Planetary Update

Pluto (pictured) is no longer a proper planet but a dwarf planet. Former asteroid Ceres and former Kuiper Belt object "Xena" (2003 UB₃₁₃) join Pluto as dwarfs. Pluto's moon, Charon, which had been considered for planethood or dwarf-planethood, is back to being just a moon.

This is really big news. Pluto had been a planet for 76 years—since its discovery. Almost every person alive who made it to the third grade was taught that Pluto was the ninth planet. Now, as Yoda says, "You must unlearn what you have learned."

Next on the agenda: Giving 2003 UB₃₁₃ a real name that doesn't require parenthesis, quotation marks, or subscripts.


See also: The Debate Over Planethood (August 16)