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In this image of Pluto taken by NASA's New Horizons spacecraft, different colors represent different compositions of surface ices, revealing a surprisingly active body.
Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

PLANET TYPE – Dwarf Planet


MASS – 0.00218 Earths

PLANET RADIUS – 0.1868 Earths


ORBITAL PERIOD – 247.94 years


DETECTION METHOD –  A new astronomic technique of photographic plates combined with a blink microscope.

1840s. Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analyzing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus’s orbit was being disturbed by another planet besides Neptune.

1894. Percival Lowell started an extensive project in search of a possible ninth planet, which he termed “Planet X”.

1906. Percival Lowell—a wealthy Bostonian who had founded Lowell Observatory in Flagstaff, Arizona.

1909. By this year, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916.

1909, March 20. Earliest precovery observations were made by the Yerkes Observatory.

1915, March 19, and April 7. Unknown to Lowell, his surveys had captured two faint images of Pluto but they were not recognized for what they were.

1929. Due to Percival’s widow, Constance Lowell, entering into a ten-year legal battle with the Lowell Observatory over her husband’s legacy, the search for Planet X did not resume until this year.

1930, January 21. A lesser-quality photograph taken on this day helped confirm the movement.

1930, February 18. After nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and 29.

1930, March 13. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory.

1930, May 1. The name “Pluto” was announced.

1931. Nicholson & Mayall, estimated Pluto’s mass to be 1 Earth.

1941. Glenn T. Seaborg named the newly created element plutonium after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following uranium, which was named after Uranus, and neptunium, which was named after Neptune.

1948. Kuiper, estimated Pluto’s mass to be 0.1 (1/10) Earth.

1976. Cruikshank, Pilcher, & Morrison, estimated Pluto’s mass to be 0.01 (1/100) Earth.

1978. The discovery of Pluto’s moon Charon allowed the measurement of Pluto’s mass for the first time: roughly 0.2% that of Earth, and far too small to account for the discrepancies in the orbit of Uranus.

1978. Christy & Harrington, estimated Pluto’s mass to be 0.0015 (1/650) Earth.

1989. Voyager 2’s flyby of Neptune.

1992. Myles Standish used data from Voyager 2′s flyby of Neptune in 1989, which had revised the estimates of Neptune’s mass downward by 0.5%—an amount comparable to the mass of Mars—to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them, the need for a Planet X, vanished.

2000, February. The Hayden Planetarium in New York City displayed a Solar System model of only eight planets, which made headlines almost a year later.

2005, July 29. Astronomers at Caltech announced the discovery of a new trans-Neptunian object, Eris, which was substantially more massive than Pluto and the most massive object discovered in the Solar System since Triton in 1846.

2006. Buie et al, estimated Pluto’s mass to be 0.00218 (1/459) Earth.

2006, September. The IAU included Pluto, and Eris and its moon Dysnomia, in their Minor Planet Catalogue, giving them the official minor planet designations “(134340) Pluto”, “(136199) Eris”, and “(136199) Eris I Dysnomia”.

2015, July 14. NASA’s New Horizons space probe flew through the Pluto system, providing much information about it.

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