Friday, June 15, 2007

The Would-Be 10th Planet

Today, astronomers at the California Institute of Technology announced new measurements of a Pluto-like object in the outer solar system, which under different circumstances could have been identified as the 10th planet. The object, officially named Eris, belongs to the new class of "dwarf planets" that now includes Pluto. Earlier measurements had securely determined that Eris was physically larger than Pluto, which was one factor in the decision to demote Pluto from planethood and create the new category. The new observations show that Eris is also 27 percent more massive than Pluto -- making the would-be 10th planet the currently undisputed king of the dwarf planets.

In 2005, a tiny moon called Dysnomia was discovered in orbit around Eris. Using both the Hubble Space Telescope and the largest telescope in the world on top of Mauna Kea in Hawaii, the scientists measured the position of Dysnomia on six different nights as it traveled around Eris in a 15-day orbit. There is a simple relation between the size of a moon's orbit and the mass of the planet it circles, formulated in the 16th century by Johannes Kepler. This allowed the CalTech astronomers to calculate the mass of Eris directly from the observations.

This discovery adds further support to the reclassification of objects like Pluto and Eris as "dwarf planets" instead of traditional planets like the other 8 in our solar system. Without the new definitions, there would probably already be 8 Pluto-like planets for school children to memorize in the sequence after Neptune, with hundreds of others likely to be discovered in the coming years. Pluto has made its mark in history, but 50 years from now it will be as anonymous as a random chunk of rock in the asteroid belt.