Early in the 20th century, slight changes in the orbits of the two most outer planets, Neptune and Uranus, caused American astronomer Percival Lowell to believe there might be another planet out there near their orbits. He reasoned that the gravitational pull of something large was affecting the orbits of those planets. But despite looking for what he called “Planet X” for 11 years until his death in 1916, Lowell never found it.
In 1929, the Lowell Observatory decided to look again for the elusive body. They hired an untrained amateur astronomer named Clyde Tombaugh who used a powerful, 13-inch telescope built just for this purpose. The Observatory relied on Lowell’s predictions and the new telescope to search the heavens for the planet that they were convinced was there.
It took a year of painstaking work, but the existence of Planet X was confirmed in February, 1930 when Tombaugh was examining photographic plates created by the telescope. The following month on what would have been Percival Lowell’s 75th birthday, the Observatory announced to the world that a new planet had been found.
Once discovered, Planet X needed a name. The name chosen was Pluto, the Roman God of the Underworld (all planets except Earth are named after gods/goddesses) who was able to make himself invisible. The name denoted both the assumed unfavorable surface conditions, being so far from the Sun’s heat, and also honored Lowell, whose initials are the first two letters of the planet’s name.
At that time, Pluto was considered to be the ninth and outer-most planet in the solar system. It was also the tiniest of the planets, being just two-thirds the size of earth’s moon.
Further study showed that Pluto is not always the planet farthest from the sun. Its orbit is more elliptical than those of the other planets. That means that sometimes Pluto is a lot nearer to the Sun than at other times, and actually crosses Neptune’s orbit. This results in Pluto being sometimes closer to the Sun than Neptune.
Its great distance from the Sun, nominally 3.67 billion miles, or 39.5 astronomical units makes Pluto very inhospitable. It is almost 40 times farther from the Sun than the Earth and takes 248 years just to make one orbit around the Sun. It requires 5.5 hours for a radio signal traveling at the speed of light to reach it. The surface temperature is minus 300 degrees F., which is about where nitrogen (which makes up most of its atmosphere) turns to liquid.
As the decades passed and astronomers learned more about Pluto, some questioned whether Pluto should really be considered a full-fledged planet. This was, in part, because it was so much smaller than the other planets and also because of its odd orbit. Also, Pluto’s biggest moon, Charon, discovered in 1978, is incredibly large in comparison to the body it is orbiting.
In the 1990s, when bigger and better telescopes began to discover other large bodies beyond Neptune and especially when another large body was discovered in 2003 that rivaled the size of Pluto, its status became seriously in doubt.
In 2006, The International Astronomical Union officially created a definition of what makes a planet: it is in orbit around the Sun; it has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and it has enough mass to “clear the neighborhood” around its orbit. Since Pluto met only the first two of these criteria, the IAU downgraded Pluto from a planet to a “dwarf planet.”
NASA had learned a lot about Pluto from studying images taken by the Hubble Space Telescope. Scientists used Hubble’s photos to discover four of Pluto’s moons, and pictures of its surface that showed dark and light areas. But even using that powerful telescope, the images were still fuzzy.
In 2006, NASA launched New Horizons, which would be the first mission to Pluto. It is a spacecraft about the size of a piano that would fly to the outer edges of our solar system. New Horizons took nine years to reach Pluto, and on July 14th of this year, it arrived at the dwarf planet and sent back spectacular pictures. By the end of 2015, New Horizons will have spent almost six months studying Pluto and its now five moons.
Until the flyby, the exact size of Pluto was not known. Its diameter has now been measured at 1473 miles or 2370 kilometers, or about 1/5 that of Earth and ½ of Mercury, the smallest planet.
New Horizons’ cameras and scientific instruments will continue to gather information about this distant dwarf. These pictures and data will help scientists know more about what this tiny body is like. After it has finished studying Pluto and its moons, New Horizons will travel deeper into the Solar System and will likely discover more about our amazing Universe and its origins.
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Marcus Goodkind of Tuckasegee, a retired aerospace engineer, worked with the National Aeronautics and Space Administration as a manager at Kennedy Space Center on all the manned programs from Mercury to Shuttle, including Apollo 11, the first manned lunar landing.