One winter morning in the first half of the 20th century, a 24-year-old nobody, an amateur astronomer from a tiny town in Illinois (who grew up in an even tinier town in Kansas) made a big splash at the Lowell Observatory in Flagstaff, Arizona. Using a 13-inch reflecting astrograph (photographic telescope), he assiduously searched the heavens for an elusive object thought to exist since the late 19th century. Finally, his efforts were rewarded. Two photographic plates taken six days apart were almost identical – except for a tiny pinprick of light that was clearly moving across the field of background stars. Comparison to an earlier plate and further photographs confirmed the discovery. On February 18, 1930, Clyde Tombaugh succeeded where the big guns – including Percival Lowell himself – had failed, and the news of the existence of a ninth planet in the solar system broke upon the world. On the suggestion of an 11-year-old girl from Oxford, England, the newly discovered planet was christened Pluto, after the Roman god of the underworld.
As one of the most mysterious objects in our solar system, Pluto has proven to be full of surprises. On June 22, 1978, astronomer James Christy of the U.S. Naval Observatory discovered that Pluto has a moon, christened Charon. Ten years later, Pluto was found to have a thin atmosphere which freezes and falls to the ground as Pluto moves in its elliptical orbit away from the Sun. A true color image of Pluto taken in 2001 shows that Clyde Tombaugh’s pinpoint of light is actually mostly brown. Only last May, the Hubble Space Telescope revealed the existence of two more moons orbiting Pluto, named Hydra and Nix.
Now, as Fox News reports, an uproar of controversy roils the scientific community in the wake of the recent discovery of a yet more distant planet, as astronomers debate what a planet really is, and whether Pluto is a planet at all. The new possible planet, currently nicknamed "Xena" and officially named 2003 UB313, also has a satellite and turns out to be even larger than Pluto, which is smaller than several moons in the solar system, including our own. In a world where a bright-line definition of "planet" is lacking, whether "Xena" is declared a planet may have implications for Pluto’s status. Should "Xena" be declared a planet? Should Pluto be demoted? If "Xena" is not declared a planet, why should the smaller Pluto retain planetary status? These are some of the questions scientists hope to resolve at the meeting of the International Astronomical Union in Prague over the next couple of weeks.
Of course, it’s always possible that even after the International Astronomical Union reaches its verdicts in Prague this month, new data may force yet another re-evaluation. On January 19th of this year, NASA launched the New Horizons unmanned spacecraft on its mission to Pluto, which it should reach in nine years. Pluto has not failed to deliver surprises over three quarters of a century, and New Horizons is sure to discover some of its best.
In the meantime, Pluto’s status has yet to be set in stone. There is precedent for demoting an object that once had planetary status: Ceres, discovered in 1801, was later reduced to asteroid (though this is being reconsidered at the Prague meeting). Personally, I hope this precedent won’t be followed in the case of Pluto. As a kid fascinated with astronomy, I grew to know and love Pluto as the ninth planet, even before we had any pictures more revealing than Tombaugh’s plates (and how bummed I was when I realized we wouldn’t get any pics from Voyager II). Besides, Pluto has three moons. If Ceres is re-promoted to planet, then Pluto will take its place as the tenth planet, making "Xena" a possible number 11. However this works out, I’ll be happy as long as Pluto gets to remain a planet, wherever it is in the pecking order.