The Last Outpost

At the very fringes of the solar system is the "dwarf planet" Pluto. About as big as our Moon, it is visible only in large telescopes. It was found as a result of the search for a mysterious unknown planet, dubbed "Planet X," but it was almost certainly not what astronomers were seeking. Here's the story behind the search:

From ancient times, people knew five visible planets: Mercury, Venus, Mars, Jupiter and Saturn. They were seen only as special "stars" that wandered from constellation to constellation. In fact, that's what "planet" means: wanderer. Ancients didn't know the planets were worlds like our own. It took Galileo to show that, and Copernicus to put the Earth in third place. Then Uranus was discovered in 1789 -- by accident -- bringing to seven the number of planets in the Sun's family.

Astronomers added up the effects of the gravity of the Sun and the first six planets, and found to their surprise that these were not enough to predict the orbit of Uranus. An unknown planet must be responsible for the slight correction needed in the orbit of Uranus. They figured out where this planet had to be, looked, and to their delight, found a new planet. They named it Neptune. This was in 1846. Now there were eight known planets.

But as happened before, the gravity of the Sun and other seven planets didn't exactly predict the orbit of Neptune. They thought there had to be a planet beyond Neptune, and called this unknown planet "Planet X." Searching for this new planet, they looked at thousands of photographs of the predicted position. In 1930. they found Pluto.

Was this only a happy accident? Pluto is much too small to have any effect on Neptune's orbit. There seem to be many objects the size and composition of Pluto at the far reaches of the solar system.

A spacecraft dubbed "New Horizons" was launhed in January, 2006, headed for an encounter with Pluto in 2015. We will certainly learn more about Pluto then, but for most of us it will always be just "The Last Outpost."

(09/26/07)