For most of human history, our solar system had 6 planets (counting the Earth), the ones that could be observed by eye, wandering across the night sky. In 1781, William Herschel’s telescope revealed a seventh — Uranus — but it was the eighth planet that became the great triumph of nineteenth century Newtonian celestial mechanics. In the 1840s, unexplained perturbations in the orbit of Uranus led French mathematical astronomer Urbain Jean Joseph Le Verrier and English astronomer John Couch Adams to independently predict the existence and approximate orbit of a distant as-yet-unseen planet. Armed with his calculations, Le Verrier convinced two German astronomers to look for his planet, they almost immediately found it, right where it was predicted to have been.
“It was just this perfect triumph of analytical method, perfect triumph of Newton’s theory, perfect triumph of a kind of inferential reasoning that is very very powerful indeed,” says Tom Levenson, Professor and Director of the Graduate Program in Science Writing at MIT, and author of the new book, The Hunt for Vulcan: How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe. Following his success with Neptune, Le Verrier readied his equations to tackle another mystery: the anomalous perihelion shift of Mercury. Convinced that a planet orbiting even closer to the Sun was responsible for the perturbation, he named this hypothetical object Vulcan, Roman god of fire, volcanoes, and the forge. Just as with Neptune, reports of Vulcan sightings followed Le Verrier’s prediction. The only problem? It was never there.
Without another planet, Newton’s theory of gravitation could not explain Mercury’s orbit, and the existence (or not) of Vulcan remained an intractable problem well beyond Le Verrier’s death in 1877. Only in 1915, with Einstein’s theory of general relativity, did Mercury’s perihelion precession receive a satisfying answer. So close to the massive Sun, the curvature of spacetime affects the orbit of Mercury with no need to invoke a Vulcan. “This really moved Einstein,” says Levenson. “He calculated the orbit of Mercury and found out that his theory predicted it perfectly — you know, spot on, just dropped out of the calculation.”
Meanwhile, in the outer solar system, reported perturbations in the orbits of Uranus and Neptune prompted searches for a yet more distant planet — what Percival Lowell termed “Planet X” — ultimately culminating in the discovery of Pluto by Clyde Tombaugh in 1930. It was quickly realized, however, that tiny Pluto was nowhere near massive enough to influence ice giants, and decades later, Voyager 2 provided a new mass estimate for Neptune that resolved the apparent perturbations ascribed to Planet X. In 2006, following the discovery of more distant, icy bodies, Pluto was reclassified as a dwarf planet.
Since the discovery of Neptune, our solar system has had eight planets, then maybe nine (Vulcan), then eight again, then nine (Pluto), then eight once more, and — soon perhaps — nine again. “From the point of view of history of science, there’s you know a ton that we can learn,” says Konstantin Batygin, Assistant Professor of Planetary Science at Caltech and co-author of a new paper predicting the existence of Planet Nine. “Neptune was, of course, predicted before it was discovered and…our goal really was to, as much as possible, work in the vein of Le Verrier and the discovery of Neptune.”
Planet Nine, if indeed it exists, explains several observed behaviors of distant Kuiper Belt Objects (KBOs) in the outer solar system. The model developed by Batygin and co-author Michael Brown suggests that Planet Nine closest approach to the Sun (perihelion) occurs between 200 and ~350 Astronomical Units, where 1 AU is the distance between the Earth and the Sun. As distant as that may seem, it pales in comparison to Planet Nine’s projected aphelion; at its furthest point from the Sun, the planet might be as far as 1200 AU. These parameters sketch out a vast ellipse that, as Kepler’s Laws tell us, would take 10,000 to 20,000 years for Planet Nine to trace out.
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| A graphical explanation of Kepler’s Laws of motion Image Credit: The Simple Physicist |
With its huge, elongated orbit, it seems utterly unlike the orderly progression of the other planets, but Batygin sees it differently. “I think actually Planet Nine makes the solar system slightly less weird,” he says. “Planets that orbit other stars typically do not have orderly orbits like the ones we have in the solar system…I think that Planet Nine is really our link to the extrasolar world.”
With each addition (or subtraction) of a planet, our understanding of our solar system and even the universe has shifted. Whether it’s Neptune and the triumph of `Newtonian celestial mechanics, or Vulcan and the discovery of general relativity, or Planet Nine and the chance to see our solar system in a new light, planetary predictions continue to shape our understanding of our place in the cosmos. With that in mind, the search is on for the newest ninth planet.