Posted by The object that turned out to be Pluto was found by someone looking for something else. Discovering Pluto was the result of a hunt for a giant planet that does not exist, carried out by a 24-year-old from a Kansas farm who had never been to university, using a machine that flicked between two photographs. It took another 85 years before anyone saw what the place actually looks like.
Percival Lowell’s ghost planet
The search began with Percival Lowell, a wealthy Bostonian who built his own observatory in Flagstaff, Arizona, in 1894. Lowell is better remembered today for his conviction that Mars was crisscrossed by artificial canals. He was also convinced that small irregularities in the orbit of Uranus pointed to an unseen ninth planet, which he called Planet X, and he spent the last decade of his life calculating where it should be and photographing the sky to find it.
He died in 1916 without success. The irregularities he was chasing later turned out to be largely an artefact of an overestimated mass for Neptune, corrected after Voyager 2 flew past it in 1989. So Planet X, as Lowell imagined it, was never there. That makes what happened next even odder.
A farm boy and a blink comparator
In 1929 the observatory restarted the search and needed someone patient. Clyde Tombaugh had sent the staff drawings of Jupiter and Mars made with a homemade telescope on his family’s farm, and they offered him a job. His task was grinding work: photograph the same patch of sky on nights a few days apart, then compare the glass plates.
The comparison was done with a blink comparator, a device that let the observer switch rapidly back and forth between two plates. Stars stay put. Anything in the Solar System shifts slightly and appears to jump. Each plate held tens of thousands of stars, sometimes hundreds of thousands near the Milky Way, and Tombaugh scanned them square by square for hours at a stretch.
On February 18, 1930, comparing plates taken on January 23 and 29 in the constellation Gemini, he saw a faint speck hop. The observatory spent weeks confirming the motion before announcing the find on March 13, which happened to be Lowell’s birthday and the anniversary of William Herschel’s discovery of Uranus. Tombaugh later went to university, studied astronomy properly and spent much of his career at New Mexico State University. He never stopped being slightly amused that he had found a planet before earning a degree.
An eleven-year-old names a world
The name came from Oxford. Venetia Burney, eleven years old, suggested Pluto over breakfast after her grandfather read the news aloud; he passed the idea to an astronomer friend, who cabled it to Arizona. The Roman god of the underworld fitted a dark, cold world, and the first two letters doubled as Percival Lowell’s initials. The staff voted, and Pluto won. Burney received a five-pound note as a reward and lived long enough to see a dust counter on New Horizons named after her.
Shrinking with every measurement
For decades, nobody really knew how big Pluto was. Early estimates assumed something roughly Earth-sized, in keeping with Lowell’s predictions. Each improved measurement made it smaller. By the 1950s astronomers suspected it was no bigger than Mars, and by the 1970s it was clearly smaller than the Moon. Some astronomers joked that, at that rate, Pluto would vanish altogether.
The breakthrough came in 1978. James Christy at the US Naval Observatory noticed that some images of Pluto showed a bump that came and went with a regular period. It was a moon, later named Charon, and its orbit finally allowed a reliable mass for Pluto. It turned out to be a tiny fraction of Earth’s, far too small to disturb Uranus. Pluto had been found because of a calculation that was wrong. Then luck helped again: between 1985 and 1990 the two bodies took turns passing in front of each other as seen from Earth, an alignment that happens only twice in each 248-year orbit, and timing those eclipses gave astronomers the first decent sizes for both.
In 1988 astronomers watched Pluto pass in front of a background star and saw the starlight fade gradually instead of blinking off, which revealed a thin atmosphere. The Hubble Space Telescope later picked out four more small moons: Nix and Hydra in 2005, Kerberos in 2011 and Styx in 2012.
The year Pluto lost its title
From 1992 onward astronomers began finding other icy bodies beyond Neptune, in what is now called the Kuiper Belt. Pluto was no longer alone out there, just the first one spotted. The crisis came with Eris, found in 2005 by a team led by Mike Brown and initially thought to be larger than Pluto (it is very close in size, and slightly more massive). Either both were planets, or neither was.
In August 2006, at its General Assembly in Prague, the International Astronomical Union voted on a definition: a planet must orbit the Sun, be massive enough to pull itself round, and have cleared its orbital neighbourhood of other material. Pluto fails the third test. It became a dwarf planet, a decision that still annoys a good part of the public and a fair number of planetary scientists, some of whom argue the definition says more about orbits than about what a world is actually like.
Nine and a half years to get there
Ironically, the spacecraft built to explore the ninth planet was already in flight when the vote happened. NASA launched New Horizons on January 19, 2006, on the fastest departure from Earth of any spacecraft at the time. A small container of Clyde Tombaugh’s ashes, who died in 1997, rode along. A gravity assist from Jupiter in February 2007 shaved years off the trip, and the probe then slept through most of the long cruise.
The flyby, on July 14, 2015, lasted only hours. New Horizons passed about 12,500 kilometres above the surface at roughly 14 kilometres per second, too fast to stop or orbit. Everything had been programmed in advance because a radio signal took about four and a half hours each way. For most of that day the spacecraft was silent, pointing its instruments at Pluto instead of its antenna at Earth, and the mission team in Maryland simply waited for a call home to confirm it had survived. It did.
What the first pictures showed
Nobody had predicted the heart. A pale, roughly heart-shaped plain dominated the images, and it was promptly named Tombaugh Regio. Its western lobe, a smooth basin of nitrogen ice now called Sputnik Planitia, had no visible craters at all, which meant the surface was young in geological terms. Nearby rose mountains of water ice several kilometres high. Charon, meanwhile, showed a vast canyon system and a dark reddish polar cap.
That is where the story of discovering Pluto hands over to a different one: the slow work of figuring out what all those images mean, which is still going on. For a place found by a mistake, it has turned out to be one of the most surprising worlds in the Solar System.
You can follow that next chapter, along with the history of other unlikely finds in astronomy, on SETIworld. There is plenty more out past Neptune worth reading about.
