Even many years later, some astronomers want to revise the definition of a planet to clarify the parameters that set planets apart from other celestial objects. But why isn't Pluto considered a ...

In 2006, the International Astronomical Union (IAU) voted on the definition of a planet. Famously, Pluto no longer met the ...

“New Horizons shattered a major paradigm of planetary science,” says Alan Stern, the mission’s principal investigator. “Pluto turns out to have as much complexity as Mars or Earth, so much so that I ...

Clyde Tombaugh discovered Pluto in 1930 at Lowell Observatory in Flagstaff. Here's how Pluto won - and lost - its planetary ...

(Correction: An earlier version of this story misstated how far Pluto is from the sun. The correct number is 3.7 billion miles.) What was discovered in Flagstaff, Arizona, and killed off in Prague?

Pluto was reclassified from a planet to a dwarf planet by the International Astronomical Union because it did not meet all three criteria of their updated definition of a planet. According to the ...