For me, its pretty simple. It is in orbit around a star. It has enough mass to have spun itself into a sphere.
And it has its own moons, icing on the cake. Its a planet.
So does that mean there are others? Who said “9” was the magic number? There might be more. Maybe we’ll get the chance to see some now that we’ve sailed on past Pluto.
Pluto was demoted because in 2007 the International Astronomical Union (IAU) had a little problem. Astronomers kept discovering more and more large objects floating in the Kuiper belt, the region of space beyond the planet Neptune. They have so far found over 1000 of these Kuiper Belt Objects (KBOs), including some rather large ones, such as Sedna, Quaoar, Orcus, Haumea, Makemake, and Eris. That last one, Eris, is actually larger than Pluto itself. So the number of planets in the solar system threatened to grow ever larger: 10 planets, 12, 13? Where do you stop?