When did Pluto stop being a planet, and why? Pluto was always in a tough spot when it came to being a planet. Just 1,477 miles across, it's only one-fifth the diameter of Earth.

Though Pluto has formally been considered a dwarf planet for almost two decades, it still has many lessons left for planetary scientists — including hints about how the solar system formed.

I grew up when Pluto was a planet. But, as you no doubt know, 18 years ago, on August 24, ... A few people suggested I do a song about Pluto, and I thought it was a fine idea.

Pluto, officially a dwarf planet, has surprising complexity on its surface. While certainly a world, not everyone agrees it is a planet.

Pluto was discovered in 1930 in Arizona, but in 2006 scientists decided to cut Pluto from the planetary line up. Here is why Pluto isn't a planet.

Pluto, therefore, is not the gravitationally dominant object in its neighborhood — and thus, not a planet, according to the new definition. Sign up for the Live Science daily newsletter now ...

The whole thing is a bit silly, writes Dave Eicher. Is Pluto a planet? It's still out there, an intriguing, distant body in our solar system.