Percival Lowell knew something else was out there. Based on his calculations, the American businessman and astronomer was convinced that an unknown ninth planet was responsible for the wobbling orbits of Uranus and Neptune. For more than a decade until his death in 1916, Lowell peered into the shroud of darkness from the observatory he founded in Flagstaff, Arizona, but he never could find his elusive “Planet X” on the edge of the solar system.
Then in the winter of 1930, as 24-year-old observatory assistant Clyde Tombaugh tediously compared photographs of a section of the night sky, he noticed a tiny speck of light on his plates had moved against the fixed background of stars. It was Planet X—right where Lowell calculated it would be. The Lowell Observatory announced its discovery of the ninth planet on March 13, 1930, the anniversary of its founder’s birth. At the suggestion of 11-year-old English schoolgirl Venetia Burney, the new planet was christened Pluto after the Roman god of the underworld, beating out other suggested names such as Minerva and Erebus.
As astronomers learned more about Pluto, however, it turned out that the solar system’s outermost planet was a celestial oddball. It had the most elliptical and tilted orbit of any planet. At the closest point on its 248-year transit of the sun, Pluto passed inside the orbit of the solar system’s eighth planet, Neptune. While at the time of discovery astronomers announced the distant planet “may be bigger than Jupiter,” Pluto turned out to be even smaller than Earth’s moon.
As more was learned about it, astronomers began to question whether Pluto had gained admission to the exclusive planetary club based on inflated credentials. Then in 1992, Massachusetts Institute of Technology scientists Jane Luu and David Jewitt discovered beyond Pluto’s orbit conclusive evidence of the Kuiper belt, a vast zone of debris left over from the formation of the solar system. Among the hundreds of celestial bodies orbiting the sun in the Kuiper belt were those similar in size and mass to Pluto. When Caltech astronomers led by Mike Brown discovered Eris, which had a greater mass the Pluto, in the Kuiper belt in 2005, it became clear that a change needed to be made to the membership of the solar system’s planetary club.
When the International Astronomical Union (IAU) gathered in Prague in August 2006, the world’s top astronomical body considered a plan to expand the solar system to 12 planets with Pluto and its moon Charon, which is half its size, recognized as a twin planet. That measure was rejected, but with a show of hands on August 24, 2006, a majority of the IAU members present instead redefined a “planet” as a celestial body that orbits the sun, is generally spherical as a result of the force of its own gravity and must “clear the neighborhood around its orbit.” This third stipulation of the new planetary definition proved Pluto’s downfall as it lacked sufficient mass to affect the orbits of Uranus and Neptune.
Pluto’s brief life as a planet was over, dead at age 76.
Along with Eris and Ceres, an asteroid between Mars and Jupiter, Pluto was reclassified as a “dwarf planet.” (In 2008, the IAU also added Makemake and Haumea to its list of recognized dwarf planets.) The decision was hailed by some astronomers as “a triumph of science over sentiment,” noted the New York Times. However, many who had grown up in a nine-planet solar system and felt a special affinity for the awkward planet that shared a name with a Disney character disagreed with the new worlds’ order and voiced their displeasure.
A sidewalk model of Pluto that stood in front of the Smithsonian Castle in Washington, D.C., became a makeshift memorial. A small vase of flowers was placed at its base. Condolence cards emblazoned with “Rest in Peace” were taped to its side along with a note reading “We’ll Miss You” signed by the eight remaining planets. “Pluto will always be a planet in my heart,” scrawled one well-wisher. Vendors did a healthy business hawking T-shirts bearing the messages “Pluto was framed” and “Back in my day, Pluto was a planet.” The decision was so controversial that the Caltech astronomer Brown, who had discovered Eris, even wrote a book “How I Killed Pluto and Why It Had It Coming.”
The loss of Pluto’s planetary status continues to be jarring for those who grew up with nine planets emblazoned in their science textbooks and on their space-themed placemats. Those who believe Pluto was unceremoniously evicted from its planetary perch question the legitimacy of the IAU’s vote, pointing out that only one-tenth of the 2,700 scientists who attended the 2006 conference were present for the ballot taken on its closing day.
No matter its classification—planet or dwarf planet—Pluto continues to fascinate as has been evident after NASA’s New Horizons spacecraft passed within a distance of 7,800 miles of its surface last July. Launched in January 2006 when Pluto was still an official planet, New Horizons carried the ashes of Tombaugh, who died in 1997 at the age of 90. More than a year after its fly-by, New Horizons continues to send back amazing high-resolution photographs of Pluto as well as data that has revealed it to be more geologically active and dynamic than previously thought. Along with soaring water-ice mountains and plains of frozen nitrogen, Pluto has been found to sport a bright heart-shaped region just above its equator—a welcome find for Pluto lovers everywhere.