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The Pluto Controversy http://apod.nasa.gov/apod/astropix.html What is a Planet???? The problem is that astronomers have never defined what a planet is. Some History • In ancient times there were seven planets – Moon – Mercury – Venus – Sun – Mars – Jupiter – Saturn Some History • After Copernican revolution there were 6 planets – Mercury – Venus – Earth – Mars – Jupiter – Saturn Some History Asteroids were originally classed as planets. • Uranus discovered in 1781 • Ceres (asteroid)) discovered in 1801. It was assumed to be the missing planet between mars and Jupiter! • Pallas 1802 (asteroid) • Juno 1804 (asteroid) • Vesta 1807 (asteroid) • Neptune discovered in 1846 • Now are there 12 planets? Some History • By 1890 over 300 objects had been found orbiting between Mars and Jupiter and were being called minor planets or asteroids. Some History • More planets were being discovered between Mars and Jupiter • New classification as asteroids – – – – Ceres Juno Vesta et al. • Now we have reduced the number of planets from 12 to 8. • Therefore, in the past astronomers have reclassified objects from being planets to being something else. Discovery of Pluto • March 13, 1930 by Clyde Tombaugh at Lowell Observatory (Percival Lowell). • ~ 6 degrees from predicted location • Inclined 17 degrees from plane of Solar System • Assumed to be like the Earth Pluto’s Moon • Charon discovered at USNO Flagstaff, AZ in July 1978. • Pluto's mass ~ 0.0021 Earth masses. • NOT SO EARTH LIKE!!!!!! My teachers lied to me!!!! Pluto is Strange • Orbit of Pluto comes inside orbit of Neptune. • Is Pluto an escaped moon of Neptune? Maybe and maybe not. Pluto is Strange • Pluto’s obit is the most highly inclined orbit of all the planets. • i = 17 degrees • Therefore, Neptune and Pluto can not collide. http://pluto.jhuapl.edu/science/everything_pluto/16_plutoOrbit.html Pluto is Strange • After discovery of Charon the mass of Pluto was determined to be 0.0021 times the Earth’s mass. Pluto is Strange http://solarviews.com/eng/pluto.htm Pluto and Charon are about the size of the USA. The Ort Cloud and Kuiper Belt • In 1950, Dutch astronomer Jan Oort hypothesized that comets came from a vast shell of icy bodies about 50,000 times farther from the Sun than Earth is. A year later astronomer Gerard Kuiper suggested that some comet-like debris from the formation of the solar system should also be just beyond Neptune. http://www.solarviews.com/eng/kuiper.htm Kuiper Belt • The Kuiper belt remained theory until the 1992 detection of a 150-mile wide body, called 1992QB1 at the distance of the suspected belt. Several similar-sized objects were discovered quickly confirming the Kuiper belt was real. http://www.solarviews.com/eng/kuiper.htm WW31 This illustration compares the orbit of the binary Kuiper Belt object, called 1998 WW31, with the orbits of Pluto and Neptune, the outermost solar system planets. The inset picture, consisting of six snapshots taken by NASA's Hubble Space Telescope, shows one member of the Kuiper Belt pair [the faint white blobs] during its elliptical orbit. The bright white object at the bottom of the oval is the other member of the pair. Quaoar, an Icy World Far Beyond Pluto Quaoar, an Icy World Far Beyond Pluto With the help of NASA's Hubble Space Telescope, astronomers have determined that 2002 LM60, an icy Kuiper belt object dubbed "Quaoar," by its discoverers, is the largest body found in the solar system since the discovery of Pluto 72 years ago. Quaoar (pronounced kwa-whar) is about half the size of Pluto. Like Pluto, Quaoar dwells in the Kuiper belt. Some Reasons Pluto may not be a Planet • Orbital inclination out of plane of Solar System • Other similar object found nearby that are considered Kuiper Belt objects. • Mass is very low (0.002) and more like that of a asteroid or comet than a planet. • Density is 0.373 times the Earth’s density. • Pluto’s size is closer to being an asteroid than a planet. Explanation: How many planets are in the Solar System? This popular question now has a new formal answer according the International Astronomical Union (IAU): eight. Last week, the IAU voted on a new definition for planet and Pluto did not make the cut. Rather, Pluto was re-classified as a dwarf planet and is considered as a prototype for a new category of trans-Neptunian objects. The eight planets now recognized by the IAU are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Solar System objects now classified as dwarf planets are: Ceres, Pluto, and the currently unnamed 2003 UB313. Planets, by the new IAU definition, must be in orbit around the sun, be nearly spherical, and must have cleared the neighborhood around their orbits. The demotion of Pluto to dwarf planet status is a source of continuing dissent and controversy in the astronomical community. What is a planet? The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. What is Pluto? • A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are listed on IAU's "dwarf planet" watch list, which keeps changing as new objects are found and the physics of the existing candidates becomes better known. • The "dwarf planet" Pluto is recognized as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects. The Redefined Solar System • • • Pluto demoted to a dwarf planet. Ceres promoted to dwarf planet and NOT an asteroid. I guess we are making this stuff up as we go. Nature of Science • Science is tentative. • Science is created by humans using logical reasoning and creativity. • Scientific knowledge is our best logical guess and may change as needed.