The search for Pluto's replacement has begun. The astronomer leading the charge is Caltech's Mike Brown, also known as the man who killed Pluto. He has already discovered 16 dwarf planets in the outer solar system, but when it comes to real planets—bodies with enough mass to make themselves round, clear their neighborhood of debris, and orbit the Sun—there's only one new contender. And they call it Planet X.

"We have found evidence that there's a giant planet in the outer solar system," Brown says. "By 'giant' we mean the size of Neptune, and when we say 'outer solar system' we mean 10 to 20 times farther away than Pluto."

"We have found evidence that there's a giant planet in the outer solar system."