![]() On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. Tombaugh’s task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. The search for Planet X did not resume until 1929, when the job was handed to Clyde Tombaugh, a 23-year-old Kansan who had just arrived at the Lowell Observatory. There are sixteen known pre-discoveries, with the oldest being made by the Yerkes Observatory on August 20, 1909. Lowell was not the first to unknowingly photograph Pluto. Unknown to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but they were not recognized for what they were. ![]() Lowell and his observatory conducted the search until his death in 1916, to no avail. In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed “Planet X.” By 1909, Lowell and Pickering had suggested several possible celestial coordinates for such a planet. This period in astronomy was one of intense planet hunting, and Pickering was a prolific planet predictor. Tombaugh, with contributions from William H. The object formerly known as the planet Pluto was discovered on Februat the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Historic American Buildings Survey (Library of Congress). Lowell Observatory, Pluto Dome, Flagstaff, AZ. So any large body that does not meet these criteria is now classed as a “dwarf planet,” and that includes Pluto, which shares its orbital neighborhood with Kuiper belt objects such as the plutinos. You may wonder what that means, “not clearing its neighboring region of other objects?” Sounds like a minesweeper in space! This means that the planet has become gravitationally dominant - there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence, in its vicinity in space. In all the billions of years it has lived there, it has not managed to clear its neighborhood. Pluto meets only two of these criteria, losing out on the third. It has “cleared the neighborhood” around its orbit.It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).So, the three criteria of the IAU for a full-sized planet are: What is a Dwarf Planet?Ī “dwarf planet,” as defined by the IAU, is a celestial body in direct orbit of the Sun that is massive enough that its shape is controlled by gravitational forces rather than mechanical forces (and is thus ellipsoid in shape), but has not cleared its neighboring region of other objects. So now we have eight planets instead of the nine we used to have. The “gas giants” of course are Jupiter, Saturn, Neptune, and Uranus. It contains the asteroid belt as well as the terrestrial planets, Mercury, Venus, Earth, and Mars. The “inner Solar System” is the region of space that is smaller than the radius of Jupiter’s orbit around the sun. In August 2006 the International Astronomical Union (IAU) downgraded the status of Pluto to that of “dwarf planet.” This means that from now on only the rocky worlds of the inner Solar System and the gas giants of the outer system will be designated as planets. NASA Jet Propulsion Laboratory, Goddard Space Flight Center. The image combines blue, red and infrared images taken by the Ralph/Multispectral Visual Imaging Camera (MVIC). NASA’s New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015. ![]() Essentially Pluto meets all the criteria except one-it “has not cleared its neighboring region of other objects.” The Rich Color Variations of Pluto. The International Astronomical Union (IAU) downgraded the status of Pluto to that of a dwarf planet because it did not meet the three criteria the IAU uses to define a full-sized planet.
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