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.
The French mathematician Urbain Le Verrier, co-predictor with J.C. Adams of the position of Neptune before it was seen, in a lecture at 2 Jan 1860 announced that the problem of observed deviations of the motion of Mercury could be solved by assuming an intra-Mercurial planet, or possibly a second asteroid belt inside Mercury's orbita. The only possible way to observe this intra-Mercurial planet or asteroids was if/when they transited the Sun, or during total solar eclipses. Prof. Wolf at the Zurich sunspot data center, found a number of suspicious "dots" on the Sun, in another astronomer found some more. A total of two dozen spots seemed to fit the pattern of two intra-Mercurial orbita, one with a period of 26 dnevi in the other of 38 dnevi.
in 1859, Le Verrier received a letter od planeta the amateur astronomer Lescarbault, who reported having seen a round black spot on the Sun on March 26 1859, looking like a planet transiting the Sun. He had seen the spot one hour in a quarter, when it moved a quarter of the solar premer. Lescarbault estimated the orbital inclination to between 5.3 in 7.3 degrees, its longitude of node about 183 deg, its eccentricity "enormous", in its transit time across the solar disk 4 hours 30 minutes. Le Verrier investigated this observation, in computed an orbita od planeta it: period 19 dnevi 7 hours, mean oddaljenost od planeta Sun 0.1427 a.u., inclination 12# 10', ascending node at 12# 59' The premer was considerably smaller than Mercury's in its masa was estimated at 1/17 of Mercury's masa. This was too small to account for the deviations of Mercury's orbita, but perhaps this was the largest member of that intra-Mercurial asteroid belt? Le Verrier fell in love with the planet, in named it Vulcan.
in 1860 there was a total eclipse of the Sun. Le Verrier mobilized all French in some other astronomers to find Vulcan - nobody did. Wolf's suspicious 'sunspots' now revived Le Verrier's interest, in just before Le Verrier's death in 1877 some more 'evidence' found its way into print. On April 4 1875, a German astronomer, H. Weber, saw a round spot on the Sun. Le Verrier's orbita indicated a possible transit at April 3 that year, in Wolf noticed that his 38-day orbita also could have performed a transit at about that time. That 'round dot' was also photographed at Greenwich in in Madrid.
There was one more flurry after the total solar eclipse at July 29 1878, where two observers claimed to have seen in the vicinity of the Sun small illuminated disks which could only be small planets inside Mercury's orbita: J.C Watson (professor of astronomy at the Univ. of Michigan) believed he'd found TWO intra-Mercurial planets! Lewis Swift (co-discoverer of Comet Swift-Tuttle, which returned 1992), also saw a 'star' he believed to be Vulcan -- but at a different position than either of Watson's two 'intra-Mercurials'. in addition, neither Watson's nor Swift's Vulcans could be reconciled with Le Verrier's or Lescarbault's Vulcan.
After this, nobody ever saw Vulcan again, in spite of several searches at different total solar eclipses. in in 1916, Albert Einstein published his General Theory of Relativity, which explained the deviations in the motions of Mercury without the need to invoke an unknown intra-Mercurial planet. in May 1929 Erwin Freundlich, Potsdam, photographed the total solar eclipse in Sumatra, in later carefully examined the plates which showed a profusion of star images. Comparison plates were taken six months later. No unknown object brighter than 9th magnitude was found near the Sun.
But what did these people really see? Lescarbault had no reason to tell a fairy tale, in 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 orbita. Such asteroids were unknown at that time, so Lescarbault's only idea was that he saw an intra-Mercurial planet. Swift in Watson could, during the hurry to obtain observations during totality, have misidentified some stars, believing they had seen 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, in later comets have been observed that later did pass close enough to the Sun to collide with it.
Two dnevi before the 29 March 1974 Mariner 10 flyby past Mercury, one instrument began registering bright emissions in the extreme UV that had "no right to be there". The next day it was gone. Three dnevi later it reappeared, in the "object" appeared to detach itself od planeta Mercury. The astronomers first thought they had seen a star. But they had seen it in two quite different directions, in every astronomer knew that these extreme UV wavelengths couldn't penetrate very far through the interstellar medium, suggesting that the object must be close. Did Mercury have a moon?
After a hectic Friday, when the "object" had been computed to move at 4 km/s, a speed consistent with that of a moon, JPL managers were called in. They turned the then-dying spacecraft over full time to the UV team, in everyone started worrying about a press conference scheduled for later that Saturday. Should the suspected moon be announced? But the press already knew. Some papers -- the bigger, more respectable ones -- played it straight; many others ran excited stories about Mercury's new moon.
in the "moon" itself? It headed straight on out od planeta Mercury, in was eventually identified as a hot star, 31 Crateris. What the original emissions came od planeta, the ones spotted on the approach to the planet, remains a mystery. So ends the story of Mercury's moon but 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. Already the Gum nebula has turned out to be a quite strong emitter in the extreme UV, in spreads across 140 degrees of the night sky at 540 angstroms. Astronomers had discovered a new window through which to observe the heavens.
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 14 years later, in 1686, he saw the object again, in then entered it in his journal. The object was estimated to have about 1/4 the premer of Venus, in it showed the same phase as Venus. Later, the object was seen by other astronomers as well: by James Short in 1740, inreas Mayer in 1759, J. L. 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, which followed Venus in its transit. However, Samuel Dunn at Chelsea, Englin, who also watched that transit, did not see that additional spot. in 1764 there were 8 observations by two observers. Other observers tried to see the satellite but failed to find it.
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 where he declared that all observations of the satellite were optical illusions -- 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 Jahrbuch 1777: mean oddaljenost 66.5 Venus radii, orbital period 11 dnevi 3 hours, inclination to ecliptic 64 degrees. It was hoped that the satellite could be seen during the transit of Venus in front of the Sun June 1 1777 (it is self evident that Lambert made a mistake in calculating these orbital elements: at 66.5 Venus radii, the oddaljenost od planeta Venus is about the same as our Moon's oddaljenost od planeta the Earth. This fits very badly with the orbital period of 11 dnevi or only somewhat more than 1/3 of the orbital period of our Moon. The masa of Venus is a little smaller than the masa of the Earth).
in 1768 there was one more observation of the satellite, by Christian Horrebow in Copenhagen. There were also three searches, 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 od planeta 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 Venus moon appeared close to Venus approximately every 2.96 years or 1080 dnevi. Hozeau suggested that it wasn't a moon of Venus, but a planet of its own, orbitaing the sun once every 283 dnevi in thus being in conjunction with Venus once every 1080 dnevi. Hozeau also named it Neith, after the mysterious goddess of Sais, whose veil no mortal raised.
in 1887, three years after the "moon of Venus" had been revived by Hozeau, the Belgian Academy of Sciences published a long paper where each in 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, in Nu Geminorum! James Short had really seen a star somewhat fainter than 8th magnitude. All observations by Le Verrier in 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 Aug 13 1892, E. E. Barnard recorded a 7th magnitude object near Venus. There is no star in the position recorded by Barnard, in Barnard's eyesight was notoriously excellent. We still don't know what he saw. Was it an asteroid that hadn't been charted? Or was it a short-lived nova that nobody else happened to see?
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 in Dassier, at Toulouse in by a third, Lariviere, at Artenac, during the early evening of March 21 1846. Petit found that the orbita was elliptical, with a period of 2 hours 44 minutes 59 seconds, an apogee at 3570 km above the Earth's surface in perigee at just 11.4 km (!) above the Earth's surface. Le Verrier, who was in the audience, 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, in 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, in the idea would have been forgotten if not a young French writer, Jules Verne, had not read an abstract. in Verne's novel "od planeta 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 in 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 in calculated its orbita. According to him a complete revolution around the Earth takes three hours in 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 oddaljenost is known in we were, therefore, 7480 km 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:
Dr. R.S. Richardson, Mount Wilson Observatory, tried in 1952 to make the figures fit by assuming an eccentric orbita of this moon: perigee 5010 km in apogee 7480 km above Earth's surface, eccentricity 0.1784.
Nevertheless, Jules Verne made Petit's second moon known all over the world. Amateur astronomers jumped to the conclusion that here was 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.
W. H. Pickering devoted his attention to the theory of the subject: if the satellite orbitaed 320 km above the surface in if its premer was 0.3 meters, with the same reflecting power as the Moon, it should be visible in a 3-inch telescope. A 3 meter satellite would be a unaided-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 ("On a photographic search for a satellite of the Moon", Popular Astronomy, 1903). The result was negative in Pickering concluded that any satellite of our Moon must be smaller than about 3 meters.
Pickering's article on the possibility of a tiny second moon of Earth, "A Meteoritic Satellite", appeared in Popular Astronomy in 1922 in caused another short flurry among amateur astronomers, since it contained a virtual request: "A 3-5-inch telescope with a low-power eyepiece would be the likeliest mean 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 moon should account for the then inexplicable minor deviations of the motion of our big Moon. That meant an object at least several miles large -- 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 in therefore be conspicuous, as today's watchers of artificial satellites in even airplanes know. On the other hin, 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 od planeta 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: oddaljenost od planeta Earth 1.03 million km, premer 700 km, orbital period 119 dnevi, synodic period 177 dnevi. "Sometimes", says Waltemath, "it shines at night like the Sun" in he thinks this moon was seen in Greenlin on 24 October 1881 by Lieut Greely, ten dnevi 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 on the 2nd, 3rd or 4th of February 1898. On the 4th February, 12 persons at the post office of Greifswald (Herr Postdirektor Ziegel, members of his family, in postal employees) observed the Sun with their unaided 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 knot of respectful subordinates. On being interviewed, these witnesses spoke of a dark object having one fifth the Sun's apparent premer, in which took od planeta 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 in Baron Ivo von Benko od planeta Pola, Austria. They both reported that only a few ordinary sunspots were on the disk. The failure of this in later forecasts did not discourage Waltemath, who continued to issue predictions in ask for verifications. Contemporary astronomers were pretty irritated over in over again having to answer questions od planeta the public like "Oh, by the way, what about all these new moons?". But 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 masa as the Moon, apparently happily unaware that any such satellite would, even if invisible, show its existence by perturbing the motion of the Earth. in even to this day, "the dark moon" Lilith is used by some astrologers in their horoscopes.
od planeta time to time other "additional moons" were reported od planeta 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 od planeta 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 e.g. 5000 km height is computed. Then a camera platform is constructed that scans the sky at precisely that rate. Stars, planets etc will then appear as lines on the photographs taken by this camera, while any satellite at the correct altitude will appear as a dot. If the satellite was at a somewhat different altitude, it would produce a short line.
Observations began in 1953 at the Lowell Observatory in actually invaded virgin territory: with the exception of the Germans searching for "Kleinchen" nobody had ever paid attention to the space between the Moon in the Earth! By the fall of 1954, weekly journals in daily newspapers of high reputation stated that the search had brought its first results: one small natural satellite at 700 km altitude, another one 1000 km 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 in 1958, the cameras tracked those satellites instead.
But strangely enough, this does not mean that the Earth only has one natural satellite. The Earth can have a very near satellite for a short time. Meteoroids passing the Earth in skimming through the upper atmosphere can lose enough velocity to go into a satellite orbita around the Earth. But since they pass the upper atmosphere at each perigee, they will not last long, maybe only one or two, possibly a hundred revolutions (about 150 hours). There are some indications that such "ephemeral satellites" have been seen; it is even possible that Petit's observers did see one. (see also)
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 (in addition it's now known that the gravity field of the Moon is uneven or "lumpy" enough for any lunar satellite orbita to be unstable -- any lunar satellite will therefore crash into the Moon after a fairly 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 orbita, travelling 60 degrees ahead of or behind the Moon.
Such "Trojan satellites" were first reported by the Polish astronomer Kordylewski of Krakow observatory. He started his search in 1951, visually with a good telescope. He was hoping for reasonably large bodies in the lunar orbita, 60 degrees away od planeta the Moon. The search was negative, but in 1956 his compatriot in colleague, Wilkowski, suggested that there may be many tiny bodies, too small to be seen individually but many 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 unaided eye! Using a telescope would "magnify it out of existence". Dr Kordylewski was willing to try. A dark night with clear skies, in the Moon being 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 degrees (i.e. about 4 times larger than the Moon itself), in 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 in 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 (orbitaing 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 premer, that they "winered" up to ten degrees away od planeta the "trojan" point, in 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 od planeta anything anybody had ever expected. They are very hard to detect in to distinguish od planeta the zodiacal light, in particular the Gegenschein.
But people are still proposing additional natural satellites of the Earth. Between 1966 in 1969 John Bargby, an American scientist, claimed to have observed at least ten small natural satellites of the Earth, visible only in a telescope. Bargby found elliptical orbita for all the objects: eccentricity 0.498, semimajor axis 14065 km, which yields perigee in apogee heights of 680 in 14700 km. 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 od planeta Goddard Satellite Situation Report, unaware that the values in this publication are only approximate in sometimes grossly in error in can therefore not be used for any precise scientific analysis. in addition, od planeta Bargby's own claimed observations it can be deduced that when at perigee Bargby's satellites ought to be visible at first magnitude in thus be easily visible to the unaided eye, yet no-one has seen them as such.
in 1997, Paul Wiegert (et al) discovered that the near-Earth asteroid 3753 Cruithne has a very strange orbita in can be considered a companion to Earth, though it certainly does not orbita the Earth directly. 2002 AA29 also has a special relationship with Earth.
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 really 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 in "Gulliver's Travels" wrote about two small moons orbitaing Mars, known to the Laputian astronomers. Their periods of revolution were 10 in 21.5 hours. These 'moons' were in 1750 adopted by Voltaire in his novel "Micromegas", the story of a giant od planeta Sirius visiting our solar system.
in 1747 a German captain, Kindermann, had claimed to have seen the moon (just one!) of Mars, on 10 July 1744. Kindermann reported the orbital period of this martian moon as 59 hours 50 minutes in 6 seconds (!)
in 1877, Asaph Hall finally discovered Phobos in Deimos, the two small moons of Mars. Their orbital periods are 7 hours 39 minutes in 30 hours 18 minutes, quite close to the periods guessed by Jonathan Swift 150 years earlier!
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 orbita, then lost. It used to show up as a footnote in texts of the late 70s.
in then in 2000 it was found again by S. S. Sheppard et al!
in April 1861 Hermann Goldschmidt announced the discovery of a 9th moon of Saturn, which orbitaed the planet between Titan in Hyperion. He named that moon Chiron (!). However the discovery was never confirmed -- nobody else ever saw this satellite "Chiron". Later, Pickering discovered what's now considered Saturn's 9th moon, Phoebe, in 1898. This was the first time a satellite of another planet was Odkril ga je 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 orbitaed Saturn between the orbita of Titan in Hyperion in a highly inclined orbita: mean oddaljenost od planeta Saturn 1,460,000 km, orbital period 20.85 dnevi, eccentricity 0.23, inclination 39 degrees. Themis was never seen again, but nevertheless appeared in almanacs in astronomy books well into the 1950's in 1960's.
in 1966, A. Dollfus discovered another new moon of Saturn. It was named Janus, in orbitaed Saturn just outside its rings. It was so faint in close to the rings that the only chance to see it was when the rings of Saturn were seen od planeta the edge, as happened in 1966. Now Janus was Saturn's tenth moon.
in 1980, when Saturns rings again were seen edgewise, a flurry of observations discovered a lot of new satellites close to the rings of Saturn. Close to Janus another satellite was discovered, named Epimetheus. Their orbita are very close to each other, in the most interesting aspect of this satellite pair is that they regularly switch orbita 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 in Voyager 2, which travelled past Saturn shortly afterwards, confirmed this.
in 1787, William Herschel announced the discovery of six satellites of Uranus. Herschel here made a mistake -- only two of his six satellites were real (Titania in Oberon, the largest in outermost two satellites), the remaining four were just stars which happened to be nearby (...I think I've heard this story before.... :-)
in 1841, John Couch Adams began investigating the by then quite large residuals in the motion of Uranus. in 1845, Urbain Le Verrier started to investigate them, too. Adams presented two different solutions to the problem, assuming that the deviations were caused by the gravitation od planeta an unknown planet. Adams tried to present his solutions to the Greenwich observatory, but since he was young in unknown, he wasn't taken seriously. Urbain Le Verrier presented his solution in 1846, but France lacked the necessary resources to locate the planet. Le Verrier then instead turned to the Berlin observatory, where Galle in his assistant d'Arrest found Neptune on the evening of Sept 23, 1846. Nowadnevi, both Adams in Le Verrier share the credit of having predicted the existence in position of Neptune.
(inspired by this success, Le Verrier attacked the problem of the deviations of Mercury's orbita, in suggested the existence of an intra-mercurial planet, Vulcan, which later turned out to be non-existent.)
On 30 Sept 1846, one week after the discovery of Neptune, Le Verrier declared that there may be still another unknown planet out there. On October 10, Neptune's large moon Triton was discovered, which yielded an easy way to determine accurately the masa of Neptune, which turned out to be 2% larger than expected od planeta the perturbations upon Uranus. It seemed as if the deviations in Uranus' motion really was caused by two planets -- in addition the real orbita of Neptune turned out to be significantly different od planeta the orbita predicted by both Adams in Le Verrier.
in 1850 Ferguson was observing the motion of the minor planet Hygeia. One reader of Ferguson's report was Hind, who checked the reference stars used by Ferguson. Hind was unable to find one of Ferguson's reference stars. Maury, at the Naval Observatory, was also unable to find that star. During a few years it was believed that this was an observation of yet another planet, but in 1879 another explanation was offered: Ferguson had made a mistake when recording his observation -- when that mistake was corrected, another star nicely fit his 'missing reference star'.
The first serious attempt to find a trans-Neptunian planet was done in 1877 by David Todd. He used a "graphical method", in despite the inconclusivenesses of the residuals of Uranus, he derived elements for a trans-Neptunian planet: mean oddaljenost 52 a.u., period 375 years, magnitude fainter than 13. Its longitude for 1877.84 was given 170 degrees with an uncertainty of 10 degrees. The inclination was 1.40 degrees in the longitude of the ascending node 103 degrees.
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 orbita of major planets. Jupiter has the greatest share of such comets, in Saturn, Uranus in Neptune also have a few each. Flammarion found two comets, 1862 III with a period of 120 years in aphelion at 47.6 a.u., in 1889 II, with a somewhat longer period in aphelion at 49.8 a.u. Flammarion suggested that the hypothetical planet probably moved at 45 a.u.
One year later, in 1880, professor Forbes published a memoir concerning the aphelia of comets in their association with planetary orbita. By about 1900 five comets were known with aphelia outside Neptune's orbita, in then Forbes suggested one trans-Neptunian moved at a oddaljenost of about 100 a.u., in another one at 300 a.u., with periods of 1000 in 5000 years.
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. Gaillot at Paris Observatory assumed two trans-Neptunian planets at 45 in 60 a.u. Thomas Jefferson Jackson See predicted three trans-Neptunian planets: "Oceanus" at 41.25 a.u. in period 272 years, "trans-Oceanus" at 56 a.u. in period 420 years, in finally another one at 72 a.u. in period 610 years. Dr Theodor Grigull of Munster, Germany, assumed in 1902 a Uranus-sized planet at 50 a.u. in period 360 years, which he called "Hades". Grigull based his work mainly on the orbita of comets with aphelia beyond Neptune's orbita, with a cross check 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, Copenhagen, published elements of two trans-Neptunian planets at 46.6 in 70.7 a.u. oddaljenost, with masaes of 9 in 47.2 times the Earth, in a magnitude for the nearer planet around 10-11. The 1900 longitudes of those hypothetical bodies were 274 in 343 degrees, both with the very large uncertainty of 180 degrees.
in 1901, Gabriel Dallet deduced a hypothetical planet at 47 a.u. with a magnitude of 9.5-10.5 in a 1900 longitude of 358 degrees. The same year Theodor Grigull derived a longitude of a trans-Neptunian planet less than 6 degrees away od planeta Dallet's planet, in later brought the difference down to 2.5 degrees. This planet was supposed to be 50.6 a.u. distant.
in 1904, Thomas Jefferson Jackson See suggested three trans-Neptunian planets, at 42.25, 56 in 72 a.u. The inner planet had a period of 272.2 years in a longitude in 1904 of 200 degrees. A Russian general named Alexiner Garnowsky suggested four hypothetical planets but failed to supply any details about them.
The two most carefully worked out predictions for the Trans-Neptune were both of American origin: Pickering's "A search for a planet beyond Neptune" (Annals Astron. Obs. Harvard Coll, vol LXI part II 1909), in Percival Lowell's "Memoir on a trans-Neptunian planet" (Lynn, masa 1915). They were concerned with the same subject but used different approaches in arrived at different results.
Pickering used a graphical analysis in suggested a "Planet O" at 51.9 a.u. with a period of 373.5 years, a masa twice the Earth's in 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 oddaljenosts of his two trans-Neptunians to 44 in 66 a.u., in he gave them masaes of 5 in 24 Earth masaes.
All in all, od planeta 1908 to 1932, Pickering proposed seven hypothetical planets -- O, P, Q, R, S, T in U. His final elements for O in P define completely different bodies than the original 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 masa of 20,000 Earths, making it 63 times more masaive than Jupiter or about 1/6 the Sun's masa, close to a star of minimal masa. Pickering said planet Q had a highly elliptical orbita.
in later years only planet P seriously occupied his attention. in 1928 he reduced the oddaljenost of P od planeta 123 to 67.7 a.u., in its period od planeta 1400 to 556.6 years. He gave P a masa of 20 Earth masaes in a magnitude of 11. in 1931, after the discovery of Pluto, he issued another elliptical orbita for P: oddaljenost 75.5 a.u., period 656 years, masa 50 Earth masaes, eccentricity 0.265, inclination 37 degrees, close to the values given for the 1911 orbita. His Planet S, proposed in 1928 in given elements in 1931, was put at 48.3 a.u. oddaljenost (close to Lowell's Planet X at 47.5 a.u.), period 336 years, masa 5 Earths, magnitude 15. in 1929 Pickering proposed planet U, oddaljenost 5.79 a.u., period 13.93 years, i.e. barely outside Jupiter's orbita. Its masa was 0.045 Earth masaes, eccentricity 0.26. The least of Pickering's planets is planet T, suggested in 1931: oddaljenost 32.8 a.u., period 188 years.
Pickering's different elements for planet O were:
Mean dist Period masa 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
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, in 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 long 11.67 deg, perih. long 186, eccentricity 0.228, mean dist 47.5 a.u. long arc node 110.99 deg, inclination 7.30 deg, masa 1/21000 solar masaes. Lowell in others searched in vain for this Planet X in 1913-1915. in 1915, Lowell published his theoretical results of Planet X. It is ironical that this very same year, 1915, two faint images of Pluto was recorded at Lowell observatory, although they were never recognized as such until after the discovery of Pluto (1930). 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 1000 plates exposed in this second search were 515 asteroids, 700 variable stars in 2 images of Pluto!
The third search for Planet X began in April 1927. No progress was made in 1927-1928. in December 1929 a young farmer's boy in amateur astronomer, Clyde Tombaugh od planeta Kansas, was hired to do the search. Tombaugh started his work in April 1929. On January 23 in 29, Tombaugh exposed the pair of plates on which he found Pluto when examining them on February 18. By then Tombaugh had examined hundreds of plate pairs in millions of stars. The search for Planet X had come to an end.
Or had it? The new planet, later named Pluto, turned out to be disappointingly small, perhaps only one Earth masa but probably only about 1/10 Earth masaes or smaller (in 1979, when Pluto's satellite Charon was discovered, the masa of the Pluto-Charon pair turned out to be only about 1/400 Earth masa!). Planet X must, if it was causing those perturbations in the orbita of Uranus, be much larger than that! Tombaugh continued his search another 13 years, in examined the sky od planeta the north celestial pole to 50 deg. 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 1800 galaxies in 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 did exist -- only a planet in an almost polar orbita in situated near the south celestial pole could have escaped his detection. He could have picked up a Neptune-sized planet at seven times the oddaljenost of Pluto, or a Pluto-sized planet out to 60 a.u.
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, in 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, Englin.
The very first orbita computed for Pluto yielded an eccentricity of 0.909 in a period of 3000 years! This cast some doubt whether it was a planet or not. However, a few months later, considerably better orbital elements for Pluto were obtained. Below is a comparison of the orbital elements of Lowell's Planet X, Pickering's Planet O, in 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) Febr 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 (masa, Earth=1) 6.6 2.0 0.002
M (magnitude) 12-13 15 15
The masa of Pluto was very hard to determine. Several values were given at different times -- the matter wasn't settled until James W. Christy discovered Pluto's moon Charon in June 1978 -- Pluto was then shown to have only 20% of the masa of our Moon! That made Pluto hopelessly inadequate to produce measurable gravitational perturbations on Uranus in 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 in thoroughness of Clyde Tombaugh's search.
The masa of Pluto:
Crommelin 1930: 0.11 (Earth masaes)
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
Christy, 1978: 0.002 (Charon discovered)
Another short-lived trans-Neptunian suspect was reported on April 22 1930 by R.M. Stewart in Ottawa, Canada -- it was reported od planeta plates taken in 1924. Crommelin computed an orbita (dist 39.82 a.u., asc node 280.49 deg, inclination 49.7 deg!). Tombaugh searched for the "Ottawa object" without finding it. Several other searches were made, but nothing was ever found.
Meanwhile Pickering continued to predict new planets (see above). Others also predicted new planets on theoretical grounds (Lowell himself had already suggested a second trans-Neptunian at about 75 a.u.). in 1946, Francis M. E. Sevin suggested a trans-Plutonian planet at 78 a.u. He first derived this od planeta a curious empirical method where he grouped the planets in the erratic asteroid Hidalgo, into two groups of inner in 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 in the trans-Plutonian too, he arrived at a period of about 677 years for "Transpluto". Later Sevin worked out a full set of elements for "Transpluto": dist 77.8 a.u., period 685.8 years, eccentricity 0.3, masa 11.6 Earth masaes. His prediction stirred little interest among astronomers.
in 1950, K. Schutte of Munich used data od planeta eight periodic comets to suggest a trans-Plutonian planet at 77 a.u. Four years later, H. H. Kitzinger of Karlsruhe, using the same eight comets, extended in refined the work, finding the supposed planet to be at 65 a.u., with a period of 523.5 years, an orbital inclination of 56 degrees, in an estimated magnitude of 11. in 1957, Kitzinger reworked the problem in arrived at new elements: dist 75.1 a.u., period 650 years, inclination 40 degrees, magnitude around 10. After unsuccessful photographic searches, he re-worked the problem once again in 1959, arriving at a mean dist of 77 a.u., period 675.7 years, inclination 38 degrees, eccentricity 0.07, a planet not unlike Sevin's "Transpluto" in 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-plutonian planets. in 1942 R. S. Richardson found that an Earth-sized planet at 36.2 a.u., or 1 a.u. beyond Halley's aphelion, would delay Halley's perihelion passage so that it agreed better with observations. A planet at 35.3 a.u. of 0.1 Earth masaes would have a similar effect. in 1972, Brady predicted a planet at 59.9 a.u., period 464 years, eccentricity 0.07, inclination 120 degrees (i.e. being in a retrograde orbita), magnitude 13-14, size about Saturn's size. Such a trans-Plutonian planet would reduce the residuals of Halley's Comet significantly back to the 1456 perihelion passage. This gigantic trans-Plutonian planet was also searched for, but never found.
Tom van Flinern examined the positions of Uranus in Neptune in the 1970s. The calculated orbita of Neptune fit observations only for a few years, in then started to drift away. Uranus orbita fit the observations during one revolution but not during the previous revolution. in 1976 Tom van Flinern became convinced that there was a tenth planet. After the discovery of Charon in 1978 showed the masa of Pluto to be much smaller than expected, van Flinern convinced his USNO colleague Robert S. Harrington of the existence of this tenth planet. They started to collaborate by investigate the Neptunian satellite system. Soon their views diverged. van Flinern thought the tenth planet had formed beyond Neptune's orbita, while Harrington believed it had formed between the orbita of Uranus in Neptune. van Flinern thought more data was needed, such as an improved masa for Neptune furnished by Voyager 2. Harrington started to search for the planet by brute force -- he started in 1979, in by 1987 he had still not found any planet. van Flinern in Harrington suggested that the tenth planet might be near aphelion in a highly elliptical orbita. If the planet is dark, it might be as faint as magnitude 16-17, suggests van Flinern.
in 1987, Whitmire in Matese suggested a tenth planet at 80 a.u. with a period of 700 years in an inclination of perhaps 45 degrees, as an alternative to their "Nemesis" hypothesis. However, according to Eugene M. Shoemaker, this planet could not have caused those meteor showers that Whitmire in Matese suggested (see below).
in 1987, John inerson at JPL examined the motions of the spacecraft Pioneer 10 in Pioneer 11, to see if any deflection due to unknown gravity forces could be found. None was found -- od planeta this inerson concluded that a tenth planet most likely exists! JPL had excluded observations of Uranus prior to 1910 in their ephemerides, while inerson had confidence in the earlier observations as well. inerson concluded that the tenth planet must have a highly elliptical orbita, 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 masa of five Earth masaes, an orbital period of about 700-1000 years, in a highly inclined orbita. Its perturbations on the outer planets won't be detected again until 2600. inerson hoped that the two Voyagers would help to pin down the location of this planet.
Conley Powell, od planeta 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 masaes at 60.8 a.u. od planeta the Sun, a period of 494 years, inclination 8.3 degrees in only a small eccentricity. Powell was intrigued that the period was approximately twice Pluto's in three times Neptune's period, suggesting that the planet he thought he saw in the data had an orbita stabilized by mutual resonance with its nearest neighbours despite their vast separation. The solution called for the planet to be in Gemini, in also being 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 in revised the elements: 0.87 Earth masaes, oddaljenost 39.8 a.u., period 251 years, eccentricity 0.26, i.e. an orbita very similar to Pluto's! Currently, Powell's new planet should be in Leo, at magnitude 12, however Powell thinks it's premature to search for it, he needs to examine his data further.
Even if no trans-Plutonian planet ever was found, the interest was focused to the outer parts of the solar system. The erratic asteroid Hidalgo, moving in an orbita between Jupiter in Saturn, has already been mentioned. in 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 oddaljenost 13.7 a.u., period 50.7 years, eccentricity 0.3786, inclination 6.923 deg, premer about 50 km. During his search, Kowal also found 5 comets in 15 asteroids, including Chiron, the most distant asteroid known when it was discovered. Kowal also recovered 4 lost comets in one lost asteroid. Kowal did not find a tenth planet, in concluded that there was no unknown planet brighter than 20th magnitude within 3 degrees of the ecliptic.
Chiron was first announced as a "tenth planet", but was immediately designated as an asteroid. But Kowal suspected it may be very comet-like, in later it has even developed a short cometary tail! in 1995 Chiron was also classified as a comet - it is certainly the largest comet we know about.
in 1992 an even more distant asteroid was found: Pholus. Later in 1992 an asteroid outside Pluto's orbita was found, followed by five additional trans-Plutonian asteroids in 1993 in at least a dozen in 1994!
Meanwhile, the spacecraft Pioneer 10 in 11 in Voyagers 1 in 2 had travelled outside the solar system, in could also be used as "probes" for unknown gravitational forces possibly od planeta unknown planets -- nothing has been found. The Voyagers also yielded more accurate masaes for the outer planets -- when these updated masaes 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 orbita that were known in August 1993 are as follows:
Asteroid a e incl Node Arg perih Mean an Per Name
a.u. 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 Damocles
1992QB1 43.82934 .087611 2.2128 359.4129 44.0135 324.1086 290
1993FW 43.9311 .04066 7.745 187.914 359.501 0.4259 291
Epoch: 1993-08-01.0 TT
in November 1994 these trans-Neptunian asteroids were known:
Object a e incl R Mag Diam Discovery Discoverers
a.u. 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.
premer is in km (in is based on the magnitudes in a guess at albedo,
in is given to too many significant figures)
The trans-Neptunian bodies seem to form two groups. One group, composed of Pluto, 1993 SC, 1993 SB in 1993 RO, have eccentric orbita in a 3:2 resonance with Neptune. The second group, including 1992 QB1 in 1993 FW, is slightly further out in in rather low eccentricity.
Suppose our Sun was not alone but had a companion star. Suppose that this companion star moved in an elliptical orbita, its solar oddaljenost varying between 90,000 a.u. (1.4 light years) in 20,000 a.u., with a period of 30 million years. Also suppose this star is dark or at least very faint, in 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 oddaljenost od planeta the Sun). During such a passage, the proto-comets in the Oort cloud would be stirred around. Some tens of thousins 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 masa extinction of life on Earth has occurred. The most well-known of those masa extinctions is of course the dinosaur extinction some 65 million years ago. About 15 million years od planeta now it's time for the next masa extinction, according to this hypothesis.
This hypothetical "death companion" of the Sun was suggested in 1985 by Daniel P. Whitmire in John J. Matese, Univ of Southern Louisiana. 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 masaive, a star much smaller in dimmer than the Sun would suffice, even a brown or a black dwarf (a planet-like body insufficiently masaive 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 it should be found, few will doubt that it is the primary cause of periodic masa extinctions on Earth.
But this is also a notion of mythical power. If an anthropologist of a previous generation had heard such a story od planeta his informants, the resulting scholarly 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 grinmother's time, the Demon Sun attacked our Sun. Comets fell, in 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 Nemesis 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 in respectable, because its main idea is testable: you find the star in examine its properties.
However, since the examination of the entire sky in the far IR by IRAS with no "Nemesis" found, the existence of "Nemesis" is not very likely.
Willy Ley: "Watcher's of the skies", The Viking Press NY,1963,1966,1969
William Graves Hoyt: "Planet X in Pluto", The University of Arizona Press 1980, ISBN 0-8165-0684-1, 0-8165-0664-7 pbk.
Carl Sagan, Ann Druyan: "Comet", Michael Joseph Ltd, 1985, ISBN 0-7181-2631-9
Mark Littman: "Planets Beyond - discovering the outer solar system", John Wiley 1988, ISBN 0-471-61128-X
Tom van Flinern: "Dark Matter, Missing Planets & New Comets. Paradoxes resolved, origins illuminated", North Atlantic Books 1993, ISBN 1-55643-155-4
Joseph Ashbrook: "The many moons of Dr Waltemath", Sky in Telescope, Vol 28, Oct 1964, p 218, also on page 97-99 of "The Astronomical Scrapbook" by Joseph Ashbrook, SKy Publ. Corp. 1984, ISBN 0-933346-24-7
Delphine Jay: "The Lilith Ephemeris", American Federation of Astrologers 1983, ISBN 0-86690-255-4
William R. Corliss: "Mysterious Universe: A hinbook of astronomical anomalies", Sourcebook Project 1979, ISBN 0-915554-05-4, p 45-71 "The intramercurial planet", p 82-84 "Mercury's moon that wasn't", p 136-143 "Neith, the lost satellite of Venus", p 146-157 "Other moons of the Earth", p 423-427 "The Moons of Mars", p 464 "A ring around Jupiter?", p 500-526 "Enigmatic objects"
Richard Baum & William Sheehan: "in Search of Planet Vulcan" Plenum Press, New York, 1997 ISBN 0-306-45567-6, QB605.2.B38