Pluto Still Not a Planet - Shouldn't Size Count?

Last November, the IAU - International Astronomical Union - came out with their long-awaited definition of what makes a Planet, and have determined that Pluto, the once coldest and most distant in our Sol system, is not a planet. What is more surprising, however, is that the same ruling also regulates Jupiter, Mars, Neptune and yes, even Earth as something other than planets.

The IAU definition of a planet is as follows:

"A celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit."

The original definition that was proposed did not contain the last line, which would've rendered asteroid Ceres, the newly named Eris (formerly known as 2003 UB313 or Xena), and Pluto's companion Charon all as planets, and leaving a dozen more distant objects waiting in the wings to earn their planetary status.

Well, we can't have school children memorizing thirty planets! That's preposterous! So, in all of their wisdom, about 435 of the 10,000 members of the IAU voted to approve the last line of new definition, which is, "and has cleared the neighborhood around its orbit."

Why was this added?

The simple answer is that the IAU did not want to have dozens of objects scarcely 500 miles in diameter classified a "planets." The general consensus was that a size limitation would be arbitrary, so to many the "neighborhood clearing" factor made sense. Not only did it eliminate many of the solar systems distant objects from the possibility of planet hood, it eliminated the largest planet, Jupiter, and also my favorite planet, Earth.

Pluto, according to the IAU, was automatically disqualified because its oblong orbit overlaps with that of Neptune. At the same time, all other Trans-Neptunian Objects (TNOs) were also eliminated because they orbit in a crowded neighborhood.

Pluto's companion, Charon is more than just a moon, together they are a true double planet.

Credit: Rev. Robert A. Vinciguerra

Copyright: Rev. Robert A. Vinciguerra