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Hypothetical Planets

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 mintues. Le Verrier investigated this observation, and computed the following orbit:

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:

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:

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:

"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,