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by Kendrick Frazier Pluto turns out not to be responsible for peculiarities in the orbits of other outer planets. Is a tenth planet of the sun still to be found? 11 iwo occasions a few nights apart in May 1795, French astronomer Joseph Lalande charted the position of what he took to be a star. Observing that the star's position seemed to be different the second night, he concluded that his first observation had been a mistake. He moved the star's position on his chart and apparently never looked for it again. If he had, he might have discovered the planet N e p t u n e . Despite Lalande's erroneous conclusion, his precise observations of N e p t u n e ' s position in 1795 have become critical to succeeding generations of planetary astronomers. T h e orbital motion that he saw is part of the evidence for an additional massive planet in the solar system, out beyond N e p t u n e . By the early 20th century, astronomers of the stature of Percival Lowell and William H. Pickering were devoting their professional lives to the search for this ninth planet of the sun. Lowell called the body he sought Planet X; to Pickering, it was Planet O. T h e y sought it without success. Then, still more than half a c e n t u r y ago, astronomer Clyde Tombaugh, fresh off a Kansas farm, discovered the distant planet Pluto. For many years it was considered to be the missing mass whose gravitational attraction perturbs the orbits of the outer planets. N o w it is known that Pluto w o n ' t do. Everything learned about Pluto since T o m b a u g h ' s discovery has diminished its size; the effect of its gravity on the orbits of the outer planets would be all b u t nil. But while Pluto can no longer be considered a candidate, more and more sophisticated observations of the planets U r a n u s and MOSAIC September/October 1981 27 associate Robert Harrington say it is "reasonably certain" that something else is out there. They estimate that there is "only about a 5 percent probability" that the answer to the discrepancies they see in the planetary orbits is something other than a perturbing body. But there are skeptics. Planetary searches have considerable glamour attached to them, and enthusiasms can easily outpace realities. Scientists have learned from history to be cautious on such matters. At the Naval Observatory's Nautical Almanac Office, the division that carries out orbital calculations, director P. Kenneth Seidelmann finds the anomalies intriguing but won't commit himself to the idea that another large planet necessarily remains to be found. Recalling that in 1977 Charles T. Kowal discovered a minor planet—the asteroid named Chiron—in orbit between Saturn and Uranus, he would, he says, " b e amazed if there is not more material out there. But I'm not sure I'd expect to find a single, solid, large, additional object." Neptune in the past 50 years seem to confirm orbits perturbed as if by some massive body. T h e perturbations that led Lowell and Pickering to posit another planet tugging at the outer planets have not been explained away; all indicate something awry in the outer solar system. Lowell honored " A single undiscovered planet could entirely explain the observed irregularities in the orbits of each of the outer planets/ 7 astronomer T h o m a s C. Van Flandern of the U.S. Naval Observatory said early in 1981. He was announcing what he and his colleagues at the observatory are calling " t h e renewal of the trans-Neptunian planet search." They use the term " t r a n s - N e p t u n i a n " rather than " t r a n s - P l u t o n i a n " for several reasons. The n e w search is an explicit renewal of the efforts of Percival Lowell to find a planet of significant mass beyond N e p t u n e . There is also debate within the astronomical community about whether Pluto, considering its small size, should even be called a planet any longer; its mass is between that of the smallest k n o w n planet, Mercury, and large asteroids. Further, Pluto is temporarily not the most distant k n o w n planet; since 1979, when Pluto's strangely elliptical orbit carried it inside the orbit of Neptune, it has not been the planet farthest from the sun. N o r will it be again until 1999. Pluto or no Pluto, Van Flandern and his 28 MOSAIC September/October 1981 Some other astronomers have even more pronounced reservations. Brian G. M a r s d e n of the Harvard-Smithsonian Center for Astrophysics finds it all very tentative. "There's a lot of uncertainty about it," M a r s d e n cautions. T o m b a u g h himself is skeptical. He has examined 70 percent of the sky in his 14 years of searching for outer planets (13 of them after his discovery of Pluto), and he is certain he didn't overlook anything as bright as the massive planet the Naval Observatory people are talking about. " I would guarantee that within the area I examined there is no tenth planet," he states bluntly. T o m b a u g h acknowledges that such a planet could be in one of the two portions of the sky he didn't get to examine. O n e is in the northern sky, in a region 45 degrees north of the ecliptic. (The ecliptic is the plane of the orbit of earth around the sun. It is also roughly the orbital plane of most of the other planets.) T h e other area is the far southern sky. At declinations of more than 50 degrees south, that region is not visible from Lowell Observatory's latitude at Flagstaff, Arizona, where T o m b a u g h carried out his search. If any undiscovered planet is in those unexamined regions, it will be extremely difficult to find, says T o m baugh, because it is then likely to be right in the middle of very bright portions of the Milky W a y . Something awry Today astronomers have twice as long a span of observations of N e p t u n e as Lowell had for his calculations. They n o w k n o w that the best calculated orbits for all of the outer planets—Jupiter, Saturn, Uranus, Neptune, Pluto—fail to agree with observations to some degree. T h e n e w , lower estimate of Pluto's mass in the calculations complicates matters even further. Seidelmann and his g r o u p have prepared a series of charts c o m p a r i n g the observations and the predicted positions of both Uranus and Neptune. In just a 10-year period, since the early 1970s, the charts show a noticeable diversion from the straight line that indicates N e p t u n e ' s expected position. "It's running off the fit already," says Seidelmann. A n d , he adds, looking at a plot of Uranus's right ascension, " t h e r e seems to be something funny here t o o . " Pluto also appears to be showing such residuals, or deviations, although the 51-year duration of observations is too short to make too much of that. A variety of such plots all s h o w the same kinds of discrepancies. "Historically, everybody who has been able to fit the observations of Neptune in the past has failed to predict its positions in the future," says Seidelmann. The planets aren't the only solar system objects hinting at an u n c h a r t e d influence. Van Flandern points out apparent anomalous influences on six long-period comets. Of course, one has to be careful in making too much of comet-orbit problems. As comets approach the sun, gases previously locked up within them heat u p , expand, and are released like a small iet thruster, altering the comet's orbit from w h a t gravitational calculations predict. But, Seidelmann points out, this thrust effect is most prevalent nearest the sun, and it also generally acts in the plane of the comet's orbit. "For long-period comets there appears to be some sort of effect when they are a w a y from the sun, and it is an effect out of the orbital p l a n e / ' All in all, something does seem to be awry. The University of Texas's J. Derral Mulholland, a specialist in celestial mechanics, has followed this debate and the evidence closely. He thinks the N a v a l Observatory group is correct in saying that the perturbations are real and that they may be due to another planet. " M y feeling that they're likely right is based on what I see when I look at the best-constructed planetary orbits," he says. Those constructions, he notes, all s h o w discrepancies in the orbits of the outer planets. " T h a t is a hard observational fact," M u l holland adds. "Something is going on. There is no chance that these are artifacts. A b solutely no c h a n c e . " Several approaches Even granting the accuracy and reality of the residuals in the outer planets' orbits, that doesn't necessarily mean some undiscovered planet is responsible for them. Seidelmann, Van Flandern, and others point to the quest for the hypothetical planet Vulcan inside the orbit of Mercury. T h e existence of such a planet seemed to be indicated by the measurement in 1859 of an otherwise unexplained advance in the perihelion (closest approach to the sun) of M e r c u r y ' s orbit. Vulcan was searched for. Some observers even thought they had found it, but the observations were all due to defects in photographic plates or the effects of wishful thinking. M a r s d e n refers to the various claimed observations of Vulcan as "a particularly sorry episode in the history of astrono m y . " T h e perilhelion advance of Mercury remained unexplained until Einstein's general theory of relativity came along and successfully accounted for the discrepancy. T h e Naval O b s e r v a t o r y is approaching the observational problem from several directions. " W e are refitting all the observational data," Seidelmann says. " W e are taking the N e p t u n e data and trying to fit segments rather than the whole thing. W e are seeing if we can change the masses of the outer planets and solve the problem. We're trying to get a good fit of the observational data without another object. And we're trying to factor in improved knowledge from other sources." One exotic example of a possible new source of information is the Viking spacecraft circling Mars. Seidelmann and Van Flandern both point out that the Viking orbiter senses the center of mass of Mars with incredible prec i s i o n ^ a matter of several meters. As it goes around the planet, Viking can record the gravitational perturbation of Mars by Uranus and other bodies. This kind of analysis should lead to better figures for the masses and residuals in future calculations of outer planet orbits. " W e need a factor of three better precision in those masses than we have now," says Van Flandern. So far, Van Flandern adds, he and his g r o u p have tried all the easy approaches and failed. Calculations on the hypothetical planet's orbit and location are proving to be incredibly difficult. " W h i l e originally I t h o u g h t we might be able to predict its position to within 20 degrees, n o w I am fearful that the prediction may have an uncertainty of 30 to 50 degrees," he says. " T h a t ' s a big h u n k of the s k y . " But he adds, " A t any rate, it's now time to press the calculations as far as we can." T h e n if they get something they can call a prediction, the search can begin. T h e natural question to ask is: If there is such a planet out there, why hasn't it already been found? Van Flandern's and Harrington's best estimates are that the planet should be of two to five earth masses, have a highly elliptical orbit that takes it some 80 to 100 astronomical units (multiples of the distance from the earth to the sun) from the sun at m a x i m u m (Pluto's mean distance is about 39 astronomical units and maximum distance about 49), and be at a visual magnitude of from 15 to 17 (objects of magnitude 6 or lower are visible to the unaided eye). Pluto was at magnitude 15.5 at the time of T o m baugh's discovery, b u t his plates were generally sensitive to stars at least as faint as magnitude 17. Grounds for skepticism T o m b a u g h , w h o at 75 still puts in an afternoon of work each day at New Mexico State University in Las Cruces, has some very definite ideas on the subject. After he detected Pluto in 1930, the Lowell Observatory officials asked him to continue the search, with the same degree of thoroughness, in the hope of finding still more planets. He did so for the next 13 years, when he was interrupted by W o r l d War II. He carefully examined two-thirds of the sky. As he says now, "I didn't find a single thing." In a way, this negative result after the discovery of Pluto may be just as important as the discovery itself: It would seem to put some severe limits on the possibility of another outer planet of significant size. T o m b a u g h takes justifiable pride in the care with which he carried out the search. The estimated number of stars in the examined areas totaled 44,675,000, give or take a million, he says. C o u n t i n g two plates per scan, that's 90 million star images. "Every one of M A Q A i n Qontornhflr/AMnhor 1QA1 9Q tricity and high inclination." The inclination of Pluto, the most highly inclined known planet, is 17 degrees. So perhaps an undiscovered planet could reside in the areas of the sky, far from the ecliptic, which Tombaugh did not examine. N o t only do the calculations seem to point to a planet with a highly inclined orbit, but there is still another line of evidence, an exotic theory proposed by Harrington and Van Flandern, that also seems to them to point well out of the plane of the ecliptic. H a r r i n g t o n and Van Flandern point out that the innermost of Neptune's two moons, Triton, is the only large, relatively close satellite in the solar system having a retrograde orbit and so highly inclined (160 degrees) to its planet's equator. And the outermost satellite, Nereid, has twice the eccentricity of any other satellite or planet in the solar system. T h e y theorize that these anomalies in Neptune's satellite system, as well as the highly inclined orbit of Pluto, can all be explained by a single encounter of Neptune sometime in the distant past with a planet then in a circular orbit, beyond N e p t u n e , and having a mass two to five times that of earth. Wandering star the 90 million images was seen individually by m e , " says T o m b a u g h . "It required a total of 7,000 h o u r s of w o r k . " T o m b a u g h discovered not only Pluto but some 775 asteroids (one of them named for him) as well. He marked a total of 3,969 asteroid images and 1,807 variable stars, and he c o u n t e d 29,548 galaxies. Concludes T o m b a u g h : " F e w astronomers have seen so much of the universe in such minute detail." Besides being a remarkable achievement in itself, the thoroughness with which T o m baugh has studied the firmament gives him a unique perspective on the search for a tenth planet. He declares unequivocally that a planet the size of the one postulated by Van Flandern and Harrington isn't in any of the areas he examined. "I could have picked up a planet as massive as they say, and it's not there. I could have picked up a planet the size of earth at a distance of 100 astro- 30 MOSAIC September/October 1981 nomical u n i t s , " he says. "I know what's there and what's not there." A skewed orbit "It could be there if it's inclined 37 degrees or more north of the ecliptic and in the area I didn't get to examine because of World W a r II," notes T o m b a u g h . " O r it could be in the extreme s o u t h e r n area of the sky, south of where I could observe from Flagstaff." The unobserved southern areas are about 50 degrees south of the celestial equator. Analyses by the Naval Observatory astronomers do hint that any planet causing the Uranus and N e p t u n e residuals should be sharply out of the plane of the ecliptic. Harrington says an inclination greater than 30 degrees, " p e r h a p s even greater than 50 degrees," is not b e y o n d reason. " T h e more we try to solve for the missing planet," says Harrington, " t h e more we need high eccen- W h a t e v e r tugged this planet out of place (perhaps, Van Flandern suggests, a star passing within 800 astronomical units of our solar system—some 80 billion miles) could have sent it off into a highly inclined orbit beyond Pluto. He and Harrington have done numerical experiments showing that such a one-time occurrence could explain quantitatively not only the orbits of Triton, Nereid, and Pluto, but also the residuals of Uranus and N e p t u n e . This exotic scenario is more than a little hard for most astronomers to accept on the basis of what is now known. Seidelmann, for instance, says he likes to keep the question of the Uranus and Neptune residuals separate from his colleague's Neptune-encounter hypothesis. Harrington and Van Flandern insist that both lines of evidence do suggest an undiscovered planet in a highly inclined orbit. Harrington thinks it likely the tenth planet would be very southerly, perhaps somewhere in the s o u t h e r n m o s t part of the Milky Way. Last year he had some people at the Cerro Tololo Inter-American Observatory in Chile make a few photographic plates for him. He blinked them and found nothing—hardly a surprise for such a preliminary first effort. Nor does he plan to examine any more plates until and unless he and Van Flandern can come u p with some sort of reasonable prediction of where best to look. • A triumph of observationtal astronomy and a triumph of celestial mechanics added planets seven and eight to the solar system. he modern era of planetary astronomy began the night of March 13, 1 7 8 1 , as the English a s t r o n o m e r William Herschel was using his 15.7-centimeter reflector telescope in an extensive review of the stars. "While I was examining the small stars in the neighborhood of H Geminorum," he began his account, "I perceived one that appeared visibly larger than the rest." Because it had a measurable diameter, it could not be a star. H e continued his observations over the next several nights and observed definite motion. He thought his discovery was a comet, but within two months other astronomers had decided correctly that it was indeed a new planet. A Finnish astronomer, A n d e r s Lexell, determined that its distance from the sun was 18.9 astronomical units (the earth is 1 astronomical unit from the s u n ) and its orbital period was between 82 and 83 years. These were close, as it turned out; the figures are 19.2 astronomical units and 84 years. Herschel had doubled the size of the known solar system. He proposed naming the planet after King George III of England, but the mythological name for Saturn's father, U r a n u s , prevailed. In contrast to the discovery of Uranus, a t r i u m p h of observation, the discovery of N e p t u n e in 1846 was a triumph of the predictive science of celestial mechanics. After U r a n u s was sighted, astronomers turned u p several instances in which it had been recorded even earlier, without the observer realizing at the time that it was a planet. T h e English astronomer John Flamsteed, for instance, had seen Uranus in 1690. But a number of these prediscovery observations appeared to have the planet in the wrong place, if its orbit is calculated b a c k w a r d in time. By the 1830s it became clear that something was amiss with the orbit of U r a n u s . Before long the possibility of the gravitational effect of another planet, as large as U r a n u s , was being seriously proposed as the source of the observed anomalies. In the 1840s, the young English astrono m e r - m a t h e m a t i c i a n John Couch Adams a n d the French mathematical astronomer Urbain Jean Joseph Leverrier both tackled the problem of trying to calculate the expected position of the u n k n o w n planet. Neither was aware of the other's efforts. Adams had an answer by September 1845, but he c o u l d n ' t get anyone to begin effective observations in the designated region of the sky. This was partly due to the notable lack of enthusiasm of the Astronomer Royal. Leverrier, too, had trouble getting his countrymen to look for the object. In frustration he finally wrote to an astronomer at the Berlin O b s e r v a t o r y , Johann Galle. Galle received the letter on September 23, 1846, and that night he and observatory director Johann Encke turned their 23-centimeter refractor to the position in the sky Leverrier had given. T h e y soon found an object of magnitude eight that wasn't on the star maps. Galle immediately wrote Leverrier, " T h e planet whose position you have pointed out actually exists." It had been found within one degree of Leverrier's predicted position. Its apparent diameter was 3.2 seconds of arc; Leverrier had calculated 3.3 seconds. Leverrier first wanted the planet named after himself, but after some months he accepted N e p t u n e . N e p t u n e might have been discovered half a century earlier if only Joseph Lalande, the French astronomer, had had more confidence in his o w n observations. In 1795 he recorded as a star on M a y 8 the position of Neptune. O n May 10 he rechecked and found the star was not in the same position he had marked. He assumed he had made a mistake and dropped the subject. For that matter, Galileo, we now believe, observed N e p t u n e during fortuitous observing conditions in 1613, when it was very close to Jupiter's position in the sky. Charles T. Kowal of Caltech's Palomar O b servatory and Stillman Drake of the University of T o r o n t o reported in September 1980 that they had found this extraordinary prediscovery observation of N e p t u n e recorded in Galileo's notebooks. D u r i n g observations of Jupiter, Galileo not only had recorded N e p t u n e ' s position on several different nights but also noticed that it had shifted positions. Kowal and Drake point out that Galileo would have found it extremely difficult to follow up on this observation of a faint, moving body without having an adequate telescope mounting. N e p t u n e soon moved out of his telescope's field of view as he was observing Jupiter and its moons. " W i t h o u t a graduated mounting on his telescope, and without Jupiter to serve as a guidepost, he would hardly have been able to find a planet and a fixed star that are not visible to the u n aided eye, let alone determine their motions." Lalande's observation shows N e p t u n e to have been in a position at least seven arcseconds from where our modern calculations indicate it ought to have been in 1795. A s tronomers have long debated whether the apparent discrepancy is an observational error or whether it is reliable and therefore an indication of the gravitational influence of an undiscovered planet. Brian M a r s d e n of the Harvard-Smithsonian Center for Astrophysics refers to it as a "slight discordance" and questions its reliability. Robert Harrington of the U.S. Naval Observatory calls it a " w h o p p i n g big residual" and b e lieves it shouldn't be too easily written off MOSAIC September/October 1981 31 Planet nine was a triumph of persistent blinking and the perseverance of Clyde Tombaugh. t was the mathematical prediction of Neptune's position, based on observed perturbations in Uranus's orbit, that finally brought planet prediction by orbit calculation into high repute. But even before N e p t u n e ' s discovery, an astronomer named Peter Hansen had suggested in 1834 that a single additional large planet still would not account for the increasing residuals—the remaining deviations^in the orbital longitude of Uranus. as incorrect analysis. Kowal takes an intermediate view. Galileo's u n k n o w i n g observation of what appears to have been N e p t u n e raises similar questions. The planet was from 40 seconds to a full minute of arc from where our best estimates say it ought to have been, if the distance scale Galileo was applying to Jupiter's moons (shown on the same page of his notes) can be applied to Neptune's position. "We don't know if he was doing it to scale/' says Kowal of Galileo's reference to the position of the planet. The other objects on the page were drawn to an indicated scale, the first time such scale measurements show up in Galileo's notebooks, but N e p t u n e ' s position was off the page and was indicated by a directional line. He then included a second drawing on the same page showing the relationship between the fixed star he had seen and the object we now k n o w is N e p t u n e ; the question is whether to assume 32 MOSAIC September/October 1981 it is at the same scale as his drawing of Jupiter and its moons. Kowal hopes to find more prediscovery observations of N e p t u n e in the notebooks of 18th-century European astronomers. (Propitious observing conditions—another conjunction of Jupiter and Neptune—occurred again in 1702.) Kowal has been involved in the high-risk search for a planet beyond Pluto since 1977, probing parts of the sky covered by Clyde T o m b a u g h but examining stars far fainter th an any T o m b a u g h could see. Using a 1.2-meter Schmidt telescope, Kowal can record star images out to the 20th magnitude; T o m b a u g h reached only the 17th. Kowal's more sensitive instruments are a mixed blessing, however, since each increase in magnitude roughly doubles the n u m b e r of star images to be examined. " M y original idea was to cover a zone 30 degrees wide, centered on the ecliptic. I am about one-third d o n e . " • In the 1870s several astronomers carried out searches without success, and in 1905 Percival Lowell began his serious search for a planet beyond N e p t u n e . He used distribution of the orbits of comets and then the residuals in the positions of U r a n u s and N e p t u n e to pinpoint his Planet X. T h e laborious search lasted until N o v e m b e r 1916. T h e death of Lowell, by then a discouraged man, brought the Lowell Observatory's planet quest to a halt. William Pickering continued to c o m p u t e outer planet orbits from the distribution of cometary orbits at least through the 1920s, even proposing one (of several) paths for his Planet O that corresponded roughly with where Pluto was later found. T h e search largely rested there for a time. Then, in 1929, two things happened: Lowell Observatory decided to resume Lowell's quest for Planet X, and Clyde T o m b a u g h arrived at the observatory. T o m b a u g h ' s family had moved from Illinois to a farm in Kansas in 1922, w h e n he was 16. He pursued his lively interest in astronomy with a 5.7-centimeter telescope given to him by an uncle. He taught himself to observe, and the more he saw of the night sky the more he yearned to see. He and his family couldn't afford to buy telescopes, so he began building them himself. The third one he made, a 23-centimeter reflector, was the best. As he says now, "This telescope changed my life." Using his new eye on the heavens, he made a series of detailed drawings of the surface of Jupiter and sent them off to the astronomers at Lowell Observatory so they could check them. " T h e y were apparently impressed." The observatory was then looking for someone to help with the observing tasks on its new 33-centimeter telescope. After a series of letters back and forth, observatory director V. M. Slipher invited T o m b a u g h to come to Flagstaff on a few m o n t h s ' trial basis. So it was that on January 14, 1929, three weeks short of his 23rd birthday, he boarded a train and headed southwest. T o m b a u g h h a d n ' t k n o w n he was to help with the resumption of the planet search, but by April he was b u s y exposing the glass photographic plates. Pairs of identical exposures of regions of the sky, 12 degrees by 14 degrees in size, were taken several days apart. One of a pair was then aligned with the other in a device called the blink comparator. First one plate, then the other, was rapidly blinked into the instrument's field of view. Distant stars would not change their positions as seen first on one plate and then rapidly on the other. But any object closer to the earth than to the vault of distant stars—and thus close enough to be part of the solar system—would blink from visibly discrepant positions on the otherwise identical plates. (See " H u n t i n g by blinking," accompanying this article.) the third plate would be examined for comparison. This quickly eliminated m a n y false leads caused by flaws on a plate. But the real key to finding Pluto was the t h o r o u g h ness of the blinking. T o m b a u g h divided the plates into thin horizontal strips and moved slowly across each, rapidly blinking each star image. He found that three alternating views per second worked best. "I was a perfectionist," he recalls. T h r o u g h o u t the fall and early winter he continued the p h o tographing and the blinking, "field by field, strip by strip, panel by panel." O n the morning of February 18, 1930, he placed plates of the region of Delta Geminorum taken on January 23 and J a n u a r y 29 into the blink comparator. By four that afternoon he was about one-fourth done. He had the guide star Delta Gem in the device's eyepiece, scanned a few fields to the left, and turned the next field into view. In his newly published autobiographical book about Pluto (Out of the Darkness, with Patrick Moore), T o m b a u g h relates what happened next: "Suddenly I spied a fifteenth-magnitude image popping out and disappearing in the rapidly alternating views. T h e n I spied another image doing the same thing about three millimeters to the left. ' T h a t ' s it,' I exclaimed to myself." He checked the individual plates. The image was to the left in the later plate. It was indeed retrograde motion. "Considering the interval between plates, the parallactic shift indicated that the object was far beyond the orbit of Neptune, perhaps a thousand million miles b e y o n d . A terrific thrill came over me. I switched the shutter back and forth studying the images.. .. This would be a historic discovery." For the next 45 minutes T o m b a u g h rechecked everything. He found the object on the next earlier, January 2 1 , plate and again on three other plates taken at the same time with a smaller camera on the same telescope mount. " N o w I felt 100 percent sure. . . . Checking each star on a plate pair for such anomalies is tedious, time-consuming work. The three senior astronomers at Lowell, busy with their travels and many other tasks, soon fell about 100 plates behind. "The plates stacked u p , " recalls Tombaugh. "So Slipher told me to do the blinking. I shuddered when he said that, because I knew what was on the plates. I k n e w what the task was." Each plate had many tens of thousands to several hundred thousand stars on it, some had more than a million, and each image had to be looked at individually. Tombaugh plunged into the job with energy but trepidation. By the time he finished, his methods were honed and he had begun what was to be his life's work. Blinking thorough O n e of the first problems, recalls T o m baugh, was to avoid being fooled by asteroids. His solution was to p h o t o g r a p h each area of the sky only when it was at opposition (directly opposite the earth across the sun). At this point all bodies in the solar system beyond earth's orbit appear from the parallax effect to shift westward (retrograde) in the sky, and the amount of the shift depends on the object's distance. An asteroid, being inside the orbit of Jupiter, would shift far too rapidly to be an undiscovered outer planet and could be ignored. A second innovation was to p h o t o g r a p h each area of the sky not twice but three times. Whenever a suspicious object turned up in the blinking of the first pair of plates, MOSAIC S e p t e m b e r / O c t o b e r 1981 33 Clyde W. Tombaugh; Lowell Observatory. 34 MOSAIC September/October 1981 I walked d o w n the hall to V. M. Slipher's office. T r y i n g to control myself, I stepped into his office as nonchalantly as possible. He looked up from his desk work. 'Dr. Slipher, I have found your Planet X.' " T h a t night was so cloudy no photographs could be taken. T h e only person to discover a planet in the twentieth century, just 24 years old, went down the hill to the only movie theater in Flagstaff and watched Gary Cooper in The Virginian. Shrinking Pluto W e k n o w n o w that Pluto was not the hypothetical Planet X. It is just too tiny. Lowell expected Planet X to be 6.6 times the mass of earth. R. L. D u n c o m b e of the University of Texas and the Naval Observatory's P. K e n n e t h Seidelmann recently chronicled the story of Pluto's declining mass. By the year of Pluto's discovery, it was clear from the calculation that its mass was less than earth's. In 1955 the best estimate stood at 0.82 earth mass. Duncombe, W. J. Klepczynski, and Seidelmann produced new estimates of 0.18 in 1968 and then 0.11 in 1971. In 1976, D. P. Cruikshank, C. B. Pilcher, and D a v i d Morrison of the University of Hawaii f o u n d evidence that Pluto is covered with methane ice and inferred that its albedo, or reflectivity, is four times as great as had been s u p p o s e d . This reduced the size (and therefore the probable mass) of Pluto still more, to 0.004 earth mass. T h e n in June 1978 James W. Christy of the Naval Observatory in Washington made an unexpected discovery. He was measuring some plates of Pluto taken with the observatory's 1.55-meter astrometric telescope at Flagstaff, located just four miles from Lowell Observatory, where Tombaugh had first seen Pluto. Each of the six images of Pluto taken on April 13 and M a y 12 showed a neverbefore-noticed elongation, sometimes above Pluto, sometimes below. Christy had discovered that Pluto has a moon. Further checks showed the same elongation on plates taken earlier. In fact, five plates taken d u r i n g a single week in June 1970 showed the elongation angle proceeding clockwise around Pluto. C h r i s t y proposed that the moon has a roughly six-day orbital period, with the orbit inclined to the line of sight such that it is most easily seen first above Pluto and three d a y s later below it. (The exact orbital period is n o w assumed to be 6.3871 days, says Christy.) Observations taken with the Flagstaff telescope in July 1978, w h e n atmospheric conditions m a d e seeing exceptionally good, Hunting by blinking H u n t i n g for a planet beyond the edge of the k n o w n solar system is a painstaking process. T h e way it is done is called blinking, which involves: (1) photographing each section of the sky at least twice, several days apart. Both plates should be exposed when the part of the sky being scanned is at opposition—directly across the sun from the earth; (2) thorough examination of each image on the photographic plates for any spot of light that might have moved against the background of more distant stars during the interval between exposure of the first and second plates of the pair. Hundreds of thousands of points of light—star images—on each plate have to be examined individually for such a p p a r e n t motion. The comparison of the pairs of photographic plates is done with an optical device called a blink-microscope comparator. T h e blink comparator presents the two plates of the pair to the eye in rapid alternation, so that the eye sees first one plate, then the other, in the exact position. All distant stars will appear as u n c h a n g i n g dots in the viewfield as the plates alternate. Any solar-system object, however, will be in a slightly different position on the two plates. This apparent motion against the background of distant stars will show u p in the blink comparator as an image " h o p p i n g " back and forth as the plates alternate. Most of this apparent motion is a consequence of the distance the earth moved in .si.' I.I . ' i >• i • • i ••!• in- i ii ••! .!•:..; -"• cn-ii. p h o t o g r a p h . As Pluto discoverer Clyde Tombaugh says, "The motion on the plate is caused by parallax. M a n y people don't realize that the shift you are looking for is created by the earth's orbital motion." When the photographic plates are taken at the opposition point, all solar-system objects beyond earth appear to make little retrograde orbital jogs against the backg r o u n d of distant stars. The amount of this retrograde motion varies with the object's proximity to earth. This provides a very quick way to distinguish the many hundreds of asteroids that show u p o n p h o t o s from the more distant outer planets. Most of the asteroids are between the orbits of Mars and Jupiter, at distances from the s u n between 2.5 and 3.5 astronomical units. Thus they show enormous apparent motion. A planet on the edge of the solar system—Pluto's mean distance is 39.4 astronomical units—would show a much smaller displacement on the blinking plates. As soon as Tombaugh noticed the image of what turned out to be Pluto, a tiny three millimeters apart on the plates he was blinking in February 1930, and he confirmed that the motion was retrograde, he k n e w he had discovered a planet beyond the edge of the t h e n - k n o w n solar system. As he recalls, "Considering the [six-day] interval between the plates, the [size of the] parallactic shift indicated that the object was far beyond the orbit MOSAIC Sentemhfir/Ortnhftr 1Qfi1 . 1 * confirmed the discovery and allowed more accurate calculation of the characteristics of the moon. The excellent seeing conditions resulted in the best photos of Pluto ever taken. They still d o n ' t show a sharp-edged disk, nor are they quite sharp enough to reveal a separation between Pluto and its m o o n , which Christy named Charon. Christy says the observatory's Flagstaff facility is trying to get better pictures, but the viewing conditions of July 1978 have yet to be surpassed. Of about 100 images of Pluto taken since then, says Christy, about one-third are sharp enough to show the elongation. " W e were very fortunate to h a v e the exceptional seeing conditions of that July." J. Derral Mulholland of the University of T e x a s has been using a new electrographic camera in an attempt to resolve the images b u t has so far been unsuccessful, due to the poor seeing conditions he encountered d u r i n g recent o b s e r v i n g a t t e m p t s at M c D o n a l d Observatory. H. J. Reitsema and Faith Vilas of the University of Arizona h a v e used a computer to produce a digital image of Charon by subtracting out the image of Pluto. In a few years, however, Pluto and C h a r o n should go into a multi-year period of frequent mutual eclipses, and astronomers should be able to get the most precise information yet on the Pluto system. Apart from the intrinsic interest in finding a n e w satellite in our solar system, the discovery of Charon had wider significance. As Christy's colleague Robert Harrington points out, if we k n o w the period and mean distance of a satellite, we can compute the mass of the planet-satellite system. Harringt o n ' s calculation n o w gives a mass for the two together of only 0.0024 earth masses, with 0.0022 of that due to Pluto and 0.0002 due to Charon. Pluto had shrunk even more. In fact, A. J. Dessler of Rice University 36 MOSAIC Sfintemher/nntnhftr1Qft1 and Christopher T. Russell of the University of California at Los Angeles wrote a whimsical paper in 1980 noting the downwardplunging trend of the estimates of Pluto's mass. Entitled " F r o m the Ridiculous to the Sublime: T h e Pending Disappearance o{ Pluto," the paper concluded that "Pluto is simply evaporating with time." W h e n this was published in the American Geophysical Union's publication EOS, the authors were surprised to learn that some readers took them seriously. Perhaps nothing should surprise anyone about Pluto anymore. T h e n e w estimate of mass resulting from the discovery of Pluto's moon was the clincher, if one was needed. As Harrington says, "Pluto is obviously not Lowell's Planet X nor William Pickering's Planet O, nor any of the other planets that have been hypothesized to explain the discrepancies between the observed and predicted motions of Uranus and N e p t u n e . " T h o m a s Van Flandern of the Naval O b servatory concurs: "Pluto is too small to account for m a n y of the observed irregularities in the motions of the outer planets." Ironically, it appears that Lowell's accurate calculation of the location of Planet X was a fluke. Astronomers now agree that it was Clyde Tombaugh's extraordinary search procedures that led to the discovery of Pluto. Pluto was found, says Brian Marsden, "not because it was predicted but because it was there." A n d there is still no real clue to the cause of the perturbations in the orbits of Uranus and N e p t u n e . • Kendrick interested sciences. Frazier is a science writer long in astronomy and the geophysical The National Science Foundation has contributed to the support of its share of the research discussed in this article through its Solar System Astronomy Programs, PET I wanted to drop you a note telling you that I have been using copies of the article "Seeing the Brain at W o r k . " It appeared in the M a y / J u n e issue of Mosaic, Volume 12, N u m b e r 3.1 have been passing out copies to patients and their families who are considering participation in our PET research. I have also sent copies to physicians who have referred patients to me or have asked about the procedure. It has proven helpful and effective in this capacity. National Norman Foster Institutes of Health Bethesda, Maryland