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50 years in the past, astronomers had been chipping away at Pluto’s mass

a photo of Charon behind Pluto

The shrinking mass of Pluto — Science Information, August 28, 1971

Pluto was the final of the planets to be found (in 1930). If astronomers proceed to make it lighter, it could be the primary to vanish.… [The latest measurement] brings Pluto right down to 0.11 of Earth’s mass, lower than an eighth of its former self.… The vast discrepancies among the many figures introduced for the mass of Pluto illustrate the actual difficulties of measuring its mass.… If a planet has satellites, its mass might be decided from learning their motions.… However Pluto has no identified satellites.


The invention of Pluto’s moon Charon in 1978 (SN: 7/15/78, p. 36) lastly allowed astronomers to precisely calculate the planet’s mass: about 0.2 % of Earth’s mass. Many years after scientists resolved Pluto’s heft, the planet acquired arguably the best demotion of all — a downgrade to dwarf planet (SN: 9/2/06, p. 149). Some astronomers have since proposed alternate definitions for the time period “planet” that, if broadly adopted, would restore Pluto to its former rank.