On 14 July 2015, the New Horizons spacecraft will make its closest approach to Pluto, passing within 6000 miles of the dwarf planet. As the piano-sized machine begins to stream high-resolution images of Pluto back to earth (which even the Hubble telescope perceives as a murky blob) I thought it would be a good to take a minute and consider the story of Pluto’s discovery in the early decades of the twentieth century.

At this time, Pluto was known as Planet X. Like most stories of discovery, the story of Planet X seems straight-forward at first, then gets more tangled the deeper one digs. It is worth disentangling. The story of Pluto reveals a bigger story about scientific discoveries and the difficulties of attributing credit.

Here’s the straight forward part. On 18 February 1930, Clyde Tombaugh sat in the Lowell Observatory and compared photographic plates taken of the same patch of sky on different days. He was looking for a misalignment of objects from plate to plate — something that would indicate the motion of a comet, asteroid, or planet against the backdrop of stationary stars. The density of stars on the plates made this a nightmarish task — a celestial Where’s Waldo with millions of objects to consider. Yet with the assistance of a blink comparator — a machine that strobes two images back and forth repeatedly — Tombaugh perceived a tiny object moving across the star field. He had discovered a distant planet circling the sun, one forty times more distant than the earth. 

This was Planet X. Since the discovery of Neptune in 1846, astronomers had searched excitedly for planets in more distant orbits. Much of this excitement grew out of the way Neptune had been discovered. In the year before it was sighted by Johann Gottfried Galle, Neptune had been predicted by Urbain Le Verrier based upon irregularities in the orbit of Uranus. Put simply, Uranus did not seem to be behaving in accordance with Newton’s laws of motion. At one point in its orbit, Uranus moving faster than predicted. At another point, it moved more slowly. The strange behavior could be explained, Le Verrier argued, by the existence of an planet beyond Uranus that exerted a gravitational pull upon the seventh planet. Le Verrier’s prediction proved correct.

As Uranus circles the sun on the interior orbit, the gravitational force of Neptune accelerates it at point a, then decelerates it at point b. These perturbations led Le Verrier to predict the existence of Neptune before it was observed in 1846.

This was the kind of discovery that brought astronomers to the edge of rapture. Finding Neptune did not arrive by luck or serendipity. It did not appear from some brute process of sorting and observation. It was predicted by the powers of human calculation. It became visible through Le Verrier’s feat of mathematical prediction. He had summoned it, and it had appeared. French physicist Francois Arago marveled at this. “He discovered a planet through the point of his pen.”

Inspired, astronomers began looking for irregularities in Neptune’s orbit as well. Meanwhile, others looked to the orbital radii of comets, which they believed might also point to the influence of a distant unknown planet. By the late 1800s, the astronomical community had become a roadside revival for the prediction of trans-Neptunian planets. As Morton Grosser points out in his 1964 Isis article “The Search for a Planet Beyond Neptune,” the quest for the trans-Neptunian planet “was a kind of celestial grail, and repeated failures to find it seemed to attract new searchers rather than to discourage those already seeking.” (It’s interesting to note that, at exactly the same time, polar explorers were approaching the North Pole with the same giddy attitude and language; see for example Elsa Barker’s 1908 poem “The Frozen Grail.”)

Percival Lowell

In 1915, Percival Lowell tried to weigh the merits of these multiple predictions, all of which were based upon different sets of observational evidence. The exercise was a daunting one, yet in working it out, Lowell seems to have crossed a threshold in his own thinking about his craft, one that makes him sound more like a philosopher of science than an astronomer hunting for planets.

The theory of a planet cannot in the nature of things be exact; and this for three reasons: 

1) The observations on which it is founded are necessarily more or less in error;

2) The theory itself may be more or less imperfect

3) An unknown body may be acting of which perforce no account has been given

Nevertheless, Lowell came down to earth long enough to make a prediction of his own. Planet X did exist. It could be located in a an orbit of forty-three astronomical units (where 1 au = distance between the sun and the earth). In mass, it would be twice as big as the earth. Lowell died in 1916 but the quest to find Planet X continued. When Tombaugh found the flickering spot of light in his blink comparator in 1930, it seemed to be vindication for Lowell’s prediction. When the name “Pluto” was offered by 11-year old Venetia Burney from Oxford England, it found approval at the Lowell Observatory. The name — representing the Roman god of the underworld — seemed suitable for a planet that was so cold, dark, and distant. Moreover, the symbol of the planet would be cast as ♇, which also functioned as a monogram for Percival Lowell.

Yet from the very beginning, Lowell’s status as discoverer was controversial. Astronomers noted that while Lowell’s prediction was in the neighborhood of Pluto’s position, it wasn’t an exact fit. Nor was it clear that Pluto was big enough to exert a gravitational effect upon Neptune big enough to explain the irregularities of Neptune’s orbit. In 1951, a paper by V. Kourganoff vindicated Lowell’s prediction, and there matters stood until 1978 when astronomer Robert Sutton Harrington of the US Naval Observatory determined that the mass of Pluto, at 1/500th the mass of the Earth, was too small to influence the orbital path of Neptune. Lovell’s prediction — through no fault of his own — fell short according to errors in observation, the first point in his 1915 article.

Accordingly, the discovery of Pluto did not follow in Neptune’s footsteps, because it was discovered as a matter of luck rather than of prediction. It seems that Tombaugh was looking at the right place, at the right time, but for the wrong reasons. So should Lowell be stricken from the record of Pluto’s discovery. Should we rename this icy dwarf planet according to other names proposed in 1930: Zeus, Minerva, or Cronus?

Then again, would Tombaugh even have been looking for Planet X if Lowell had not made such a persuasive case for finding it there? Certainly there was a degree of luck in finding Pluto. Yet, it was a discovery that also required powerful equipment, careful practice, and a dogged conviction that Lowell was right. In this, Pluto takes its place next to a number of scientific and geographical discoveries — from Columbus’s “discovery” of America” to Kepler’s search for a divine planetary arrangement. Unlike Neptune’s “discovery at the point of a pen,” perhaps Planet X’s epitaph should read “Look long enough and you will find it.”