We like to picture our solar system as an organized and mostly explored space. But much of our own backyard is still terra incognita, full of barely glimpsed worlds. Throughout history, astronomers have made spurious claims of theoretical moons and planets that might exist in our solar system. As opposed to strange and largely unscientific theories — such as a “twin Earth” forever hidden by the Sun — some of these celestial bodies have earned enough credence to make their way into astronomy textbooks, sometimes for decades. And while astronomers have discounted many of these theoretical bodies as more evidence is collected, the hunt continues for a few of these proposed worlds.
5. Second Moons of Earth
The curious claim that Earth may have more than one moon was first proposed by 19th century astronomers. Assertions that our own planet may host additional moons in addition to “the” Moon cropped up well into the early 20th century. One of the first claims came courtesy of Frédéric Petit in 1846, although “Petit’s Moon” was said to have a perigee of just 11 kilometers above the Earth’s surface at closest approach, which would have placed it grazing our atmosphere! Georg Waltemath created a stir when he made formal announcements for two extra moons of the Earth in 1898; the informal name “Lilith” even crept into the astronomical lexicon of the early 20th century. It is now known that the Earth does, on occasion, capture and release small asteroids as temporary moons. Thus far, the only documented natural “temporary moon” of the Earth has been an asteroid designated 2006 RH120, which orbited the Earth for nine months in 2006 and 2007. About 5 meters in size, such an asteroid would only be visible in the largest telescopes. The asteroid enters Earth orbit roughly every 20 years, and is expected to return in 2028.
4. The Strange Tale of Vulcan
No, we’re not talking about Spock’s homeworld … The prediction and discovery of Neptune by French astronomer Urbain Le Verrier stands as a watershed moment in early 18th century astronomy. For an encore, Le Verrier also predicted the existence of a world orbiting the Sun interior to the planet Mercury to explain its anomalous motion. In 1859, a French country doctor claimed to spy a fleeting world crossing the disk of the Sun, and the legend of Vulcan was born. Astronomers searched for the infamous world near the Sun during solar eclipses and predicted transits, and Vulcan was widely considered to be a real world in the 19th century. Einstein’s theory of general relativity eventually explained the anomalous orbit of Mercury, and talk of Vulcan faded into obscurity. But it’s interesting to note that there’s still an effort underway to detect asteroids near the Sun, dubbed Vulcanoids.
3. The Hunt For Planet X
Following the planet Neptune’s discovery in 1846, most astronomers were convinced even more distant planets awaited discovery. One astronomer in particular, Percival Lowell, seemed convinced that a “Planet X” must exist, given discrepancies in the predicted orbits of Neptune and Uranus. Clyde Tombaugh thought he’d discovered Planet X when he found Pluto in 1930, although we now know Pluto is extremely tiny, unable to affect the orbits of one of the gas giant planets.
The apparent anomalous motions of Neptune have been explained with more precise observations, yet the search for Planet X still makes news. In early 2014, researchers announced that observations from NASA’s Wide-Field Infrared Survey Explorer (WISE) spacecraft turned up evidence of thousands of new celestial bodies, including nearby stars that had been completely overlooked. But there was no sign of a Planet X, prompting one astronomer to say, “The outer solar system probably does not contain a large gas planet …”
2. A Massive World Named Tyche
Though the search for Planet X may be all but over, the discovery of distant bodies such as the dwarf planet Eris and the planetoid Quaoar far out in the Kuiper Belt in highly elliptical orbits suggest that a large gas giant-sized body may still lurk in the distant outer solar system. In 1999, University of Louisiana researchers made news when they proposed a possible massive planet nicknamed Tyche far out in our solar system. Thus far, Tyche has yet to turn up in dedicated searches of the sky, and another leading theory for the wacky, perturbed orbits of bodies seen far out in the distant solar system is the close passage of a star near our solar system early in its history. If this is the case, we’re lucky the Earth and inner worlds stayed put!
1. The Nemesis Hypothesis
As opposed to the Nibiru/Mayan apocalypse nonsense that made its rounds in 2012, the idea of Nemesis is a real proposal put forward in 1984 by astronomers that a large brown dwarf companion to the Sun might exist on a long, eccentric 26-million-year orbit. Even longer orbits have been suggested, and hypotheses to this effect have been made to tie in the concept with several mass extinctions found in the geological record. In this scenario, passages of Nemesis through the Oort Cloud into the inner solar system would bring a rain of comets toward Earth in its wake. Yet recent research seems to have ruled out Nemesis; infrared surveys such as the NEOWISE infrared space telescope have scoured the skies and ruled out the idea of a nearby red dwarf companion to our Sun, through the possibility always exists that a low mass (in the 10-20x Jupiter mass range) brown dwarf may still lurk deep in the depths of space. Or is it plural, as in nemeses?
One More: Moons For Mercury and Venus
To date, no one has discovered a moon orbiting Venus of Mercury, but astronomers have tried to supply these planets with satellites through the years. We’ve only gotten a full detailed map of the surface of the planet Mercury over the past couple years, as NASA’s MESSENGER spacecraft has entered orbit around the innermost world. It was thought, for a very brief time during the first flyby of Mercury by the Mariner 10 spacecraft in the 1970s, that astronomers had detected a moon of the innermost world: a press release was even prepared to this effect during those hectic few hours. A high flux of ultraviolet radiation detached from the planet Mercury was detected as the spacecraft approached the planet, which was hypothesized as being reflected off of an unseen moon. The culprit? Likely the background binary star 31 Crateris. To date, no moon of Mercury has been uncovered despite systematic searches.
Venus has also been thought to have its own moon on occasion. Dubbed Neith, this was a claimed sighting made by astronomers over the years, probably caused by internal reflections in inferior telescopes and/or background stars passing near the planet. Astronomer Giovanni Cassini was a staunch Neith proponent, having claimed to have sighted the moon in 1672 and 1686. Venus is a dazzlingly bright object in the eyepiece of a telescope, and can often create optical “lens flare” ghosts.