following the movement of objects in our solar system have indicated that that fundamental understanding might be entirely wrong.

In 2005, astronomer Mike Brown of Caltech discovered an object larger than Pluto in the Kuiper Belt, which itself triggered a scientific revolution: since there were objects larger than Pluto beyond its orbit, the then-ninth planet was demoted to “dwarf-planet” status by the International Astronomical Union. Fast forward to 2016, and Brown came back with solid evidence of another planet beyond Pluto, this one the size of Neptune. The difficulty comes in, however, with just how far away the massive planet is: so distant that we effectively can’t see it. In fact, the way Brown and his colleagues discovered the planet is only through looking at models for the orbits of existing planets and known bodies in our solar system. As scientists look into how to prove or disprove the planet’s existence, a bigger question continues to loom: just how big is our solar system, and are there even more planet-sized objects, even further out, that we just can’t see?