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Planet WHAT? When Ceres is discovered, it is called a planet. But when astronomers later realize it's just the first object discovered in the asteroid belt, Ceres is re-classified as an asteroid. In other words, planets have been demoted before. Learn more about Ceres here. Image credit: ESA/ATG medialab
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The 24-year-old astronomer Clyde Tombaugh discovers Pluto, and Pluto is called the 9th planet in our solar system. Learn more about Pluto here. Image credit: NASA, ESA, and M. Buie (Southwest Research Institute)
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An 11-year-old girl, Venetia Burney, names Pluto after the Roman god of the underworld. Learn more in the book “The Kid Who Named Pluto: And the Stories of Other Extraordinary Young People in Science.” Image credit: Hendrik Goltzius (1558-1617)
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The scientific journal Nature reports the discovery of an object in the Kuiper (sounds like "KY-per") Belt, a ring of objects predicted to exist just beyond the orbit of Neptune. Also residing in the Kuiper Belt is Pluto. Should all Kuiper Belt Objects (KBOs) be called planets? Or should none of them, including Pluto, be a planet? Learn more about the Kuiper Belt here. Image credit: NASA
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Responding to rumors that Pluto’s status may be changed, the International Astronomical Union (IAU) makes this statement: Pluto is still our 9th planet! Read the IAU's statement here. Image credit: International Astronomical Union
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Dr. Neil de Grasse Tyson, the director of the Hayden Planetarium in New York, reports that his museum has quietly demoted Pluto by excluding it from a planet exhibit. Dr. Tyson says schoolchildren have sent him hate mail. Read a news story about the Hayden’s demotion of Pluto here. [Image credit: Morehead Planetarium and Science Center]
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Astronomer Mike Brown discovers an object that is similar in size to Pluto. This object, later named Eris, is talked about as a possible 10th planet. The discovery sets into motion the eventual official demotion of Pluto as a planet. Learn more about Eris here. Image credit: NASA, ESA, and M. Brown (California Institute of Technology)
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Pluto is still considered a planet when NASA launches the New Horizons robotic spacecraft to Pluto and the Kuiper Belt. <a
href='http://www.nasa.gov/mission_pages/newhorizons/launch/newhorizons-allvideos.html' >Watch the launch video here (choose “Liftoff!”)</a> Image credit: NASA Kennedy -
In a controversial vote, the International Astronomical Union says that an object has to pass 3 tests to be a planet:
1) Must orbit the Sun and cannot be a satellite (moon).
2) Have enough mass (be big enough) to have pulled itself into a round shape.
3) Have cleared the neighborhood around its orbit. Image credit: IAU/Lars Holm Nielsen -
Because Pluto is just one of many objects within the Kuiper Belt, it fails the International Astronomical Union’s "clear its neighbhoood" rule for planetary status. Pluto is officially reclassified as a dwarf planet. The newly discovered Eris and the asteroid Ceres are also declared dwarf planets. Image credit: The International Astronomical Union/Martin Kornmesser
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After nearly ten years, the New Horizons spacecraft will arrive at Pluto and study its atmosphere and surface. One of its scientific instruments, the Student Dust Counter, was built by students to measure the space dust peppering New Horizons as it travels through the solar system. Learn about the New Horizons mission here. Image credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
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