Latest Search for Hypothetical Planet at Edge of Solar System Comes Up Empty
Deeply ingrained in popular culture, it has been the object of obsession for scientists and conspiracy theorists alike: a hypothesized hidden planet lurking beyond the orbit of Pluto, which was long-thought to be the cause of observed perturbations in the orbits of the outer giant planets, past mass extinction events on Earth, and other modern-day doomsday, apocalyptic scenarios. Detailed all-sky surveys in recent years made with NASA’s Wide-field Infrared Survey Explorer, or WISE, although revealing the presence of thousands of stellar and substellar objects within the Sun’s wider neighborhood, have shown that the long-sought-for “Planet X” is nowhere to be found.
The notion of a massive, unobserved planet orbiting the Sun beyond the known reaches of the Solar System goes back to the 19th century, even before the discovery of Neptune in 1846. The discovery of the blue-tinted ice giant planet itself came as a result of a series of efforts by 19th-century astronomers to explain the discrepancies between the predicted and observed orbit of Uranus that were caused by Neptune’s gravitational tug. When astronomers realised that Neptune was experiencing similar perturbations in its orbit, they suspected that the most likely cause was the presence of a yet another unobserved planet, even farther out from the Sun.
American astronomer Percival Lowell was central in the hunt for this hypothesized trans-Neptunian massive planet during the early 20th century, which he dubbed “Planet X.” With the discovery of Pluto by Clyde Tombaugh in 1930 the matter was initially thought to be resolved. When astronomers finally realised that Pluto’s mass was too small and insignificant to have any effect on the orbits of Uranus and Neptune, the search for Planet X was rekindled. It took the detailed observations of NASA’s Voyager 2 spacecraft during its fly by of Neptune in 1989 for astronomers to realise that the observed orbital discrepancies were due to errors made in the calculation of Neptune’s mass by their 19th-century counterparts. With the help from the Voyager 2 data, more precise measurements of the ice giant mass were made, accounting for the observed orbital anomalies.
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