Discovery
After the death of Percival Lowell, the search for Planet X was paused while the family sorted out the affairs of the observatory and its owner. All work at Lowell Observatory remained on hold between 1916 and 1927 while Lowell’s widow, Constance Lowell, became involved in a lengthy court case involving her late husband’s will. By the spring of 1927, the legal battles were complete and Percival Lowell’s nephew, Roger Lowell Putnam pictured to the left, took over as sole trustee of the observatory. Putnam sought to restore his uncle’s search for a ninth planet and enlisted the help of V.M. Slipher, who was now the observatory’s director. The two shared several goals for this new phase of the search; first, they hoped that the discovery of a new planet would serve as an example of the important work started by Percival Lowell and help the observatory achieve a respected status amongst other astronomers. Second, they believed that planetary research would reinvigorate the spirit of the observatory, which had been dampened by the death of its founder. With these goals in mind, Putnam and Slipher turned their focus to the more practical matters of the search: finding the right equipment and crew.
In 1925, another member of the Lowell family, Guy Lowell, had purchased thirteen-inch glass discs from a fellow astronomer. The glass discs had no use at the time, but Putnam and Slipher decided that they could be used in the construction of a new telescope. They got in contact with Robert Lundin from Alvan Clark & Sons, whose company had worked with the observatory on several projects in the past, and contracted them to build the telescope with the thirteen-inch glass discs. Putnam and Slipher had also secured $10,000 from Percival Lowell’s brother, Abbott Lawrence Lowell, for equipment including that telescope and the dome to house it. To the right are images taken in 1928 during the construction of the dome. Below that image is a photograph of the completed telescope and dome taken several years later.
While the equipment necessary to continue the search was under construction at the observatory, one of the leading figures in the discovery was making telescopes of his own over nine hundred miles away in Kansas. Clyde Tombaugh had a passion for astronomy dating back to his teenage years. His Uncle Lee owned a three-inch telescope and introduced him to famous astronomers such as Galileo, William Herschel, and Percival Lowell. Tombaugh became inspired to build his own telescope in 1924 after he subscribed to the magazine Popular Astronomy. It took him nearly two years to complete his first eight-inch reflector and it was the first of several attempts. His next telescope was a seven-inch reflector he finished in 1927. He sold this telescope to his Uncle Lee and used the funds for his next project, a nine-inch telescope which is pictured below.
In 1928, Tombaugh read about Lowell Observatory’s research on Mars in an edition of Popular Astronomy and decided to send some of his own drawings to V.M. Slipher. Slipher and Tombaugh exchanged several letters about their research and interests in astronomy before V.M. Slipher offered a temporary position at the observatory to the young astronomer. They were in need of extra hands, especially in their renewed planetary expedition, and Tombaugh happily accepted the job. He arrived in Flagstaff on January 15, 1929 and began working in the administration building doing various tasks. Tombaugh shoveled coal, cleared snow, and gave tours to guests before he joined the astronomers in their pursuit of Planet X. He is pictured to the right in the entryway of the newly finished thirteen-inch telescope where he spent much time on the hunt for the new planet.
Examining plates was no easy task and neither was capturing an exposure of the night sky. The telescope domes were unheated to avoid damaging the exposures, which meant that Tombaugh spent long nights in the cold waiting to get the perfect images. Once the images were developed, he spent more hours at the blink comparator looking for the faintest movement. On January 21, 23, and 29 he photographed a portion of the sky around the star Delta Geminorum where Percival Lowell had predicted Planet X might be. Tombaugh started examining the plates almost three weeks later while working on other projects, and on February 18, 1930 he detected something that caught his attention. Around 4:00 p.m., after a long day of examining plates on the blink comparator, he noticed movement that he believed to be a possible planet and immediately started taking measurements and comparing other slides taken that week. Tombaugh then hurried to tell Dr. Carl Lampland and V.M. about what he had captured The slides he captured on January 23rd and 29th are pictured below. Planet X is much too small to see without zooming in on the image, which emphasizes the exceptional skill, patience, and attention to detail that Clyde Tombaugh possessed. Below the discovery plates is a scan of the envelope which held the plates. On the cover, Clyde Tombaugh details the conditions that the images were taken under and a brief account of the discovery from his own perspective. Tombaugh also lists other astronomical objects, such as asteroids, captured on the slide. At the very bottom Tombaugh eagerly exclaims "Planet 'X' (Pluto)— at last found!!!"
The staff at Lowell Observatory continued their examination of the object Clyde Tombaugh discovered to ensure that it was, in fact, Percival Lowell’s predicted planet. To the left, a letter dated February 23, 1930 from an unknown employee to Roger Putnam details what the young astronomer has captured, but the author states that further observations are necessary to confirm what has been found. The letter also reminds Putnam that astronomers years prior had believed they found Planet X, but had really captured an asteroid. For weeks, the observatory kept quiet about what Tombaugh had captured as they took more images and measurements, but by mid March they were ready to share their findings. Several conversations took place over what date to make the announcement before the team decided on Percival Lowell’s birthday, March 13th.The date was chosen as a tribute to the man who started the early calculations and had set the search in motion. On March 12, 1930, Roger Putnam sent a telegram to Harvard College Observatory revealing that the observatory had discovered the planet predicted by Percival Lowell. The telegram is pictured below and includes the object’s position and magnitude. Harvard made the announcement the following day, per Putnam’s request, and issued a card with the details of its location and potential orbit. The announcement card is featured to the left alongside Putnam’s telegram.
Below Putnam’s telegram and the Harvard announcement card is a document produced by an unknown staff member at the observatory who reflects on the discovery. They explain the delay in announcing their findings, but also share that they had received criticism for not sharing the news sooner. Despite the judgement, the author believes that the observatory had acted responsibly to determine the nature of their findings before making claims about what they had discovered. In addition to the calculations that were performed to determine Pluto’s orbit, size, and its distance from the sun, this document also sheds light on the public's initial response to the identification of a new planet. Click on the document to read all three pages of the account.
While conducting more observations and taking additional images of Planet X, the team at Lowell Observatory also found that images of the planet had unknowingly been taken in 1915 by Thomas Gill. A member of the first phase of the search, Gill had been taking routine scans of the sky using the Brashear Lens of the Swathmore telescope when he captured the earliest photographs of Pluto. Unfortunately, Gill did not detect the planet, nor did the rest of the team, and the plate was archived until astronomers dug it out fifteen years later for a comparison to the 1930 Planet X plates. To the left is a photograph of Thomas Gill at the Swathmore Telescope a year after he captured the image of Planet X. The image to its right is a scan of the 1915 plate with the earliest known image of the planet.
The discovery of Pluto forever changed Clyde Tombaugh’s life. He continued to study astronomy and earned both his bachelor’s and master’s degrees at the University of Kansas. Tombaugh taught at Northern Arizona University for a brief period during World War II before he relocated to New Mexico State University in Las Cruces. Over the span of his career, Tombaugh discovered hundreds of asteroids and became a leading figure in astronomical research. In 1980, he wrote the book Out of the Darkness: The Planet Pluto with fellow astronomer Patrick Moore that tells the story of the discovery and the other scientific research being conducted on Pluto at the time. The book’s cover is pictured below. Above this image is an audio interview with Tombaugh recorded in 1991 by a student at Northern Arizona University, Tanya Rae. The interview took place at the unveiling of Lowell Observatory’s Pluto Walk, which is a path with markers demonstrating the proportional distance between the sun and all the planets in the solar system. The two talk about the discovery, including the emotions that Tombaugh felt after realizing he had found Percival Lowell’s Planet X. He also shares details about the search process and what inspired him to study astronomy as a young boy. Below, a telegram from Clyde's parents Muron and Adella Tombaugh highlights the familial support that allowed the young astronomer to pursue his ambitions. From the young Kansas boy who built his own telescopes on his family’s farm to a professional astronomer credited with the discovery of the largest object in the Kuiper Belt, Tombaugh’s story is one that continues to inspire astronomers and non-astronomers alike.
