February 18 marks the “birthday” of Pluto, a celestial body that has given astronomers great difficulty over the past 83 years.

A nameless mystery planet emerged as a theory in the 19th century, after astronomers discovered the planet Neptune, based on irregularities in the orbit of Uranus. Based on their mass estimates for Neptune, they decided that these irregularities had to be partially caused by gravitational interference from yet another celestial body.

This hypothetical celestial body was dubbed “Planet X” by Percival Lowell, a wealthy Boston man who established an observatory in Flagstaff, Arizona to search for it. The first 15 years of the 20th century were spent searching in vain. When Lowell died, a legal struggle tied up the observatory’s funding. On February 18, 1930, nearly a year after they began operating again, a young man named Clyde Tombaugh discovered the object.

The name “Pluto” was chosen in May 1930 by members of the observatory, inspiring the name of a new element, plutonium, and, possibly, the newly created canine companion to Walt Disney’s Mickey Mouse.

But since that auspicious start, the stature of this dark ball of rock, ice and frozen nitrogen has fallen steadily. In 1931, astronomers estimated it had the same mass as Earth. By 1948, that figure had been whittled down to one-tenth Earth’s mass. Another three decades and Pluto was one-100th the mass of Earth. Two years later, it was one-500th the mass. In the late 1980s, a NASA probe produced data on Neptune and its gravitational effect on Uranus that actually invalidated the theoretical need for a Planet X. Pluto’s discovery turned out to be, more or less, a fluke.

Most recently, Pluto lost its status as an official planet. Another celestial body was discovered in 2005 within our solar system, named Eris, that turned out to be slightly larger than Pluto. The debate over whether to call Eris a planet resulted in the drafting of a stricter definition for “planet” and the reclassification of Pluto as a “dwarf planet.”

In the U.S., the popular backlash to this decision was a bit of a shock. Astronomy is not typically the most celebrated of subjects in American popular culture. But some part of the national identity seemed to be bound up in Pluto. After all, hadn’t it been discovered by an American, in an American observatory? It’s a fascinating discussion, and Queens Library has accounts by both the astronomer whose discovery doomed Pluto to demotion, and by Neil DeGrasse Tyson, the witty astrophysicist and bestselling author who frequently breaks down the big questions of the universe for the common man.


So wish the solar system’s favorite dwarf planet a happy birthday. Who knows what the next decade will bring?