Last week, a panel from the IAU proposed a new planetary definition, one that eases the requirements for a mass to gain the status of "planet".
Under the proposal, any spherical object that orbits the sun without being a satellite to another planet qualifies. And its that definition that would add three new planets to our current nine-planet solar system.
The first addition would be Ceres, the large asteroid that resides between Mars and Jupiter. The second would be Charon - Pluto's moon and the third, 2003 UB313, otherwise dubbed "Xena" by its discoverer, Michael Brown.
Historically, planets were defined as "non-fixed stars" and only planets were known (in addition to Earth) until 1781 when Uranus was discovered. Neptune came in 1846 and Pluto in 1930. But during the late 1700's Johann Titius noted the spacing of the planets - each is two times the distance from the Sun as its predecessor - and that left a curious gap between Mars and Jupiter. In the early 1800's, the Kupier asteroid belt was discovered as well as its largest inhabitants, Ceres, Pallas, Vesta and Juno.
By 1807, the list of planets had reached eleven: Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn and Herschel (the original name for Uranus, after its discoverer, used until the mid-1800's.
But Herschel had already noted the difference between the original "planets" and these new discoveries and though they had been added to the planetary list, they were dubbed Asteroids.
The fifth asteroid Astraea was discovered in 1845 and then just two years later, three more. By 1851 there were 15 asteroids and it wasn't long before the asteriods were demoted to minor planets, reducing our lineup to seven - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - until Pluto's discovery in 1930.
So, what about the three new additions?
While some scientists are excited about the possibilities under the new definition, many are concerned that it will open the door for an unending list of planet contenders. Even Michael Brown himself noted that besides Xena, he has 14 other discoveries that would qualify under the new definition. This controversy lead to a second proposal - to demote Pluto off the planetary list, leaving us with only eight.
What do you think? Keep Pluto? Add the others? The IAU is set to vote on August 24, 2006.
