50 years ago, scientists had hints of a planet beyond Pluto

Cometary evidence of a planet beyond Pluto Science News, May 6, 1972

There have been suggestions that our solar system might have a tenth planet…. In the April Publications of the Astronomical Society of the Pacific, a mathematician … presents what he says is “some very interesting evidence of a planet beyond Pluto.” The evidence comes from calculations of the orbit of Halley’s comet.


The 1972 evidence never yielded a planet, but astronomers haven’t stopped looking — though it became a search for Planet 9 with Pluto’s 2006 switch to dwarf status. In the mid-2010s, scientists hypothesized that the tug of a large planet 500 to 600 times as far from the sun as Earth could explain the peculiar orbits of some objects in the solar system’s debris-filled Kuiper Belt (SN: 7/23/16, p. 7). But that evidence might not stand up to further scrutiny (SN: 3/13/21, p. 9). Researchers using the Atacama C­osmology Telescope in Chile to scan nearly 90 percent of the Southern Hemisphere’s sky had no luck finding the planet, the team reported in December 2021.

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