Pluto Still a Mystery 75 Years Later

As Pluto Still a Mystery 75 Years Later, Pluto was only discovered fairly recently:

Pluto was quickly heralded as the ninth planet in the solar system when it was spotted Feb. 18, 1930 by Clyde Tombaugh, a young amateur astronomer at Lowell Observatory.

It's not clear that Pluto deserves the title of "planet":

But astronomers also have found about 1,000 other small icy objects beyond Neptune rotating around the sun. There may be as many as 100,000 of these bodies in what's called the Kuiper Belt, said Bob Millis, director of Lowell Observatory.

Pluto, with its elongated orbit and odd orbital plane, seems to behave more like other Kuiper Belt objects than other planets, some astronomers say. They also point out Pluto is very small, smaller than Earth's moon.