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EP: Fachverband Extraterrestrische Physik

EP 10: Poster

EP 10.10: Poster

Mittwoch, 28. März 2007, 18:00–20:00, H46

What is the number of planets in the solar system ? — •Rainer Lütticke — Universität Wuppertal, fmt — Astronomisches Institut der Ruhr-Universität Bochum

Although the large solar system bodies (SSBs) exist since some billion years and their number is constant, the known number of planets changed in the last 2000 years depending on astronomical knowledge, detection of SSBs, and definition of the term planet. In ancient times, for Greek astronomers "wandering" lights on the sky were planets. The number of planets in this geocentric model was 7 (Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn). From the late 16th century onward the number changed to 6 when in the heliocentric model the term planet was used for SSBs moving around the Sun. In 1781 the detection of Uranus raises the number again to 7. Discoveries of SSBs orbiting between Mars and Jupiter increased the number. 1801: 8 (Ceres), 1802: 9 (Pallas), 1803: 10 (Juno), 1807: 11 (Vesta), 1845: 12 (Astraea). The 13th planet was in 1846 Neptune. Further detections of SSBs lead to a number of 23 planets at the end of 1851. Because such a high (still raising) number was not acceptable by the astronomers the definition of the term planet changed over the following ∼10 years: A planet had to be a "large" SSB. Therefore the number dropped to 8. Pluto detected in 1930 was the 9th planet. In August 2006 IAU decided a new definition of the term planet after the detection of SSBs in the size range of Pluto and even larger (Eris). By that definition Pluto is no longer a planet because it has not "cleared the neighbourhood around its orbit". Therefore the actual official number of planets is 8.