Eight billion miles (14 billion kilometers) from Earth, at the solar system’s ragged edge, lies Eris — a planet-sized oddball of a world that emerged unexpectedly from the darkness 20 years ago.

The number of planets that orbit the sun depends on what you mean by “planet,” and that’s not so easy to define ...

And I just think it's the coolest world out there, that we know of. Brown discovered the dwarf planet Eris, a breakthrough that helped prompt a new definition of planethood. Eris is my favorite ...