Nearly every living person in the world today has only known a sky with nine planets. With the discovery of Sedna in 2003, it appeared that the age of double-digit planets was here.
Then they found hundreds of smaller objects in Sedna's neighborhood, the Kuiper Belt, including several more objects like Sedna, which itself was practically similar to Pluto. Hmm. How many planets do we really need here, after all?
This week's annual convention of the International Astronomical Union considered that question, and the answer they returned was, "eight." Not "ten," not "five-hundred and nine." Just eight.
Poor benighted Pluto. Its bad enough that there is a Disney cartoon dog named after it (and what the hell is Goofy? Save that for another post) Once riding high as the latest, if not greatest, planet to be discovered, Pluto has now been demoted. The IAU released a new definition of what constitutes a planet, and Pluto no longer fits the bill. The word "planet" originally designated a celestial body which moved against the static stellar background. Now it has been more narrowly defined.
The decision establishes three main categories of objects in our solar system.
Planets: The eight worlds from Mercury to Neptune.
Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite."
Small Solar System Bodies: All other objects orbiting the Sun.
Another competing definition of "planet" would have inadvertantly included in the largest asteroid Ceres, and so was rejected.
Enjoy our current eight-planet-ness while it lasts, I expect this to be overturned later when the other 99% of astronomers weigh in on this subject.
I'll end with a rare astronomy joke:
Q.How do you find Pluto with your telescope?
A.First look for the big arrow.