You might think astronomers know the solar system pretty well. And they do. But they might not know its whole story. In fact, it’s possible another planet lurks beyond Neptune
In fact, it’s possible another planet lurks beyond Neptune, or even a faint, distant companion star to the Sun. Hypothetical planets in the solar system — along with real ones — have turned up in some pretty strange places.
Of course, the bright naked-eye planets — Mercury, Venus, Mars, Jupiter, and Saturn — were all known in antiquity and revered as gods because they showed free will to move among the stars. The first telescopically discovered planet was Uranus, found by William Herschel in 1781. After orbital calculations suggested a massive tug on Uranus being applied farther out, Johann Galle, with some help from Heinrich D’Arrest, discovered Neptune in 1846.
Perturbations in Neptune’s orbit suggested yet another, more distant planet, and many searches were conducted as early as 1877; Pluto was finally found by Clyde Tombaugh in 1930 by comparing pairs of photographic plates and detecting its motion. Oddly enough, the perturbations weren’t really there, or at least Pluto wasn’t massive enough to cause them. In 2006, astronomers decided Pluto wasn’t up to planetary standards, and so demoted it to a “dwarf planet.”
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Does the number 8 really constitute the whole inventory of the Sun’s planets? Many mathematical and observational exercises have led astronomers to suspect other major bodies orbit the Sun. As early as 1841, astronomers commenced a search for various “Planet Xs.” The first one turned out to be Neptune. The second was Pluto, after some seven different trans-Neptunian planets (with different masses and orbits) had been proposed by the most active searcher, E. C. Pickering, alone.
But even after Pluto’s discovery, astronomers predicted planets beyond, mostly on theoretical grounds. In 1946, Francis Sevin predicted the existence of “Transpluto,” a planet 7 billion miles (11 billion kilometers) from the Sun (Pluto’s average orbital distance is 3.6 billion miles).