Then felt I like some watcher of the skies
When a new planet swims into his ken
— John Keats
So here we have the major elements of the Solar System as we presently know it:
That's the known. But there's also the legendary. Just as geography has lost continents like Atlantis and Lemuria, and zoology has cryptids like Bigfoot and Nessie, so astronomy has its legends — dubious or discarded planets. We worked our way out from the Sun to interstellar space, so let's work our way back in as we consider the legendary planets:
The first legend is also the newest. It goes by several names, but "Nibiru" is popular just now. Nibiru was supposed to be a planet on course to hit or near-miss the Earth by the end of 2012, causing the axis of rotation to shift and many other catastrophes. I've heard it was visible by telescope from the southern hemisphere. Astronomers object that any such body – that big, arriving that soon – would be clearly visible without a telescope, and neither eye nor telescope has spotted any such thing. The reply was that it's all being hushed up. 2012 has come and gone, of course, but Nibiru theorists have simply rescheduled.
Nemesis is the same idea as Nibiru, only grander. Paleontologists David Raup and Jack Sepkoski noticed that mass extinctions seem to happen every 26 million years, and suggested this was because of a very big, very remote object in orbit around the Sun — a red dwarf star or super-Jovian planet or brown dwarf, swinging through the Oort cloud every 26 million years, knocking comets out of orbit. Some come showering down here into the inner Solar System, and one or two of those hit Earth, causing a mass extinction. No one has found Nemesis, though, and it isn't really certain that mass extinctions are as regular as all that.
The Kuiper belt does not trail off, as you'd expect, but stops abruptly. This is called the "Kuiper cliff" and the cause is unknown. One proposed cause is an undiscovered planet, theorized by Patryk Lykawka and named "Tyche" by him, after the Greek goddess of luck, the "good sister" of the disastrous goddess/planet Nemesis.
"Planet X" was the name for the unknown planet beyond Neptune, until they found Pluto. Then it was the name for any hypothetical planet beyond Pluto, especially when it became clear Pluto wasn't big enough to cause deviations in Neptune's orbit. Then the deviations went away on closer examination. The name has fallen out of use. Trans-Plutonian planets are still a possibility, though, if they're far enough out. (Or a reality, if you include dwarf planets like Eris.) Science fiction has often thrown in a tenth planet as a future astronomical discovery, often naming it "Persephone" or "Proserpina," after the goddess who is Pluto's wife in Greco-Roman myth.
These are two hypothetical moons of Saturn, each observed by one astronomer but no others. "Chiron" (not to be confused with the body in the centaur belt with the same name) was observed and named by Hermann Goldschmidt in 1861, who claimed it as the ninth moon of Saturn. The now-recognized ninth moon, Phoebe, was discovered in 1898 by William Henry Pickering. In 1905, Pickering claimed to discover a tenth moon, which he named "Themis." But no one else has ever seen them, and they're believed to be observational errors.
This time, it isn't the planet's existence that is questionable, but its history. In the 1950s, Immanuel Velikovsky came up with a very ... colorful theory about the history of the Solar System:
Several thousand years ago, a huge comet erupted out of Jupiter and careened around the Solar System, having several near-misses with Earth. It knocked Mars out of its orbit, and Mars also had near-misses with Earth. These and other dramatic events caused repeated catastrophes, remembered now as events such as the Ten Plagues of Egypt, Joshua's standing sun, the destruction of Sodom and Gomorrah, Noah's Flood, and so forth. Mars, Earth, and the comet then settled down into their present, smooth, nearly circular orbits. The comet is now Venus.
You will guess that mainstream scholarship didn't buy it. But Carl Sagan reported that a historian said to him that, though Velikovsky's history and archeology were obvious bunk, he thought the astronomy sounded interesting. Sagan was struck by this because, though the astronomy was obvious bunk to him, he had thought the history sounded interesting. Velikovsky sounds plausible if you don't know enough.
Velikovsky himself was neither an astronomer, a historian, nor an archeologist, but a psychologist. He attributed rejection of his ideas to people not wanting to face the traumas of the race's past.
Foolishly, the scientific community tried to censor Velikovsky's books when they came out. They failed and Velikovsky was able to pose as a martyr and second Galileo for the rest of his life. Carl Sagan, bless him, saw that Velikovsky had been badly treated, even if it hadn't taken, and actually got him a hearing before the AAAS in 1974. Nobody's minds were changed.
The asteroid belt between Mars and Jupiter was once taken for the remains of a shattered planet. Theory moved on, and it was taken as left-over rubble from the dawn of the Solar System, stuff that never got gathered into a planet. Theory has moved part way back, now, and the asteroids are taken as the remains of several little planets that collided and shattered. Howsobeit, the slain planet lives on in science fiction and pseudo-science, under various names.
Another fifth planet, this one was proposed by NASA scientists John Chambers and Jack Lissauer. It is supposed to have occupied an unstable orbit between Mars and the asteroid belt, to have caused an age of heavy meteor bombardment in the early Solar System, and to have crashed into the Sun billions of years ago.
Another hypothetical extinct planet from early cosmic history. The currently favored theory for the origin of the Moon is that it formed from the debris when a Mars-sized planet smacked into the young Earth. Geochemist Alex Halliday proposed the name "Theia" for that body, after the titaness who was the mother of Selene, the moon goddess of Greek myth.
The early Solar System was an exciting place.
How could you miss a moon around your own planet? Well, maybe if it was very small. In 1846, Frederic Petit claimed to have discovered a tiny additional moon on a close, quick, elliptical orbit. It was never confirmed. In 1898, Georg Waltemath began proposing a number of little moons, here and there. One of them was taken up by astrologer Walter Gornold in 1918, who named it "Lilith" and said it escaped general notice because it was so dark.
I wonder if Waltemath's moons are the inspiration for Basidium, the "Mushroom Planet," a tiny and normally invisible moon that is the home of elf-like aliens in a children's SF series by Eleanor Cameron.
Giovanni Cassini, the astronomer who discovered the divisions in Saturn's rings, spotted a small object close to Venus in 1672, and again in 1686, whereon he announced that he had found a moon circling Venus. Several other astronomers observed it, too, but could not fit their observations onto a consistent orbit. Other astronomers failed to find it at all. Nowadays, it's thought to have been misidentified stars or stray reflections. It was named after an Egyptian goddess.
Planets have elliptical orbits. When they stray from the ellipse, it Means Something. Mercury strays rather noticeably. In all other such cases, the deviations mean the gravity of an undiscovered planet, so Le Verrier, the astronomer who discovered Neptune based on orbital deviations by Uranus, theorized that Mercury's orbit was shifting because of the gravity of a planet orbiting still closer to the Sun, which he named "Vulcan" after the Roman god of fire. People looked for Vulcan and even thought they had spotted it from time to time, but never came up with a consistent orbit. Then, in 1915, Einstein published his General Theory of Relativity, revised our ideas of gravitation, and explained Mercury's orbit based on that new understanding of gravity. So the deviation in the orbit did Mean Something, but not what we first thought.
If there is any connection to the planet in "Star Trek," I don't know about it.
Our last legend is the earliest. Philolaos (c. 470 BC — c. 385 BC) was the first astronomer to posit that the Earth moves. But he didn't have it move around the Sun. Philolaos was a Pythagorean philosopher, and so believed mathematics was foundational to the nature of the universe — the origin of that conviction in modern science. But their approach to numbers was very different and strongly mystical. Their favorite numbers were 1, 4, and 10. There were only eight major bodies apparent in the universe — Earth and the seven visible planets (which included the Sun and Moon) — so the cosmos "needed" two more to get to ten. Philolaos proposed the Central Fire and Counter-Earth. All other bodies revolve around the Central Fire. Earth goes around once a day, passing the Sun, which only shines by reflected light. Earth keeps the same face toward the Central Fire, and we live on the outer hemisphere, so we never see it. Between us and the Central Fire there orbits Counter-Earth, Anticthon in Greek.
The Central Fire has dropped out. The modern version of Counter-Earth is on the far side of the Sun from us. But if it were really there, we would have seen its effects on the orbits of other planets, not to mention its effects on space probes and its being visible from the space probes.
But Counter-Earth lives on in science fiction. Marvel Comics, John Norman's "Gor" series, "Doctor Who?", several movies, and a fantasy novel by Avram Davidson all use the Counter-Earth.
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Copyright © Earl Wajenberg, 2012