Astronomers Say Farewell to Planet Pluto

By Linda Orlando

The International Astronomical Union has stripped Pluto of its title as a planet. Thursday in Prague, the group announced their decision, which capped off a week of heated arguments over what exactly makes a planet a planet. Scientists for centuries have clashed over the definition of what a planet is, so the new guidelines will help to classify things more distinctly. Unfortunately, despite its being considered a planet since its discovery in 1930, the new classifications have resulted in Pluto getting the boot.

The new guidelines created by the prestigious international group that makes such decisions spell out the basic tests that any celestial object will have to meet before it can be considered to be a planet. The new rules say that a planet must be "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." Poor little Pluto is automatically disqualified under this description, because its oblong orbit overlaps with Neptune's.

Pluto will henceforth be joining a new category of "dwarf planets," which are similar to what astronomers have long referred to as "minor planets." A third class of still lesser objects that orbit the sun will be called "small solar system bodies," which will be applied to asteroids, comets, and other natural satellites.

The proceedings were overseen by Jocelyn Bell Burnell, a specialist in neutron stars from Northern Ireland. Burnell comforted those who disagreed with the decision by waving a stuffed Pluto beneath an umbrella and saying, "It could be argued that we are creating an umbrella called ‘planet’ under which the dwarf planets exist." But Burnell’s attempt at humor didn’t sit well with those who disagreed with the decision, and there were many in that camp.

Just last week, the group of 2,500 astronomers from 75 countries proposed that Pluto remain a planet and its largest moon and two other objects should be welcomed into the planetary fold, giving our solar system two new planets instead of taking one away. That plan bit the dust pretty quickly, with astronomers split into factions that engaged in days of heated debates, which eventually led to Thursday’s resolution to demote Pluto.

Under the new guidelines, two of the objects that were at one time on the threshold of being named as planets will now join Pluto in the "dwarf planet" arena. The asteroid Ceres, which was a planet in the 1800s before it got demoted, and 2003 UB313, an icy object slightly larger than Pluto that was nicknamed "Xena" by its discoverer, Michael Brown of the California Institute of Technology, will now be called dwarf planets. Charon, the largest of Pluto’s three moons, has dropped out of the running for any special designation.

So the elite cosmic club in our solar system now consists of only eight planets, not nine. Brown was happy with the international group’s decision, saying that celestial bodies such as Pluto and the other dwarfs don’t’ deserve planet status, since calling them planets would "take the magic out of the solar system."

Earlier this year NASA sent up the New Horizons spacecraft on a 9.5-year journey to Pluto, to unearth its secrets and help us learn more about it. Had they known they were aiming at a soon-to-be outcast, wonder if they would have spent the money?