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Hypothetical Planets

by Paul Schlyter (pausch@saaf.se)

There have been a number of objects that were once thought to exist by astronomers, but which later 'vanished.' Here are their stories.

Vulcan, the intra-Mercurial planet, 1860-1916, 1971

During the 19th century, astronomers were puzzled over unexplained deviations in the motion of Mercury. The French mathematician Urbain Jean Joseph Le Verrier, who (along with John Couch Adams) had predicted the position of Neptune based on deviations in the motion of Uranus, believed similar forces were at work. During a lecture on January 2, 1860, he announced that the solution to Mercury's deviations could be explained by assuming the existence of an intra-Mercurial planet, or possibly a second asteroid belt, inside Mercury's orbit.

The only possible way to observe intra-Mercurial bodies was when they transited the Sun, or during total solar eclipses. Professor Wolf at the Zurich sunspot data center found a number of suspicious "dots" on the Sun, and a second astronomer found additional ones. A total of two dozen spots seemed to fit the pattern of two intra-Mercurial orbits with periods of 26 and 38 days.

In 1859, Le Verrier received a letter from the amateur astronomer Lescarbault, who reported having seen a round black spot on the Sun on March 26, 1859. Lescrabault thought the object was a planet transiting the Sun. He had seen the spot for about 75 minutes, during which time it moved a quarter of the solar diameter. Lescarbault estimated the object had an orbital inclination of between 5.3° and 7.3°, a longitudal node of about 183°, an "enormous" eccentricity, and a transit time across the solar disk of 4 hours, 30 minutes. Le Verrier investigated this observation, and computed the following orbit:

  • a period of 19 days, 7 hours;
  • a mean distance from Sun of 0.1427 AU;
  • an inclination of 12° 10'; and,
  • an ascending node at 12° 59'.
The diameter was considerably smaller than Mercury's and its mass was estimated at one-seventeenth of Mercury's mass. This was too small to account for the deviations of Mercury's orbit. However, Le Verrier theorized that this might be the largest member of an intra-Mercurial asteroid belt. He named it Vulcan.

In 1860, there was a total eclipse of the Sun. Le Verrier mobilized astronomers throughout the world to find Vulcan. No one did. Wolf's suspicious 'sunspots' now revived Le Verrier's interest, and additional 'evidence' found its way into print just before Le Verrier's death in 1877. On April 4, 1875, German astronomer H. Weber saw a round spot on the Sun. Le Verrier's orbit indicated a possible transit on April 3 that year. Wolf noticed that his 38-day orbit also could have performed a transit at about that time. That 'round dot' also was photographed by astronomers in Greenwich and Madrid.

There was one more flurry of sightings after the total solar eclipse on July 29, 1878. Two observers claimed to have seen small, illuminated disks in the vicinity of the Sun, objects which could only be small planets inside Mercury's orbit. J.C Watson, professor of astronomy at the University of Michigan, believed he had found two intra-Mercurial planets. Lewis Swift (co-discoverer of Comet Swift-Tuttle, which returned in 1992), also saw a 'star' he believed to be Vulcan. However, it was in a different position than either of Watson's two 'intra-Mercurials.' Neither Watson's nor Swift's sightings could be reconciled with Le Verrier's or Lescarbault's 'Vulcan.'

Nobody ever saw Vulcan again, in spite of several searches at different total solar eclipses. In 1916, Albert Einstein published his General Theory of Relativity, which explained the deviations in Mercury's motions without the need to invoke an unknown intra-Mercurial planet. In May 1929, Erwin Freundlich photographed the total solar eclipse in Sumatra; the plates showed a profusion of star images. Comparison plates were taken six months later. No new objects brighter than 9th magnitude were found near the Sun.

But what did these people really see? Lescarbault had no reason to lie, and even Le Verrier believed him. It is possible that Lescarbault happened to see a small asteroid passing very close to the Earth, just inside Earth's orbit. Such asteroids were unknown at that time, so Lescarbault believed that he saw an intra-Mercurial planet. Swift and Watson could, during the hurry to obtain observations during totality, have misidentified some stars as Vulcan.

"Vulcan" was briefly revived around 1970-1971, when a few researchers thought they had detected several faint objects close to the Sun during a total solar eclipse. These objects might have been faint comets. Comets have been observed to pass close enough to the Sun and eventually collide with it.

Mercury's Moon, 1974

Two days before the March 29, 1974, Mariner 10 flyby of Mercury, one instument began registering bright, extreme ultraviolent (UV) emissions that had "no right to be there." The next day, the emissions were gone. Three days later, they reappeared, apparently emanating from an "object" that seemingly detached itself from Mercury. The astronomers first thought they had seen a star. But, they had seen the emissions in two quite different directions, and every astronomer knew that these extreme UV wavelengths couldn't penetrate very far through the interstellar medium. This suggested that the object must be relatively close. Did Mercury have a moon?

After a hectic Friday, during which the "object" had been computed to move at 4 kilometers (2.4 miles) per second, a speed consistent with that of a moon, Jet Propulsion Laboratory (JPL) managers were called in. They turned the then-dying spacecraft over full time to the UV team, and everyone started worrying about a press conference scheduled for later that Saturday. Should the suspected moon be announced? But the press already knew. Some newspapers -- the bigger, more respectable ones -- played it straight; many others ran excited stories about Mercury's new moon.

And the "moon" itself? It headed straight on out from Mercury, and was eventually identified as a hot star, 31 Crateris. The origins of the original emissions remain a mystery. So ended the story of Mercury's moon. At the same time, a new chapter in astronomy began: extreme UV turned out not to be so completely absorbed by the interstellar medium as formerly believed. The Gum nebula has turned out to be a emitter in the extreme UV, and spreads across 140° of the night sky at 540 angstroms. Astronomers had discovered a new window through which to observe the heavens.

Neith, the Moon of Venus, 1672-1892

In 1672, Giovanni Domenico Cassini, one of the prominent astronomers of the time, noticed a small companion close to Venus. Did Venus have a satellite? Cassini decided not to announce his observation, but when he saw it again 14 years later, he entered the observation in his journal. The object was estimated to have about one-quarter the diameter of Venus, and it showed the same phase as Venus.

The object was later seen by other astronomers: James Short in 1740, Andreas Mayer in 1759, and Joseph Louis Lagrange in 1761. (Lagrange announced that the orbital plane of the satellite was perpendicular to the ecliptic.) During 1761, the object was seen a total of 18 times by five observers. The observations of Scheuten on June 6, 1761 was especially interesting. He saw Venus in transit across the Sun's disk, accompanied by a smaller dark spot on one side that followed Venus in its transit. However, Samuel Dunn at Chelsea, England, who also watched that transit, did not see the additional spot. In 1764, there were 8 observations by two observers. Other observers failed to find the satellite.

Now the astronomical world was faced with a controversy. Several observers had reported seeing the satellite while several others had failed to find it in spite of determined efforts. In 1766, the director of the Vienna observatory, Father Hell, published a treatise in which he declared that all observations of the satellite were optical illusions. He believed the image of Venus is so bright that it is reflected in the eye, back into the telescope, creating a secondary image at a smaller scale.

Others published treatises declaring that the observations were real. J. H. Lambert of Germany published orbital elements of the satellite in Berliner Astronomischer Jarhbuch 1777:

  • mean distance, 66.5 Venus radii;
  • orbital period, 11 days, 3 hours; and,
  • inclination to ecliptic, 64°.
It was hoped that the satellite could be seen during the transit of Venus in front of the Sun on June 1, 1777. In retrospect, it is clear that Lambert made a mistake in calculating these orbital elements. At 66.5 Venus radii, the distance from Venus is about the same as our Moon's distance from the Earth. This does not fit with the orbital period of 11 days, which is about one-third of the orbital period of our Moon. (The mass of Venus is slightly smaller than the mass of the Earth.)

In 1768 , Christian Horrebow made one more observation of the satellite from Copenhagen. There were also three searches, including one made by one of the greatest astronomers of all time, William Herschel. All three of them failed to find any satellite. Quite late in the game, F. Schorr from Germany tried to make a case for the satellite in a book published in 1875.

In 1884, M. Hozeau, former director of the Royal Observatory of Brussels, suggested a different hypothesis. By analysing available observations, Hozeau concluded that the moon appeared close to Venus approximately every 2.96 years. Hozeau suggested that this was a separate planet, with a 283-day orbit around the Sun that placed it in conjunction with Venus every 1,080 days. Hozeau also named the new planet Neith, after the mysterious goddess of Sais, whose veil no mortal raised.

In 1887, three years after Hozeau had revived interest in the subject, the Belgian Academy of Sciences published a long paper in which each and every reported observation was investigated in detail. Several observations of the satellite were really stars seen in the vicinity of Venus. Roedkier's observations "checked out" especially well -- he had been fooled, in succession, by Chi Orionis, M Tauri, 71 Orionis, and Nu Geminorum. James Short had really seen a star somewhat fainter than 8th magnitude. All observations by Le Verrier and Montaigne could be similarly explained. Lambert's orbital calculations were demolished. The very last observation, by Horrebow in 1768, could be ascribed to Theta Librae.

After this paper was published, only one more observation was reported, by a man who had earlier made a search for the satellite of Venus but failed to find it. On August 13, 1892, Edward Emerson Barnard recorded a 7th magnitude object near Venus. There is no star in the position recorded by Barnard, and Barnard's eyesight was notoriously excellent. We still don't know what he saw. Was it an asteroid that had not been charted? Or was it a short-lived nova that nobody else happened to see?

The Earth's Second Moon, 1846-present

In 1846, Frederic Petit, director of the observatory of Toulouse, stated that a second moon of the Earth had been discovered. It had been seen by two observers, Lebon and Dassier, at Toulouse and by a third, Lariviere, at Artenac, during the early evening of March 21, 1846. Petit found that the orbit was elliptical, with:

  • a period of 2 hours, 44 minutes, 59 seconds;
  • an apogee of 3,570 kilometers (2,218 miles); and,
  • a perigee of just 11.4 kilometers (7 miles).
Le Verrier, who was in the audience when Petit made the announcement, grumbled that one needed to take air resistance into account, something nobody could do at that time. Petit became obsessed with this idea of a second moon, and 15 years later announced that he had made calculations about a small moon of Earth which caused some then-unexplained peculiarities in the motion of our main Moon. Astronomers generally ignored this, and the idea would have been forgotten if a young French writer, Jules Verne, had not read an abstract. In Verne's novel From the Earth to the Moon, Verne lets a small object pass close to the traveller's space capsule, causing it to travel around the Moon instead of smashing into it:
"It is," said Barbicane, "a simple meteorite but an enormous one, retained as a satellite by the attraction of the Earth."

"Is that possible," exclaimed Michel Ardan, "the earth has two moons?"

"Yes, my friend, it has two moons, although it is usually believed to have only one. But this second moon is so small and its velocity is so great that the inhabitants of Earth cannot see it. It was by noticing disturbances that a French astronomer, Monsieur Petit, could determine the existence of this second moon and calculated its orbit. According to him a complete revolution around the Earth takes three hours and twenty minutes. . . . "

"Do all astronomers admit the the existence of this satellite?" asked Nicholl.

"No," replied Barbicane, "but if, like us, they had met it they could no longer doubt it. . . . But this gives us a means of determining our position in space . . . its distance is known and we were, therefore, 7,480 kilometers above the surface of the globe where we met it."

Jules Verne was read by millions of people, but not until 1942 did anybody notice the discrepancies in Verne's text:

  1. A satellite 7,480 kilometers (4,648 miles) above the Earth's surface would have a period of 4 hours, 48 minutes, not 3 hours, 20 minutes.
  2. Since it was seen from the window from which the Moon was invisible, while both were approaching, it must be in retrogade motion, which would be worth remarking. Verne doesn't mention this.
  3. In any case, the satellite would be in eclipse and thus be invisible. The projectile doesn't leave the Earth's shadow until much later.
Dr. R.S. Richardson of Mount Wilson Observatory tried in 1952 to make the figures fit by assuming an eccentric orbit of this moon: a perigee of 5,010 kilometers (3,113 miles), an apogee of 7,480 kilometers (4,648 miles), and an eccentricity of 0.1784.

Nevertheless, Jules Verne made Petit's second moon known all over the world. Amateur astronomers jumped to the conclusion that here was an opportunity for fame -- anybody discovering this second moon would have his name inscribed in the annals of science. No major observatory ever checked the problem of the Earth's second moon, or if they did they kept quiet. German amateurs were chasing what they called Kleinchen ("little bit"). Of course they never found Kleinchen.

William Henry Pickering devoted his attention to the theory of the subject. If the satellite orbited 320 kilometers (200 miles) above the surface and its diameter was 0.3 meters (1 foot), with the same reflecting power as the Moon, it should be visible in a 7.6-centimeter (3-inch) telescope. A 3-meter (10-foot) satellite would be a naked-eye object of magnitude 5. Though Pickering did not look for the Petit object, he did carry on a search for a secondary moon - a satellite of our Moon. The result was negative and Pickering concluded that any satellite of our Moon must be smaller than about 3 meters (10 feet).

Pickering's article on the possibility of a tiny second moon of Earth, "A Meteoritic Satellite," appeared in Popular Astronomy in 1922. It caused another short flurry of activity among amateur astronomers, since it contained a virtual request: "A 3-5-inch telescope with a low-power eyepiece would be the likeliest means to find it. It is an opportunity for the amateur." But again, all searches remained fruitless.

The original idea was that the gravitational field of the second satellite should account for inexplicable, minor deviations of the motion of our Moon. That meant an object at least several miles long -- but if such a large second moon really existed, it would have been seen by the Babylonians. Even if it was too small to show a disk, its comparative nearness would have made it move fast and therefore be conspicuous, as today's watchers of artificial satellites and even airplanes know. On the other hand, nobody was much interested in moonlets too small to be seen.

There have been other proposals for additional natural satellites of the Earth. In 1898, Dr. Georg Waltemath from Hamburg claimed to have discovered not only a second moon but a whole system of midget moons. Waltemath gave orbital elements for one of these moons:

  • distance from Earth, 1.03 million kilometers (640,000 miles);
  • diameter, 700 kilometers (435 miles);
  • orbital period, 119 days; and
  • synodic period, 177 days.
"Sometimes," Waltemath said, "it shines at night like the Sun". He believed this moon was seen in Greenland on October 24, 1881, by Lieutenant Greely, ten days after the Sun had set for the winter.

Public interest was aroused when Waltemath predicted his second moon would pass in front of the Sun sometime during February 2-4, 1898. On February 4, 12 persons at the post office of Greifswald (Herr Postdirektor Ziegel, members of his family, and postal employees) observed the Sun with their naked eye, without protection of the glare. It is easy to imagine a faintly preposterous scene: an imposing-looking Prussian civil servant pointing skyward through his office window, while he reads Waltemath's prediction aloud to a group of respectful subordinates. On being interviewed, these witnesses spoke of a dark object having one fifth the Sun's apparent diameter, and which took from 1:10 to 2:10 Berlin time to traverse the solar disk. It was soon proven to be a mistake, because during that very hour the Sun was being scrutinized by two experienced astronomers, W. Winkler in Jena and Baron Ivo von Benko from Pola, Austria. They both reported that only a few ordinary sunspots were on the disk.

The failure of this and later forecasts did not discourage Waltemath, who continued to issue predictions and ask for verifications. Contemporary astronomers were pretty irritated over and over again having to answer questions from the public such as, "Oh, by the way, what about all these new moons?". However, astrologers caught on. In 1918, the astrologer Sepharial named this moon Lilith. He considered it to be black enough to be invisible most of the time, being visible only close to opposition or when in transit across the solar disk. Sepharial constructed an ephemeris of Lilith, based on several of Waltemath's claimed observations. He considered Lilith to have about the same mass as the Moon, apparently happily unaware that any such satellite would, even if invisible, show its existence by perturbing the motion of the Earth. And even to this day, "the dark moon," Lilith, is used by some astrologers in their horoscopes.

From time to time, other "additional moons" were reported from observers. The German astronomical magazine "Die Sterne" reported that a German amateur astronomer named W. Spill had observed a second moon cross our first moon's disc on May 24, 1926.

Around 1950, when artificial satellites began to be discussed in earnest, everybody expected them to be just burned-out upper stages of multistage rockets, carrying no radio transmitters but being tracked by radar from the Earth. In such cases, a bunch of small, nearby natural satellites would have been most annoying, reflecting radar beams meant for the artificial satellites. The method to search for such natural satellites was developed by Clyde Tombaugh: the motion of a satellite at an altitude of 5,000 kilometers (3,100 miles) height is computed. A camera platform was then constructed that scans the sky at precisely that rate. Stars, planets and other celestrial objects would then appear as lines on the photographs taken by this camera, while any satellite at the correct altitide would appear as a dot. If the satellite was at a somewhat different altitude, it would produce a short line.

Observations were began in 1953 at the Lowell Observatory and actually invaded virgin territory: with the exception of the Germans searching for "Kleinchen," nobody had ever paid attention to the space between the Moon and the Earth. By the fall of 1954, weekly journals and daily newspapers of high reputation stated that the search had brought its first results: one small natural satellite at 700 kilometers (435 miles) altitude, another one 1,000 kilometers (620 miles) out. One general is said to have asked, "Is he sure they're natural?" Nobody seems to know how these reports originated. The searches were completely negative. When the first artificial satellites were launched in 1957 and 1958, the cameras tracked those satellites instead.

But strangely enough, this does not mean that the Earth has only one natural satellite. The Earth can have a very near satellite for a short time. Meteoroids passing the Earth and skimming through the upper atmosphere can lose enough velocity to go into a satellite orbit around the Earth. But since they pass the upper atmosphere at each perigee, they will not last long; the number of revolutions might be anywhere from one to 100 for a maximum of 150 hours. There are some indications that such "ephemeral satellites" have been seen; it is even possible that Petit's observers did see one.

In addition to ephemeral satellites there are two more possibilities. One is that the Moon had a satellite of its own, but despite several searches none has been found. It is now known that the gravity field of the Moon is uneven, or "lumpy," enough for any lunar satellite orbit to be unstable. Any satellite will therefore crash into the Moon after a farily short time, a few years or possibly a decade. The other possibility is that there might be Trojan satellites, i.e. secondary satellites in the lunar orbit, travelling 60° ahead of or behind the Moon.

Such "Trojan satellites" were first reported by the Polish astronomer Kordylewski of Krakow observatory. He began a visual search in 1951 using a good telescope. He was hoping for reasonably large bodies in the lunar orbit, 60° away from the Moon. The search was negative. However, in 1956 his compatriot and colleague, Wilkowski, suggested that there might be many tiny bodies too small to be seen individually but numerous enough to appear as a cloud of dust particles. In such a case, they would be best visible without a telescope, i.e., with the naked eye. Using a telescope would "magnify it out of existence." Dr Kordylewski was willing to try. A dark night with clear skies, with the Moon below the horizon, was required.

In October 1956, Kordylewski saw, for the first time, a fairly bright patch in one of the two positions. It was not small, subtending an angle of 2° (i.e. about 4 times larger than the Moon itself). It also was very faint, only about half as bright as the notoriously difficult Gegenschein (counterglow - a bright patch in the zodiacal light, directly opposite to the Sun). In March and April 1961, Kordylewski succeeded in photographing two clouds near the expected positions. They seem to vary in extent, but that may be due to changing illumination. J. Roach detected these cloud satellites in 1975 with the OSO (Orbiting Solar Observatory) 6 spacecraft. In 1990, they were again photographed, this time by the Polish astronomer Winiarski, who found that they were a few degrees in apparent diameter, that they "wandered" up to 10° away from the "trojan" point, and that they were somewhat redder than the zodiacal light.

So, the century-long search for a second moon of the Earth seems to have succeeded, after all, even though this 'second moon' turned out to be entirely different from anything anybody had ever expected. These objects are very hard to detect and to distinguish from the zodiacal light, in particular the Gegenschein.

But, people are still proposing additional natural satellites of the Earth. Between 1966 and 1969, American scientist John Bargby claimed to have observed at least ten small natural satellites of the Earth, visible only in a telescope. Bargby found elliptical orbits for all the objects: eccentricity of 0.498, and semimajor axis of 14,065 kilometers (8,740 miles), which yields perigee and apogee heights of 680 and 14700 kilometers (432 and 9,135 miles), respectively. Bargby considered them to be fragments of a larger body which broke up in December 1955.

He based much of his suggested satellites on supposed perturbations of artificial satellites. Bargby used artificial satellite data from the Goddard Satellite Situation Report, unaware that the values in this publication are only approximate and sometimes grossly in error; therefore, they cannot be used for any precise scientific analysis. In addition, from Bargby's own claimed observations it can be deduced that when at perigee Bargby's satellites ought to be visible at first magnitude and thus be easily visible to the naked eye, yet no one has seen them as such.

The Moons of Mars, 1610 - 1877

The first to guess that Mars had moons was Johannes Kepler in 1610. When trying to solve Galileo's anagram referring to Saturn's rings, Kepler thought that Galileo had found moons of Mars instead.

In 1643, the Capuchin monk, Anton Maria Shyrl, claimed to have seen the moons of Mars. We now know that would be impossible with the telescopes of that time - probably Shyrl got deceived by a star nearby Mars.

In 1727, Jonathan Swift wrote in Gulliver's Travels about two small moons orbiting Mars, known to the Lilliputian astronomers. Their periods of revolution were 10 and 21.5 hours. Voltaire adopted these 'moons' in his 1750 novel Micromegas, the story of a giant from Sirius visiting our solar system.

In 1747, a German captain, Kindermann, claimed to have seen one moon of Mars, on July 10, 1744. Kindermann reported the orbital period of this Martian moon as 59 hours, 50 minutes, and 6 seconds.

In 1877, Asaph Hall finally discovered Phobos and Deimos, the two small moons of Mars. Their orbital periods are 7 hours, 39 minutes amd 30 hours, 18 minutes, quite close to the periods guessed by Jonathan Swift 150 years earlier.

The 14th Moon of Jupiter, 1975-1980

In 1975, Charles Kowal at Palomar (discoverer of Comet 95 P/Chiron) photographed an object thought to be a new satellite of Jupiter. It was seen several times, but not enough to determine an orbit, then lost. It used to show up as a footnote in texts of the late 1970's.

Saturn's Ninth and Tenth Moons, 1861, 1905-1960, 1966-1980

In April 1861, Hermann Goldschmidt announced the discovery of a nineth moon of Saturn, which orbited the planet between Titan and Hyperion. He named that moon Chiron. However, the discovery was never confirmed -- no one ever saw this satellite "Chiron" again. Pickering discovered what's now considered Saturn's 9th moon, Phoebe, in 1898. This was the first time a satellite of another planet was discovered by photographical observations. Phoebe is also Saturn's outermost moon.

In 1905, Pickering though he had discovered a tenth moon, which he named Themis. According to Pickering, it orbited Saturn between Titan and Hyperion in a highly inclined orbit:

  • mean distance from Saturn, 1,460,000 kilometers (907,250 miles);
  • orbital period, 20.85 days;
  • eccentricity, 0.23; and,
  • inclination, 39°.
Themis was never seen again, but nevertheless appeared in almanacs and astronomy books well into the 1950's and 1960's.

In 1966, A. Dollfus discovered another new moon of Saturn. It was named Janus, and orbited Saturn just outside its rings. It was so faint and close to the rings that the only chance to see it was when the rings of Saturn were seen from the edge, as happened in 1966. Janus is Saturn's tenth moon.

In 1980, when Saturns rings again were seen edgewise, a flurry of observations discovered many new satellites close to the rings of Saturn. Close to Janus another satellite was discovered, named Epimetheus. Their orbits are very close to each other, and the most interesting aspect of this satellite pair is that they regularly switch orbits with each other. It turned out that the "Janus" discovered in 1966 really were observations of both of these co-orbital satellites. Thus the 'tenth moon of Saturn' discovered in 1966 really turned out to be two different moons. The spacecraft Voyager 1 and Voyager 2, which travelled past Saturn shortly afterwards, provided confirmation.

Six Moons of Uranus, 1787

In 1787, William Herschel announced the discovery of six satellites of Uranus. Herschel made a mistake; only two of his six satellites were real (Titania and Oberon, the largest and outermost two satellites). The remaining four were stars which happened to be nearby.

Neptune's Discovery, 1846

By 1841, scientists realized that there were large, unexplained perturbations in the motion of Uranus. John Couch Adams began investigating these disturbances. He presented two different solutions to the problem, assuming that the deviations were caused by the gravitation from an unknown planet. Adams tried to present his solutions to the Greenwich observatory, but since he was young and unknown, he wasn't taken seriously. In 1845, Urbain Le Verrier started to investigate the moons as well. Urbain Le Verrier presented his solution in 1846, but France lacked the necessary resources to locate the planet.

Le Verrier then turned to the Berlin observatory, where Galle and his assistant d'Arrest found Neptune on the evening of September 23, 1846. Both Adams and Le Verrier share the credit of having predicted the existence and position of Neptune.

The Search for Trans-Neptunian Worlds, 1846 - 1930

On September 30, 1846, one week after the discovery of Neptune, Le Verrier declared that there might be still another unknown planet out there. On October 10, Neptune's large moon Triton was discovered. Triton provided an easy way to accurately determine the mass of Neptune, which turned out to be 2 percent larger than expected from the perturbations upon Uranus. It seemed as if the deviations in Uranus's motion were caused by two planets. In addition, the real orbit of Neptune turned out to be significantly different from the orbits predicted by both Adams and Le Verrier.

David Todd made the first serious attempt to find a trans-Neptunian planet. He used a "graphical method", and despite the inconclusivenesses of the residuals of Uranus, he derived elements for a trans-Neptunian planet:

  • mean distance 52 AU;
  • period, 375 years; and,
  • a magnitude fainter than 13.

Evidence from Comets

In 1879, Camille Flammarion added another hint as to the existence of a planet beyond Neptune: the aphelia of periodic comets tend to cluster around the orbits of major planets. Jupiter has the greatest share of such comets, and Saturn, Uranus and Neptune also have a few each. Flammarion found two comets: 1862 III with a period of 120 years and an aphelion at 47.6 AU; and 1889 II, with a somewhat longer period and an aphelion at 49.8 AU. Flammarion suggested that the hypothetical planet probably moved at 45 AU.

One year later, in 1880, professor Forbes published a memoir concerning the aphelia of comets and their association with planetary orbits. By about 1900, five comets were known with aphelia outside Neptune's orbit, and then Forbes suggested one trans-Neptunian moved at a distance of about 100 AU, and another one at 300 AU, with periods of 1,000 and 5,000 years, respectively.

Estimates, 1900 - 1905

During the next five years, several astronomers/mathematicians published their own ideas of what might be found in the outer parts of the solar system. However, no one captured any images of these supposed planets.

Gaillot at the Paris Observatory assumed two trans-Neptunian planets at 45 and 60 AU. Thomas Jefferson Jackson See predicted three trans-Neptunian planets:

  • "Oceanus" at 41.25 AU with a period 272 years;
  • "trans-Oceanus" at 56 AU with a period 420 years; and,
  • a planet at 72 AU with a period 610 years.
Dr. Theodor Grigull of Munster, Germany, assumed in 1902 that a Uranus-sized planet existed at 50 AU with period 360 years, which he called Hades. Grigull based his work mainly on the orbits of comets with aphelia beyond Neptune's orbit, with a cross check of whether the gravitational pull of such a body would produce the observed deviations in Uranus motion. In 1921, Grigull revised the orbital period of Hades to 310-330 years, to better fit the observed deviations.

In 1900, Hans-Emil Lau of Copenhagen published elements of two trans-Neptunian planets at 46.6 and 70.7 AU distance, with masses of 9 and 47.2 times the Earth, and a magnitude for the nearer planet around 10-11. The 1900 longitudes of those hypothetical bodies were 274° and 343°, both with the very large uncertainty of 180°.

In 1901, Gabriel Dallet deduced a hypothetical planet at 47 AU with a magnitude of 9.5-10.5 and a 1900 longitude of 358°. The same year, Theodor Grigull derived a longitude of a trans-Neptunian planet as less than 6° away from Dallet's planet, and later brought the difference down to 2.5°. This planet was supposed to be 50.6 AU distant.

In 1904, Thomas Jefferson Jackson See suggested three trans-Neptunian planets, at 42.25, 56 and 72 AU. The inner planet had a period of 272.2 years and a longitude in 1904 of 200°. A Russian general named Alexander Garnowsky suggested four hypothetical planets but failed to supply any details about them.

Pickering's Predictions

The two most carefully worked out predictions for the trans-Neptune planets were both of American origin: Pickering's "A search for a planet beyond Neptune," and Percival Lowell's "Memoir on a trans-Neptunian planet". They were concerned with the same subject but used different approaches and arrived at different results.

Pickering used a graphical analysis and suggested a "Planet O" at 51.9 AU with a period of 373.5 years, a mass twice the Earth's and a magnitude of 11.5-14. Pickering suggested eight other trans-Neptunian planets during the forthcoming 24 years. Pickerings results caused Gaillot to revise the distances of his two trans-Neptunians to 44 and 66 AU, and he gave them masses of 5 and 24 Earth masses.

From 1908 to 1932, Pickering proposed seven hypothetical planets - O, P, Q, R, S, T and U. His final elements for O and P define completely different bodies than the orginal ones, so the total can be set at nine, certainly the record for planetary prognostication. Most of Pickerings predictions are only of passing interest as curiosities. In 1911, Pickering suggested that planet Q had a mass of 20,000 Earths, making it 63 times more massive than Jupiter or about 1/6 the Sun's mass, close to a star of minimal mass. Pickering said planet Q had a highly elliptical orbit.

In later years only planet P seriously occupied his attention. In 1928, he reduced the distance of P from 123 to 67.7 AU, and its period from 1400 to 556.6 years. He gave P a mass of 20 Earth masses and a magnitude of 11. In 1931, after the discovery of Pluto, he issued another elliptical orbit for P: distance of 75.5 AU, period of 656 years, mass of 50 Earth masses, eccentricity of 0.265, and inclination of 37°. These values were close to the ones given for the 1911 orbit. His Planet S, proposed in 1928 and given elements in 1931, was put at 48.3 AU distance (close to Lowell's Planet X at 47.5 AU) with a period of 336 years, a mass equal to five Earths, and a magnitude 15. In 1929, Pickering proposed planet U with a distance of 5.79 AU and a period of 13.93 years, calculations that placed it barely outside Jupiter's orbit. Pickering gave Planet U a mass of 0.045 Earth masses and an eccentricity of 0.26. The least of Pickering's planets is planet T, suggested in 1931: distance of 32.8 AU and a period of 188 years.

Pickering's different elements for planet O were: 

      Mean dist  Period      Mass     Magnitude  Node Incl Longitude
1908    51.9     373.5 y   2 Earth's  11.5-13.4              105.13
1919    55.1     409   y                 15      100   15
1928    35.23    209.2 y   0.5 Earth's   12

Lowell's Search for "X"

Percival Lowell, most well known as a proponent for canals on Mars, built a private observatory in Flagstaff, Arizona. Lowell called his hypothetical planet Planet X, and performed several searches for it, without success. Lowell's first search for Planet X came to an end in 1909, but in 1913 he started a second search, with a new prediction of Planet X:
  • epoch, 1850-01-01;
  • mean longitude, 11.67°,
  • perihileon longitude, 186°,
  • eccentricity, 0.228,
  • mean distance, 47.5 AU;
  • long arc node, 110.99°,
  • inclination 7.30° and,
  • mass, 1/21,000 solar masses.
Lowell and others searched in vain for this Planet X in 1913-1915. In 1915, Lowell published his theoretical results of Planet X.

It is ironic that this very same year, 1915, two faint images of Pluto was recorded at Lowell observatory, although no one would realize it for another 15 years. Lowell's failure of finding Planet X was his greatest disappointment in life. He didn't spend much time looking for Planet X during the last two years of his life. Lowell died in 1916. On the nearly 1,000 plates exposed in this second search were 515 asteroids, 700 variable stars and 2 images of Pluto.

Pluto

The third search for Planet X began in April 1927. No progress was made in 1927-1928. In December 1929, a young farm boy and amateur astronomer, Clyde Tombaugh from Kansas, was hired to do the search. Tombaugh started his work in April 1929. On January 23 and 29, 1930, he exposed the pair of plates on which he found Pluto when examining them on February 18. By the time of his discovery, Tombaugh had examined hundreds of plate pairs and millions of stars.

The naming of Pluto is a story by itself. Early suggestions of the name of the new planet were: Atlas, Zymal, Artemis, Perseus, Vulcan, Tantalus, Idana, Cronus. The New York Times suggested Minerva, reporters suggested Osiris, Bacchus, Apollo, Erebus. Lowell's widow suggested Zeus, but later changed her mind to Constance. Many people suggested the planet be named Lowell. The staff of the Flagstaff observatory, where Pluto was discovered, suggested Cronus, Minerva, and Pluto. A few months later the planet was officially named Pluto. The name Pluto was originally suggested by Venetia Burney, an 11-year-old schoolgirl in Oxford, England.

The very first orbit computed for Pluto yielded an eccentricity of 0.909 and a period of 3,000 years. This cast some doubt on whether it was actually a planet. However, a few months later, considerably better orbital elements for Pluto was obtained. Below is a comparison of the orbital elements of Lowell's Planet X, Pickering's Planet O, and Pluto: 

                          Lowell's X    Pickering's O    Pluto

a (mean dist)              43.0           55.1           39.5
e (eccentricity)           0.202          0.31           0.248
i (inclination)            10             15             17.1
N (long asc node)          (not pred)     100            109.4
W (long perihelion)        204.9          280.1          223.4
T (perihelion date)        Feb 1991       Jan 2129       Sept 1989
u (mean annual motion)     1.2411         0.880          1.451
P (period, years)          282            409.1          248
T (perihel. date)          1991.2         2129.1         1989.8
E (long 1930.0)            102.7          102.6          108.5
m (mass, Earth=1)          6.6            2.0            0.002
M (magnitude)              12-13           15            15
With the discovery of Pluto, it would seem that the search for Planet X had come to an end. Or had it? The new planet turned out to be disappointingly small; at the time, it was estimated that Pluto's mass was only about 10 percent that of the Earth's mass. Over the years that followed, the mass estimates included: 
    Crommelin 1930:     0.11      (Earth masses)
    Nicholson 1931:     0.94
    Wylie, 1942:        0.91
    Brouwer, 1949:      0.8-0.9
    Kuiper, 1950:       0.10
    1965:             < 0.14    (occultation of faint star by Pluto)
    Seidelmann, 1968:   0.14
    Seidelmann, 1971:   0.11
    Cruikshank, 1976:   0.002
The matter wasn't settled until James W. Christy discovered Pluto's moon Charon in June 1978. Christy was able to confirm Cruikshank's estimate that Pluto's mass was only 1/1000 that of Earth. To put it another way, the ninth planet has only about 20 percent of the mass of our Moon.

The Search for Planet X, 1930 - Present

Pluto's low mass means the planet is hopelessly inadequate to produce measureable gravitational perturbations on Uranus and Neptune. Pluto could not be Lowell's Planet X - the planet found was not the planet sought. What seemed to be another triumph of celestial mechanics turned out to be an accident, or rather a result of the intelligence and thoroughness of Clyde Tombaugh's search.

Tombaugh continued his search another 13 years, and examined the sky from the north celestial pole to 50° south declination, down to magnitude 16-17, sometimes even 18. Tombaugh examined some 90 million images of some 30 million stars over more than 30,000 square degrees on the sky. He found one new globular cluster, 5 new open star clusters, one new supercluster of 1,800 galaxies and several new small galaxy clusters, one new comet, about 775 new asteroids, but no new planet except Pluto. Tombaugh concluded that no unknown planet brighter than magnitude 16.5 existed. Only a planet in an almost polar orbit and situated near the south celestial pole could have escaped his detection. He could have picked up a Neptune-sized planet at seven times the distance of Pluto, or a Pluto-sized planet out to 60 AU.

During this period, other astronomers searched for additional planets. Another short-lived trans-Neptunian suspect was reported on April 22, 1930 by R.M. Stewart in Ottawa, Canada. It was reported from plates taken in 1924. Crommelin computed an orbit with a distance of 39.82 AU, an ascending node of 280.49°, and an inclination 49.7°. Tombaugh searched for the "Ottawa object" without finding it. Several other searches were made, but nothing was ever found.

Pickering continued to predict new planets. Others also predicted new planets on theoretical grounds (Lowell himself had already suggested a second trans-Neptunian at about 75 AU). In 1946, Francis M. E. Sevin suggested a trans-Plutonian planet at 78 AU. He first derived this from a curious empirical method by which he grouped the planets and the errratic asteroid Hidalgo, into two groups of inner and outer bodies: 

   Group I:     Mercury   Venus   Earth    Mars   Asteroids  Jupiter
   Group II:      ?       Pluto   Neptune  Uranus  Saturn    Hidalgo
He then added the logarithms of the periods of each pair of planets, finding a roughly constant sum of about 7.34. Assuming this sum to be valid for Mercury and the trans-Pluton, too, he arrived at a period of about 677 years for "Trans-Pluto". Sevin later worked out a full set of elements for "Trans-Pluto": a distance of 77.8 AU, a period of 685.8 years, an eccentricity of 0.3, and a mass of 11.6 Earth masses. His prediction stirred little interest among astronomers.

In 1950, K. Schutte of Munich used data from eight periodic comets to suggest a trans-Pluto planet at 77 AU. Four years later, H. H. Kitzinger of Karlsruhe, using the same eight comets, extended and refined the work, finding the supposed planet to be at 65 AU, with a period of 523.5 years, an orbital inclination of 56°, and an estimated magnitude of 11.

In 1957, Kitzinger reworked the problem and arrived at new elements: a distance of 75.1 AU, a period of 650 years, an inclination of 40°, and a magnitude around 10. After unsuccessful photographic searches, he re-worked the problem once again in 1959, arriving at a mean distance of 77 AU, a period of 675.7 years, an inclination of 38°, and an eccentricity of 0.07. This planet was not unlike Sevin's "Trans-Pluto," and in some ways similar to Pickering's final Planet P. No such planet has ever been found, though.

Halley's Comet has also been used as a "probe" for trans-Pluto planets. In 1942, R. S. Richardson found that an Earth-sized planet at 36.2 AU, or 1 AU beyond Halley's aphelion, would delay Halley's perihelion passage so that it agreed better with observations. A planet at 35.3 AU of 0.1 Earth masses would have a similar effect. In 1972, Brady predicted a planet at 59.9 AU with a period of 464 years, an eccentricity of 0.07, an inclination of 120° (i.e. being in a retrogade orbit), and a magnitude of 13-14. This planet would be about the size of Saturn. Such a trans-Plutonian planet would reduce the residuals of Halley's Comet significantly back to the 1456 perihelium passage. This gigantic trans-Plutonian planet was also searched for, but never found.

Recent Searches

Tom van Flandern examined the positions of Uranus and Neptune in the 1970s. The calculated orbit of Neptune fit observations only for a few years, and then started to drift away. The orbit for Uranus fit the observations during one revolution but not during the previous revolution.

In 1976, van Flandern became convinced that there is a tenth planet. After the discovery of Charon in 1978 showed the mass of Pluto to be much smaller than expected, van Flandern convinced his U.S. Naval Observatory colleague Robert S. Harrington of the existence of this tenth planet. They started to collaborate by investigate the Neptunian satellite system. Their views soon diverged. Van Flandern thought the tenth planet had formed beyond Neptune's orbit, while Harrington believed it had formed between the orbits of Uranus and Neptune. Van Flandern thought more data was needed, such as an improved mass for Neptune furnished by Voyager 2.

Harrington started to search for the planet by brute force. He began in 1979, and by 1987 he had still not found any planet. Van Flandern and Harrington suggested that the tenth planet might be near aphelion in a highly elliptical orbit. If the planet is dark, it might be as faint as magnitude 16-17, suggests van Flandern.

In 1987, John Anderson at the Jet Propulsion Laboratory examined the motions of the spacecraft Pioneer 10 and Pioneer 11, to see if any deflection due to unknown gravity forces could be found. None was found; Anderson concluded that a tenth planet most likely exists.

Anderson concluded that the tenth planet must have a highly elliptical orbit, carrying it far away to be undetectable now but periodically bringing it close enough to leave its disturbing signature on the paths of the outer planets. He suggests a mass of five Earth masses, an orbital period of about 700 to 1,000 years, and a highly inclined orbit. Its perturbations on the outer planets won't be detected again until 2600. Anderson hoped that the two Voyagers would help to pin down the location of this planet.

Conley Powell of JPL also analyzed the planetary motions. He also found that the observations of Uranus suddenly did fit the calculations much better after 1910 than before. Powell suggested a planet with 2.9 Earth masses at 60.8 AU from the Sun, a period of 494 years, an inclination of 8.3° and only a small eccentricity.

Powell was intrigued that the period was approximately twice Pluto's and three times Neptune's period, suggesting that the planet he thought he saw in the data had an orbit stabilized by mutual resonance with its nearest neighbors despite their vast separation. The solution called for the planet to be in Gemini, and to be brighter than Pluto when it was discovered. A search was performed in 1987 at Lowell Observatory for Powell's planet - nothing was found. Powell re-examined his solution and revised the elements: 0.87 Earth masses, a distance of 39.8 AU, a period of 251 years, and an eccentricity 0.26. The orbit was very similar to Pluto's orbit. Currently, Powell's new planet should be in Leo, at magnitude 12; however, Powell thinks it's premature to search for it until the data is analyzed further.

Asteroid & Comet Discoveries

Even if no trans-Pluto planet is ever found, the interest in finding one has focused astronomers' attention on the outer parts of the solar system. During 1977-1984, Charles Kowal performed a new systematic search for undiscovered bodies in the solar system, using Palomar Observatory's 48-inch Schmidt telescope. In October 1987, he found the asteroid 1977 UB, later named Chiron, moving at mean distance of 13.7 AU, a period of 50.7 years, an eccentricity of 0.3786, and an inclination of 6.923°. Chiron has a diameter of about 50 kilometers (31 miles).

During his search, Kowal also found 5 comets and 15 asteroids, including Chiron, the most distant asteroid known when it was discovered. Kowal also recovered 4 lost comets and one lost asteroid. Kowal did not find a tenth planet, and concluded that there was no unknown planet brighter than 20th magnitude within 3° of the ecliptic.

Chiron was first announced as a "tenth planet," but was quickly designated as an asteroid. But Kowal suspected it may be very comet-like, and later it has even developed a short cometary tail. In 1995, Chiron was also classified as a comet - it is certainly the largest comet ever discovered.

In 1992, an even more distant asteroid was found, Pholus. Later that year, an asteroid outside Pluto's orbit was found, followed by five additional trans-Pluto asteroids in 1993 and at least a dozen in 1994.

Meanwhile, the spacecraft Pioneer 10 and 11 and Voyagers 1 and 2 had travelled outside the solar system, and were used as "probes" to investigate gravitational forces that might be caused by unknown planets. However, nothing has been found.

The Voyagers also yielded more accurate masses for the outer planets. When these updated masses were inserted in the numerical integrations of the solar system, the residuals in the positions of the outer planets finally disappeared. It seems like the search for "Planet X" finally has come to an end. There was no "Planet X" (Pluto doesn't really count), but instead an asteroid belt outside Neptune/Pluto was found.

The asteroids outside Jupiter's orbit that were known in August 1993 are as follows: 

Asteroid    a      e      Incl     Node   Arg perih Mean an Per  Name
           AU           deg      deg      deg      deg     yr

 944     5.79853 .658236 42.5914  21.6567  56.8478  60.1911 14.0 Hidalgo
2060    13.74883 .384822  6.9275 209.3969 339.2884 342.1686 51.0 Chiron
5145    20.44311 .575008 24.6871 119.3877 354.9451   7.1792 92.4 Pholus
5335    11.89073 .866990 61.8583 314.1316 191.3015  23.3556 41.0 1991DA

1992QB1 43.82934 .087611  2.2128 359.4129  44.0135 324.1086 290  "Smiley"
1993FW  43.9311  .04066   7.745  187.914  359.501    0.4259 291  "Karla"

                  Epoch:  1993-08-01.0  TT
In November 1994, these trans-Neptunian asteroids were known: 

Object     a     e    incl  R Mag  Diam  Discovery  Discoverers
          AU         deg          km      Date

1992 QB1  43.9  0.070  2.2   22.8   283   1992 Aug  Jewitt & Luu
1993 FW   43.9  0.047  7.7   22.8   286   1993 Mar  Jewitt & Luu
1993 RO   39.3  0.198  3.7   23.2   139   1993 Sep  Jewitt & Luu
1993 RP   39.3  0.114  2.6   24.5    96   1993 Sep  Jewitt & Luu
1993 SB   39.4  0.321  1.9   22.7   188   1993 Sep  Williams et al.
1993 SC   39.5  0.185  5.2   21.7   319   1993 Sep  Williams et al.
1994 ES2  45.3  0.012  1.0   24.3   159   1994 Mar  Jewitt & Luu
1994 EV3  43.1  0.043  1.6   23.3   267   1994 Mar  Jewitt & Luu
1994 GV9  42.2  0.000  0.1   23.1   264   1994 Apr  Jewitt & Luu
1994 JQ1  43.3  0.000  3.8   22.4   382   1994 May  Irwin et al.
1994 JR1  39.4  0.118  3.8   22.9   238   1994 May  Irwin et al.
1994 JS   39.4  0.081  14.6  22.4   263   1994 May  Luu & Jewitt 
1994 JV   39.5  0.125  16.5  22.4   254   1994 May  Jewitt & Luu 
1994 TB   31.7  0.000  10.2  21.5   258   1994 Oct  Jewitt & Chen
1994 TG   42.3  0.000  6.8   23.0   232   1994 Oct  Chen et al.
1994 TG2  41.5  0.000  3.9   24.0   141   1994 Oct  Hainaut 
1994 TH   40.9  0.000  16.1  23.0   217   1994 Oct  Jewitt et al.
1994 VK8  43.5  0.000  1.4   22.5   273   1994 Nov  Fitzwilliams et al.
The trans-Neptunian bodies seem to form two groups. One group, composed of Pluto, 1993 SC, 1993 SB and 1993 RO, have eccentric orbits and a 3:2 resonance with Neptune. The second group, including 1992 QB1 and 1993 FW, is slightly further out and in rather low eccentricity.

Nemesis, the Sun's companion star, 1983-present

Suppose our Sun was not alone but had a companion star. Suppose that this companion star moved in an elliptical orbit, its solar distance varying between 90,000 AU (1.4 light years) and 20,000 AU, with a period of 30 million years. Also suppose this star is dark or at least very faint, and because of that we haven't noticed it yet.

This would mean that once every 30 million years that hypothetical companion star of the Sun would pass through the Oort cloud (a hypothetical cloud of proto-comets at a great distance from the Sun). During such a passage, the proto-comets in the Oort cloud would be stirred around. Some tens of thousands of years later, here on Earth we would notice a dramatic increase in the the number of comets passing the inner solar system. If the number of comets increases dramatically, so does the risk of the Earth colliding with the nucleus of one of those comets.

When examining the Earth's geological record, it appears that about once every 30 million years a mass extinction of life on Earth has occurred. The most well-known of those mass events is, of course, the dinosaur extinction some 65 million years ago. The theory predicts there will be another mass extinction in 15 million years.

This hypothetical "death companion" of the Sun was suggested in 1985 by Daniel P. Whitmire and John J. Matese of the University of Southern Lousiana. It has even received a name, Nemesis. One awkward fact of the Nemesis hypothesis is that there is no evidence whatever of a companion star of the Sun. It need not be very bright or very massive. A star much smaller and dimmer than the Sun would suffice, even if it was a brown or a black dwarf (a planet-like body insufficiently massive to start "burning hydrogen" like a star). It is possible that this star already exists in one of the catalogues of dim stars without anyone having noted something peculiar, namely the enormous apparent motion of that star against the background of more distant stars (i.e., its parallax). If Nemesis should be found, few will doubt that it is the primary cause of periodic mass extinctions on Earth.

In 1987, Whitmire and Matese suggested a tenth planet at 80 AU with a period of 700 years and an inclination of perhaps 45°, as an alternative to their "Nemesis" hypothesis. However, according to Eugene M. Shoemaker, this planet could not have caused those meteor showers that Whitmire and Matese suggested.

Nemesis is also a notion of mythical power. If an anthropologist of a previous generation had heard such a story from his informants, the resulting scholary tome would doubtless use words like 'primitive' or 'pre-scientific.' Consider this story:

There is another Sun in the sky, a Demon Sun we cannot see. Long ago, even before great grandmother's time, the Demon Sun attacked our Sun. Comets fell, and a terrible winter overtook the Earth. Almost all life was destroyed. The Demon Sun has attacked many times before. It will attack again.
This is why some scientists thought this theory was a joke when they first heard of it - an invisible Sun attacking the Earth with comets sounds like delusion or myth. It deserves an additional dollop of skepticism for that reason: we are always in danger of deceiving ourselves. But even if the theory is speculative, it's serious and respectable, because its main idea is testable: you find the star and examine its properties.

However, the existence of Nemesis is not very likely. The Infrared Astronomical Satellite (IRAS) examined the entire sky in the far infrared (IR) spectrum. However, it did not find any evidence of a star that would fit the description of "Nemesis."

References

Ashbrook, Joseph. "The many moons of Dr Waltemath." Sky and Telescope, Vol. 28, p. 218, October 1964.

Corliss, William R. "Mysterious Universe: A handbook of astronomical anomalies." Sourcebook Project, 1979.

Hoyt, William Graves. "Planet X and Pluto." University of Arizona Press, 1980.

Jay, Delphine. "The Lilith Ephemeris." American Federation of Astrologers, 1983.

Ley, Willy. "Watcher's of the skies." Viking Press New York, 1969.

Littman, Mark. "Planets Beyond - discovering the outer solar system." John Wiley, 1988.

Sagan, Carl and Ann Druyan. "Comet." Michael Joseph Ltd, 1985.

Van Flandern, Tom. "Dark Matter, Missing Planets& New Comets. Paradoxes resolved, origins illuminated." North Atlantic Books, 1993.

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