Posted by Vicki Sieglen on 17 Aug 2006 at 2:15 pm.
Filed under Astronomy.
Currently in Prague, Czech Republic, The International Astronomical Union (IAU) is discussing a new definition for the word “planet”. By the end of next week the solar system may have three new planetary additions taking our current planet total from 9 planets to 12. The three additions to earn the crown planet-hood would be the largest asteroid Ceres (which orbits between Mars and Jupiter in the asteroid belt), Pluto’s “moon” Charon, and UB313 or Xena as named by its discoverer.
Ceres, back in the early 1800’s, was once considered a planet, but was later stripped of the title because of other bodies located with similar orbits found in the asteroid belt. Pluto’s “moon” Charon would receive planet-hood because Charon is roughly half the diameter of Pluto itself. That is why sometimes you may have heard Pluto and Charon referred to as the double planet. (When it comes to the Earth-Moon system, the Moon is approximately one third the size of Earth). As for UB313, its orbit lies further from the sun than Pluto’s, but UB313 is actually bigger than Pluto. In turn, the size of UB313 as led many people to argue that it to should be considered a planet.
The new planet definition has been in the works for 2 years by astronomers from around the world. The definition they are proposing is the following: a planet is any object that orbits a star that itself is not a moon of another planet or star, and is large enough that gravity will force it to hold a nearly spherical shape.
This new definition will bring about a new way of classifying planets. The planets that we are familiar with; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune will be called classical planets. Planets the are smaller than Mercury will be named dwarf planets. Then we have the class of planets call plutons; Pluto would serve as a prototype for this category. To fall into the plutons category a planet must have an orbit that takes longer than 200 years to go around the sun and hold a very elongated orbit. Its orbit must also be greatly tilted beyond that of the classical planets. As it turns out, Pluto, UB313, and Ceres fall into both the dwarf planet category as well as the pluton category.
So with this definition and classification system Pluto would indeed remain a planet; however, we would need to contemplate adding more bodies that could meet this criteria taking the number of planets to possibly over 60.
Alan Boss, an astronomer at Carnegie Hall of Washington, stated to Astronomy magazine:
What is the value of a definition when you add 50 bodies? I think it sort of cheapens the word ‘planet’.
This week the world’s astronomers will discuss this new definition of the word planet and are expected to make a vote on Thursday, August 24th.
