Pluto and the patterns of planetary discovery

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PLUTO AND THE PATTERNS OF PLANETARY DISCOVERY KENNETH W. KEMP THE PATTERNS OF PLANETARY DISCOVERY Only three timesin recordedhistory hasa major additionto the solar systembeen made- once in the eighteenthcentury, oncein the nineteenth,and oncein our own. But the histories of the first two discoverieswere very different from one another. This paper discusseswhether the third instanceof planetary discovery was more like the first or more like the second. William Herschel’s discovery of Uranus in 1781 was a natural result of his “review of the Heavens”and a tribute to his skill as an observer, but as a planetary discovery it was essentiallyfortuitious. Indeedsofar from Herschel’smind was eventhe possibilityof discoveringa new planet, that he at first believedonly that he had discovereda new comet. The history of the discovery of Neptune in 1846 is strikingly different. The impossibilityof working out a -------------------------

KennethW. Kemp receivedhis Ph.D. degreein Philosophy from the University of Notre Dame in 1984. He currently teachesphilosophyat the College of St. Thomasin St. Paul, MN. 19

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satisfactory theory of Uranus led John Couch Adams and Urbain JeanJosephLeverrier to predict the existenceof a new planet. JohannGottfried Galle found the new planet within 24 hours of his receipt of Leverrier’s requestto searchfor it. The discoveryof two major planetswithin a century could not help but raisethe questionwhethertherewere not yet more planetsout there, waiting to be discovered. Despite the fact that Neptune’sorbit was too poorly known to revealdiscrepanciesof the kind found in the earlier caseof Uranus, and despite the fact that severalanalysesof the theory of Uranus indicated substantialagreementbetween the theoretical and observed positionsof Uranus, by 1875the searchfor yet anotherplanet was underway. Basingtheir work usually on the residualdiscrepanciesin the orbit of Uranus, but supplementingit occasionallyby consideration of the non-random distribution of cometary aphelia or by analysis of Neptune’s orbit, a successionof astronomers,as well as a number of cranks, set out to find a trans-Neptunianplanet(Hoyt 1980,p.74). Most of the serious proposalswere followed by a searchfor the predictedplanet, but no successwas achieveduntil 1930. Then, fourteen years after the death of that most persistent of planet searchers, Percival Lowell, an astronomer at his observatory (Clyde Tombaugh)locatedPluto within about6” of whereLowell had predictedit would be found.

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UNSUCCESSFUL SEARCHES FOR A NINTH PLANET

If the story were to be stoppedthere, asit sometimesis, the discovery of Pluto would soundlike a close analogueof the discoveryof Neptune. And indeedit hasoften beentaken to be just that (e.g. Reaves 1951), most recently by Patrick Moore (1981), who arguesthat: If Lever-tierand Adamsare regardedas the discoverers of Neptune,then logically Lowell was the discovererof Pluto. But the earliest searcher,David Peck Todd, on the other hand, seemsto haveseenhis enterpriseas lying somewherein the middle ground. He wrote (Todd 1880): While the magnificent researchesof Leverrier and Adamson the perturbationsof Uranusare masterpieces of analytic skill, I felt they should not be taken as modelsin the presentinvestigation. . . . Theprovisional treatment of the residualsof Uranus was undertaken then as a preliminary to the proposedtelescopicsearch to determinewhetherthat searchwasworth undertaking; and if so, at what point approximatelyit was to begin. Here we have the intention to find a new planet, which is much more specificthan Herschel’smere interest in what was out there, but much lessspecific than Leverrier’s and Galle’s searchfor a particular object in a specific place. Todd later becamemore confident of the existenceof the object, despite an unsuccessful, albeit aborted, telescopic search in the

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interim. Forbes’ independent,but similar, work led Todd to write (Todd 1886): I say the trans-Neptunianplanet becauseI regard the evidenceof its existenceas well-founded, and further, because,sincethe time when I was engagedupon this search. . . as is well-known, the independentresearches in cometary perturbations by Professor Forbes conducted him to a result identical to my own, - a coincidencenot lightly to be setasideas pure accident. The most striking note of similarity betweenTodd’s work and that of Leverrier and Adams is that, despite Todd’s disclaimerand his hesitancyaboutthe reliability of his results, he did ventureto publisha setof orbital elementsfor his hypothetical planet. In this he was followed by virtually every subsequentinvestigator. Thesepredictedlocationsplayed an important role in telescopic searcheswhich followed each prediction. The most comprehensivesearch,in plan if not in execution, was that one undertakenby Todd himself. However, limited resourcesforced even him greatly to reducethe scopeof his project and further adversecircumstancesforced him to suspendeventhis more limited search. All subsequent searches,down to thoseconductedby the Lowell Observatory, were similarly focused. If one of these searcheshad been successful, the discovery of Pluto would have been, in interestingrespects,similar to that of Neptune- a mathematical predictionfollowed by a successfultelescopicsearch. But none of the searcheswas successfuland the caseof Percival Lowell is somewhatmore complicated. It was preciselythe elements

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of “complication” in the story that led to the discovery of Pluto. THE EFFORTS OF PERCIVAL LOWELL

Lowell’s work can be divided into two, chronologically overlapping,phases. The first of these,carried out from 1905 to 1907, was purely observational. At his Flagstaff observatory, Lowell initiated a photographicsurvey of the invariable plane, sweepinga band 5” wide aroundthe entire sky. Duplicate photographsof the sameregion, takenat intervalsof about two weeks, were superimposedand compared, first with a magnifyingglass,andlater with a modifiedHartmann comparator. Despiteits scope,the effort turned up nothing. (Due to its unusuallylarge inclination, Pluto had not beenwithin the areacoveredby Lowell’s searchin the period in question.) At about the same time as he began his observational program, Lowell initiatedanotherline of investigation,this one mathematical. Hoping to provide his observationalwork with somefocus, Lowell attemptedto calculatethe orbital elements of the unknown planet. At this point, Lowell saw himself explicitly in the tradition of the discoverersof Neptune, for in the memoir summarizingthis phaseof his project, he wrote (Lowell 1915,p.l): Ever sincecelestial mechanicsin the skillful handsof Leverrier andAdamsled to the world-amazeddiscovery of Neptune,a belief hasexistedbegottenof that success that still other planetslay beyond, only waiting to be

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found. Leverrier himself, with the farsight of genius, was firmly of this view, though unfortunately oversanguineof the happy date of its demonstration. In consequencesince his time many attempts have been madeto indicate the position of one or more of these unknowns, attempts for the most part of no scientific value becausenot founded on rigorous mathematical investigation. For so complicatedis the problem that all elementary meansof dealing with it only lead to error. The soleroadto any hopeof capturelies through the methodicalapproachof laboriousanalysis. But although the extensivephotographyundertakenas a result of the various predictions madeduring this period did include an image of Pluto, it was not recognized,and so the secondphaseof Lowell’s searchendedin failure. THE DISCOVERY OF PLUTO

Lowell died in 1916just as the secondphaseof his work was winding down, but he had takenprecautionsto assurethat his work would be continued. Not only hadhis commitmentto finding a trans-Neptunianplanetleft a strongimpressionon his staff at the Lowell Observatory,but in his will he left what he hopedwould be fundssufficient for the Observatoryto carry on with his projects. In 1929, after a delay during which the Observatory awaited improved instrumentation, the Lowell Observatory began a third search for the trans-Neptunian planet. Although the photographic survey undertaken in executionof this searchwas begunin Lowell’s favored (and as

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it turns out, the correct) region of the sky, the image of Pluto was once again overlookedand the survey proceededaround the entire ecliptic before, upon return to Lowell’s favored region, a trans-Neptunianplanetwas finally found. Unfortunately, however,althoughthe newplanetwas found within 6” of one of the two locationspredictedby Lowell (the discovery position was also remarkably close to one of the many predictedby W. H. Pickering)it was recognizedalmost immediately that, being so faint, the planet had to be much smaller than Lowell hadpredicted-- too small, it seemed,even to have producedthe effects on the basisof which its location had beenpredicted. This point was made forcefully by Yale astronomer E. W. Brown (1931), although the English astronomerA.C.D. Crommelin protested(1940) that: the proof of the puddingis in the eatingand the fact that the elementsof the orbit were predictedwith considerableprecisionoverrides a priori argumentsagainst the possibility of suchan achievement... . .. .

Brown’ s view prevailed. Indeed, evenastronomersat the Lowell Observatory,while continuingto supportthe claim that the planetwas Lowell’s, must havefelt the uncertaintyas well, for within monthsthey not only resumedthe examinationof the remainingphotographs,but extendedthe photographicsurvey to belts immediately surrounding those photographedin the initial survey. In assessingthe resumption of the search, however, the possibility of another motivation must not be overlooked. Fifty yearsafter his discovery, Tombaughwrote

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(1980): Pluto seemedto be a new kind of object. Were there other similar objects in the vast outer reachesof the solar system? There was one way to find out - a systematicthorough searchover very large areasof the sky. In the years sincethe discovery, generalscientific opinion has vacillated on the question of whether Lowell should be creditedwith predictingthe existenceof Pluto, or its location. Kourganoff (1941, p.60) undertooka point-by-point refutation of Brown’s argument, and concluded: The thesisof “pure chance”in regard to the discovery of Pluto is absolutely untenable. Pluto was “discovered” in 1915by Lowell and “rediscovered”in 1919 by Pickering before its “physical” discovery by Tombaughat the Lowell Observatory. But what strengthKourganoff’s work had was as a refutation of a refutation, andastronomersremaineduneasyaboutthe exceedinglysmall massof the planet. For Lowell’s predictions about the location of the new planet were based on a hypothetical planet six times as massiveas the Earth, while early post-discoveryanalysesof the perturbationsof Neptune and Uranusindicateda sizeof only one-halfto oneEarth mass (Jackson 1930; Nicholson & Mayall 1931). Observational data on Pluto itself could not support even as large a size as that; imagebrightnessand size indicatedonly about 0.1 Earth masses(Bower 1931). In the last twenty years, asperturbation

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analyses(Duncombeef al. 1968)havemanagedto work with valuesthat small, new observationalevidencehas forced still further reductionsin the estimates. The surface albedo and reflectanceof Pluto (Cruikshank 1976) supporteda value no larger than 0.004 Barth masses(Duncombe 1980). Finally, the 1978 discovery of Pluto’s satellite by U.S. Naval Observatory astronomers, which made possible the first gravitationaldeterminationof the planet’smassindependentof any putativeplanetaryperturbations,settledthe matter. Lowell ObservatoryhistorianWilliam Hoyt writes (1980, p.247): The discovery of Charon, and the consequent determination of Pluto’s very small mass, made it certainthat Plutocould not havebeenPercivalLowell’s predictedPlanetX. Elements for another trans-Ncptunian planet thus do occasionallycontinue to be proposedin order to eliminate remaining residualsin various planetary orbits (e.g. Brady 1972) and Moore recently (1981, p.486) went so far as to suggestthat Lowell’s planet may still await discovery. THE IRRELEVANCE OF THE MATHEMATICAL PREDICTION But, it shouldnot havetakenuntil the determinationof the massof Pluto to realizethat the discoveryof Pluto was in its most important respectsmore like the discoveryof Uranusthan like the discoveryof Neptune. Kourganoff (quotedin Reaves

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1951, p.59), who held the oppositeview, argued: In any great scientific discoverythere are many factors essentialto success. For the discovery of Pluto they were: 1. The verificationof the working hypothesis of the accuracyof the old observations[of Uranusand Neptune]. 2. The accuratetheory of Leverrier-Lowell which transformed the observations of Uranus, old andnew, into somethinguseful. 3. The careful searchfor the trans-Neptunian planet at the Lowell Observatory - the enthusiasmof PercivalLowell. Of these,it would be the first and second,the mathematical factors, which would makethe discoveryPluto similar to that of Neptune. The actual searchfor Neptunetook Galle all of a few hours. But it is preciselythe mathematicalfactors which, pace Kourganoff, turn out to be irrelevantto Pluto’s discovery. Twice the theoretical predictions had led to the photographingof Pluto, once in 1915and againin 1929,and twice inspection of the plates had failed to turn up the planet. Without a careful and non-trivial observationaleffort, the planet could not be found. The prediction was therefore insufficient for the discoveryof Pluto, But more significantly, the prediction, thoughit might have played some role in motivating the search, was not, strictly speaking, even necessary. Pluto was discovered only by photographing the entire invariable plane and by doing a careful and exhaustivesearchof the resulting plates, They

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could havecompletedtheir project and discoveredtheir planet without Lowell’s predictionsever having existed. Tombaugh has said that he would have found any trans-Neptunianplanet the size of Pluto out to a distanceof 60 A.U. (Tombaughand Moore 1980,p. 182). Larger planetscould havebeendetected at even greaterdistances. In this sense,the discovery is very much like Herschel’s and very little like Leverrier’s and Galle’s. This is not to say that the planet would have been discoveredwithout Percival Lowell. His personalenthusiam for the project won him the respectof his staff; this respect kept the project alive even after his death. Moreover, had it not beenfor his financial contribution, there would havebeen no observatory. But if Lowell had done all that he did (including displayinghis enthusiasmabout PlanetX), but had not madean actualprediction, is it not plausiblethat the search would havebeenconductedanyway? I think so. After all, the searchfor the planet was extendedeven to areasfar removed from the areawhere Lowell predictedthat it would be found. The influenceof the prediction itself, even on Lowell’s staff, was therefore limited. So in all honestyit must be recognized that his observatory could have said to him, as he once so uncharitably wrote to one of his computers (Hoyt 1980, p. 102): “I have now completed my investigation (I regret without being able to useany of your computations).”

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THE IMPORTANCE OF CAREFUL OBSERVATION But if Percival Lowell’s contribution was primarily charismaticand financial, where did the rest of the discovery come from? Kourganoff emphasizesthe “careful searchat the Lowell Observatory,” and indeedthat is right. The skill and carefulnesswhich a successfulsearchrequiredcan be seenby focusingon three featuresof the researchprogram. The first key to a successfulsearchwas the emphasison photographyat the oppositionpoint. Althoughimagesof Pluto canbe seenon Lowell Observatoryphotographsmadein 1929, they were not identified when examined. One reason (Tombaughand Moore 1980, p. 111) was that the plates . . . . . . were taken at a large angular distance from the oppositionpoint . . . Pluto was nearits stationarypoint and would not haveexhibiteda detectableshift without a longer interval of time betweenthe two platesof the pair. Moreover, the tangentialvector would havebeen too uncertainto indicatethat the body was beyond the orbit of Neptune. Second, the photographyitself was no mere mechanical process. Not only did it haveto be carefully planned,but pairs of plateshad to havethe samemagnitudelimit if they were to be comparedsuccessfully. Magnitude limit is a function not only of exposuretime but of climatic conditions. Prolonging exposuretime to compensatefor climatic variation enabled Clyde Tombaugh to save many plates and avoid much repetition. His proficiencyin sodoingcontributedsignificantly to the expeditiousexecutionof the search.

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The final elementof the searchis the examinationof the plates themselves. This is much more time-consumingand difficult than the taking and processingof the plates “. . . even with the highly efficient meansof today . . .“, accordingto an early report co-authoredby Lowell astronomerV. M. Slipher (Putnam and Slipher 1932). The examinationof the plates which the searchproduced required individual inspectionof sometwo million star images. The discoveryplate itself, by no meansthe richestin star images,containedapproximatelya thousandstarsper squaredegree. Any but the most thorough examinationof the platescould (anddid!) result in overlooking the planet. It was only when the observatorycommitted itself to sucha search-- a searchin which weekswere devotedto the star-rich regionsof Gemini (Lowell’s favored region) alone-that the new planetwas finally found. CONCLUSION

Although the caseof Pluto differs from that of Uranus with regard to the motivation of the observer, it differs from the caseof Neptunein that mathematicalpredictionper se played no apparentrole in the actualdiscovery:not only did mathematical prediction prove to be insufficient, it evenproved not to be necessary.And this beingthe factor that makesthe Neptune story what it is, the story of Pluto must be regarded as fundamentallydifferent.

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Kenneth W. Kemp ACKNOWLEDGEMENTS

I wish to thank ProfessorMichael Crowe and Mr. Peter Wilson of the University of Notre Dame, and an anonymous reviewer, for their helpful commentson earlier drafts of this paper. While they may not agreewith all conclusionsof the paper, their commentsand objectionswere extremely helpful. REFERENCES Bower, E.C. 1931, “On the Orbit and Massof Pluto with an Ephemerisfor 1931-2”, Lick Obs’y Bull., 15, 171. Brady, J.L. 1972, “The Effect of a TransneptunianPlaneton Halley’s Comet”, PublAstr. Sot. Pad&, 84, 322. Brown, E.W. 1931, “Criterion for the Prediction of an Unknown Planet”, Monthly Notices of the Roy.Astron.Soc.,92, 80.

Crommelin, A.C.D. 1940, “Pluto, the Lowell Planet”, Joum. Brit.Astron.Assoc., 40, 265.

Cruikshank, D.P. et al. 1976, “Pluto: Evidencefor Methane Frost”, Science,194, 835. Duncombe,R.L. et al. 1968, “Orbit of Neptune& the Mass of Pluto”, Astron.J., 73, 830. Duncombe, R.L. 1980, “A History of the Determinationof Pluto’s Mass”, Icarus, 44, 12.

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Hoyt, W. 1980,PlanetsX & Pluto (Tucson:The University of Arizona Press). Jackson, J. 1930, “The Orbit of Neptune”, Mon.Not.Roy. Astron.Soc., 90, 728.

Lowell, P. 1915, “Memoir on a Trans-NeptunianPlanet”, Lowell Obs.iUem., 1, 1. Moore, P. 1981, “SomeThoughtson Planet‘X’“, JoumBrit. AstronAssoc., 91, 483.

Nicholson, S.B. & N.U.Mayall 1931, “Position, Orbit, & Massof Pluto”, Astrophys.J., 73, 1. Putnam, R.L. & V. M. Slipher 1932, “SearchingOut Pluto Lowell’s Trans-NeptunianPlanet X”, The Scientific Monthly, 34, 5.

Reaves,G. 1951, “Kourganoff’s Contributionsto the History of theDiscoveryof Pluto”, PublAstron.Sot. Pa@, 63, 49.

Todd, D.P. 1880, “Preliminary Account of a Speculative& Practical Search for a Trans-Neptunian Planet”, Amer.Journ. Sci., 20, 225. - - -

1886, “Telescopic Search for the Trans-Neptunian Planet”, Pm. Amer.Acad.Arts & Sci., 21, 228.

Tombaugh,C.W. & P. Moore 1980,Out of theDarkness: The PlanetPZuto(New York: New AmericanLibrary), 152.