In Search of Planet X

Pluto
NASA
Pluto, July 2015; photographed by NASA’s New Horizons spacecraft, with enhanced color

Nearly every civilization has recorded celestial events, partly in the hope that they might explain terrestrial ones, but also to place themselves in the grand cosmic scheme. The Nebra Sky Disk, a twelve-inch bronze disk with gold inlay, dated to 1600 BCE and discovered in the Saxony-Anhalt region of Germany, depicts the sun, moon, and what appears to be the star cluster Pleiades. By the second millennium BCE, the Babylonians knew of the inner planets—Mercury and Venus—as well as the outer planets—Mars, Jupiter, and Saturn.

An understanding of the organization of our solar system, however, is relatively recent. It was only in 1543 that Nicolaus Copernicus radically reordered the cosmos with De revolutionibus orbium coelestium, upsetting the geocentric model that had been in place since antiquity. For Copernicus, the universe was finite and bounded, with the sun surrounded by the planets of our solar system and fixed stars. Today we know that our own galaxy is one of perhaps trillions in the universe, and that the universe is not only expanding but that its expansion is accelerating. Moreover, we have many independent lines of evidence suggesting that dark matter and dark energy—two mysterious entities whose nature remains elusive—shape the universe as we know it, and that the matter and energy we can observe make up just over 4 percent of all the contents of the cosmos. Such disorienting revelations have been typical of modern astronomy and cosmology over the last century, as technological advances have increased our ability to gather data, probe greater distances, and reconstruct events that took place billions of years ago. These major refinements in our knowledge have required the continuous redrawing of our cosmic map and extended our vision well beyond our solar system.

Two recent books, Discovering Pluto: Exploration at the Edge of the Solar System by Dale P. Cruikshank and William Sheehan and Chasing New Horizons: Inside the Epic First Mission to Pluto by Alan Stern and David Grinspoon, offer ringside views of the exploration of the outer solar system, from the discovery more than two hundred years ago of planets beyond Saturn to the launch in 2006 of NASA’s New Horizons space mission to study Pluto’s environs. Discovering Pluto begins with the discoveries of Uranus and Neptune, the seventh and eighth planets from the sun. The former was first identified by the English astronomer William Herschel in 1781. Later observation revealed that Uranus, whose orbit takes eighty-four years to complete, did not follow the course astronomers had expected it would take.

First, there was an astrometric anomaly: the measured position in the orbit deviated from the predicted orbit of Uranus as determined by Newton’s law of gravity. By the 1830s, it was established that the orbit of Uranus also seemed to diverge from Johannes Kepler’s second law,…


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