On the 23rd and 24th of January, 1930, a young astronomer working in Flagstaff, Arizona, scanned a small patch of the night sky. He was taking pictures of star positions, looking for anomalies that would signal movement somewhere at the edge of the solar system. He took the pictures then set them aside, not realizing that he’d found exactly what he was looking for: a ninth planet in the solar system.

In Search of Planets

The hunt for Pluto grew out of the search for other planets in the solar system. In the 18th Century, as the field of astronomy expanded with better and more powerful telescopes, astronomers began to scan the heavens to look for additional planets. Sir William Herschel had first observed Uranus, which was originally believed to be a star or comet, in March of 1781 as part of a larger movement by astronomers to discover new features in the solar system. After Uranus was formally identified as a planet, astronomers raced to identify others. In 1801, a new planet called Ceres was discovered in the gap between Mars and Jupiter. It was later downgraded to a new class of astronomical objects, an asteroid, but Ceres’ discovery helped prompt the prediction of other planets throughout the Solar System.

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As astronomers observed Uranus and its orbit, they noticed small variations: something large, and further out in the solar system, was affecting the gas giant. In 1821, astronomer Alexis Bouvard published a detailed table of Uranus’s orbit. Drawing from his data, a pair of astronomers, John Couch Adams from England and Urbain Le Verrier from France independently predicted the location of a new planet. Using the data provided by Le Verrier, a German astronomer named Johann Gottfried Galle began to search for this unknown body, and on September 23rd, 1846, he observed it near where it was predicted. Neptune was the furthest planet discovered thus far, but, shortly after its discovery, astronomers began to predict that other planets lay beyond in the far reaches of the Solar System.

To many astronomers, the orbits of Uranus and Neptune seemed to indicate the existence of another body—something altering their movements around the sun. Between 1877 and 1930, a number of theories were proposed. They predicted the existence of anywhere from one to five planets beyond Neptune. However, one astronomer, Jean Baptiste Aimable Gaillot, reran calculations on the planetary orbits, and determined that these theories were put forth in error. The astronomy community largely abandoned the idea of another planet beyond Neptune. However, there were lingering optimists. One such astronomer was American William H. Pickering, who predicted a Planet O far out beyond Neptune, as well as Planets P through U, all of which were debunked. Another American, Percival Lowell, theorized the existence of celestial body he called Planet X.

Planet X

Lowell was a businessman from the Cambridge area who had fostered a life-long interest in astronomy. In 1893, he began to put his interests to use, funding an observatory in Flagstaff, Arizona. In 1905, he began searching for the solar system’s 9th planet. He worked off of the original observations of Uranus and Neptune, and made assumptions based on the Titius-Bode rule, which described the relative positions of planets in the solar system. The assumption helped lead astronomers to Ceres in 1801, but had been of limited use with other planets. Using his astronomy, Lowell spent the rest of his life searching the skies for his quarry. He died in 1916, never discovering the planet he had spent years looking for.

Following Lowell’s death in 1916, his estate and the observatory that bore his name spend a number of years engaged in a legal battle involving Lowell’s widow, which halted the search for the planet at the observatory. When the matter was settled in 1927, Lowell’s nephew, Roger Lowell Putnam, resumed the search for his Uncle’s obsession: Planet X. In 1929, he brought aboard a young astronomer named Clyde William Tombaugh to aid in the search. Over the next couple of years, Tombaugh took over three hundred thousand images of the heavens. It was painstaking work: To discover moving objects, he had to compare each image to one another in sequence, to try and discover individual objects that moved from one point to another between nights.

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On February 18th, he examined two pictures that had been taken the previous month, and found his moving object: a faint bright point that jumped from picture to picture. The solar system’s ninth planet had been discovered.

The observatory announced the discovery to the world on March 13th, a date selected to coincide not only with Percival Lowell’s birthday, but the discovery of Uranus a century and a half earlier. Word of a new planet attracted the attention of the world, and the observatory used the opportunity to validate their founder’s theories, which to that point had not been taken entirely seriously. At the time of the announcement, the planet was unnamed, and recommendations were solicited. Over a thousand recommendations flooded in, and they quickly settled on Pluto, which had been suggested by 11-year-old Venetia Burney.

The discovery proved controversial. The planet had been observed several times in the past; Lowell, it turned out, had imaged it, himself. It seems Pluto had gone unrecognized, in part, due to its faintness in the night sky. What’s more, while Pluto was found more or less where estimates had predicted, those predictions had presupposed a larger, gaseous body (Lowell, for his part, had predicting a planet six times the mass of Earth), something akin to the other outer planets. Pluto seemed too small to exert significant force on its nearest orbital neighbors.

Over the coming decades, however, observatories began to monitor Pluto and revise its size as they took more detailed observations. With more powerful telescopes, astronomers in the 1950s and ‘60s determined that Pluto wasn’t dim, after all. It was, in fact, capable of reflecting a considerable amount of light; it just had a smaller mass than predicted.

In 1978, astronomers at the U.S. Naval Observatory discovered that Pluto wasn’t alone: it had a moon. Looking closely at photographs of the planet, astronomer James Christy discovered a bulge on the side that came and went. The low resolution of pictures meant that it was difficult to see. Named Charon, the moon was even smaller than its parent planet at 750-miles in diameter. The discovery allowed astronomers to measure the planet’s mass and revise the figure further to 0.2% that of Earth’s. Four additional moons were discovered decades later: Nix and Hydra in 2005, Kerberos in 2011, and Styx in 2012.

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Pluto has remained a curiosity at the edge of the Solar System. Both Voyager missions missed the planet while exiting the solar system, but with the activation of the Hubble Space Telescope, scientists had a new tool at their disposal to observe the planet. In 1990, the Hubble Space Telescope took the first images of the pair for the first time. At the same time, scientists began to discover new objects at the end of the solar system, now known as the Kuiper Belt.

What’s a Planet, Anyway?

It was the discovery of a new body beyond Pluto in 2005 that prompted a new discussion on the definition of objects in the solar system. Since 2003, scientists had been scanning the skies with new, computer programs that would make locating a planet a bit easier, and in between 2004 and 2005, several new objects were discovered: Haumea, Makemake, and Eris. Eris was found to be larger than Pluto, and initially thought to be a potential tenth planet.

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The question of classification sparked a debate, and the International Astronomical Union convened to settle the issue. Traditionally, the definition of objects in the solar system had shifted: rocky inner planets, larger outer gas giants and moons had been long known, but with the discovery of smaller rocky bodies in 1801, a new definition was needed, and asteroids were formally recognized as a new type of body in the system. While Pluto was initially described as a planet, its size was continually revised down, and it became clear that some of the solar system’s moons were much larger than it. The discovery of other bodies in the solar system around the same size suggested that a new classification was needed.

After much debate, the IAU came up with three criteria to classify a planet:

A planet:

  • is in orbit around the Sun,
  • has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
  • has “cleared the neighbourhood” around its orbit.

Pluto, the IAU determined, did not meet the classification because of its size relative to the objects around it: therefore, it fell into a new category: Dwarf Planet. While Pluto’s classification was changed, it wasn’t alone: Ceres was re-categorized as a Dwarf planet as well, and it’s thought that numerous other objects in the Kuiper Belt will meet the same classification.

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The reclassification of Pluto was met with considerable social outrage from civilians brought up to know Pluto as a planet, but also from fellow astronomers, who pointed out some flaws in the definition: planets such as Earth and Mars could technically be classified as ‘dwarf planets’. Additionally, state legislatures in New Mexico and Illinois passed resolutions proclaiming that Pluto would always be considered a planet. In 2008, a new definition was released, a ‘plutoid’, which described a trans-Neptunian dwarf planet.

New Horizons

In 1990, Astronomers began planning a mission to Pluto. But planners had trouble getting the project off the ground, and it was cancelled. However, by the mid-2000s, the project was underway, and after some delays, the New Horizons mission launched on January 19th, 2006.

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The mission was designed to fly out to Pluto and fly by at a close distance. At the speed required to reach the outer areas of the solar system, it would be impractical to slow, and an additional part of its mission would be to explore parts of the Kuiper Belt.

In December 2014, the spacecraft was reactivated, and over the next couple of months, began to beam back information at the rate of one kilobyte per second. Over the course of June and July 2015, the probe returned the first detailed pictures of the dwarf planet to date. It will pass by the planet on July 14th, before exploring the outer reaches of the solar system. Onboard, it carries the ashes of Clyde Tombaugh.

Image credits: Nasa, Wikipedia, US Naval Observatory.