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Sunday, June 27, 2010

Pluto is not Planet X

Posted by Darin Hayton on 06/27 at 01:54 PM

This month’s issue of Sky & Telescope published a short article helping amateur astronomers see Pluto: “See Pluto in 2010.” Apparently, this “is one of the best years in Pluto’s 249-year orbit around the Sun to see the dwarf planet.” If you have a good 8-inch telescope gathering dust, download the charts and go look for Pluto, “the most distant chunk of solid material that can be seen through the eyepiece of normal backyard telescopes.” Sky & Telescope’s “See Pluto in 2010” is the impetus to dust off the review of Pluto literature that seems to have stalled a few months back

William Graves Hoyt, Planets X and Pluto

William Graves Hoyt’s book, Planets X and Pluto was published shortly after James Christy discovered Charon, which allowed astronomers to calculate more accurately Pluto’s mass, and long before other Kuiper Belt Objects started crowding the neighborhood. Consequently, unlike more recent literature that focuses on the debates about Pluto’s planetary status, Hoyt’s book concentrates on the early debates about whether or not Percival Lowell predicted Pluto. After a brief history of planetary discovery—Herschel’s discovery of Uranus and Leverrier’s & Adams’ discovery of Neptune—Hoyt turns to the efforts to find a ninth planet beyond Neptune.

As Hoyt points out, American astronomy was just emerging when Neptune was discovered. Harvard was installing the nation’s first large telescope. American astronomers, notably Benjamin Peirce at Harvard and Sears Cook Walker at the U.S. Coast Survey, were quick to compute elements for Neptune and then claimed that Leverrier and Adams had not predicted Neptune, that Neptune’s discovery by Galle was merely a happy accident. Other astronomers, including the American Benjamin Apthorp Gould, calculated Neptune’s elements and argued that Leverrier and Adams had predicted the planet. The core question here animates much of Hoyt’s book: What is the relationship between theoretical calculations and observation?

The central chapters of Planets X and Pluto follow Lowell’s efforts to calculate the location of Planet X. As early as 1902 Lowell seems to have been convinced that a ninth planet existed somewhere beyond Neptune. By 1905 Lowell was trying to calculate planet x’s position and, at the same time, he directed a photographic survey of the sky hoping to locate the planet. During the last decade of his life Lowell spent considerable time and effort trying to locate his elusive planet x. Although he approached his search using theoretical and observational tools, increasingly he seemed committed to his theoretical methods.

After he died in 1916, Lowell’s search for planet x stalled while his estate and his observatory tried to work out the details and priorities. After more than a decade, the search for planet x resumed. This time the search focused on a program of observation directed by Lowell’s last calculations. Famously, the Lowell Observatory hired Clyde Tombaugh to carry out the survey of the sky. Tombaugh’s key qualifications included his moderate but not extensive training and, consequently, his ability to follow directions. Vesto Slipher praised Tombaugh, claiming to have:

been negotiating with a young man of the self-made variety that we hope will be able to do the observing with the new telescope and do other assisting about the Observatory. … His training and schooling is limited to high school, but he has since done a good deal of reading in astronomy and seems to have helped himself to some considerable knowledge and expertise with instruments. … It seems to me that we would probably get more real assistance from the young man described above than we would from the highly trained variety for the reason that the latter care only to take up new pieces of work for themselves rather than help us with lines the Observatory has been doing.1

Just over a year after arriving in Flagstaff, Tombaugh discovered Pluto. He became, on 18 February 1929, “the only American to discover a major planet, an event which had occurred only twice before in history.”2 Together, these two quotations point to an important theme in the literature on Pluto: Pluto’s discovery by a “self-made” American.

The balance of Hoyt’s book looks at the early controversy around who deserves credit for having discovered Pluto. In particular, to what extent does Percival Lowell deserve credit for having calculated Pluto position. Typically, Lowell has received the lion’s share of credit—the astronomer Bart Bok even wrote in the foreword: “I feel full credit should go to Lowell and his collaborators for the discovery.”3 Even Tombaugh has claimed that Lowell deserves much of the credit. In the early decades following Pluto’s discovery, however, there seems to have been considerable debate about Lowell’s contribution to the discovery. Astronomers at the Lowell Observatory, naturally, stressed Lowell’s contribution and emphasized the close fit between his predictions and where Tombaugh ultimately found Pluto. Other astronomers were more divided. The English astronomer A.C.D. Crommelin and the American Henry Norris Russell claimed that Lowell’s predictions turned out to be too close to be accidental. Other astronomers, most vocally Ernest Brown from Yale, argued that given Pluto’s small size and mass, Lowell could not have predicted its position from Uranus’s perturbations. According to Brown, the fact that Tombaugh discovered Pluto close to where Lowell had predicted was, in the end, purely accidental. When Christy finally discovered Charon and astronomers could use it to calculate Pluto’s mass, it became clear that “Pluto could not be Percival Lowell’s predicted Planet X.”4 For Hoyt, this brings the controversy to a close: Lowell did not discover Pluto, Tombaugh did.

By focusing on debates about whether or not Lowell’s Planet X was Pluto, Hoyt’s Planets X and Pluto tells an interestingly different story. At the same time, it repeats some of the tropes common to this literature. Pluto, in this account, was discovered by a self-made, young American. Although Hoyt’s book predates the recent controversies over Pluto’s planetary status, it is clear that for Hoyt Pluto is bound up in a particular nationalist narrative and a validation of American astronomy.

Notes—
1Quoted in William Graves Hoyt, Planets X and Pluto, 179.
2Ibid., 193.
3Ibid., xii.
4Ibid., 247.

Tags: clyde tombaugh, neptune, percival lowell, pluto, uranus, william graves hoyt

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