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Number of Planets

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Raymond A. Serway, & Jerry S. Faughn. College Physics Sixth Edition. Pacific Grove: Thomson Learning Inc, 2003.
Table 7.2
Escape Speeds for the Planets and the Moon
Planet v (km/s)
Mercury 4.3
Venus 10.3
Earth 11.2
Moon 2.3
Mars 5.0
Jupiter 60.0
Saturn 36.0
Uranus 22.0
Neptune 24.0
Pluto 1.1
9 Planets
Planet, Microsoft Encarta Online Encyclopedia, 2007. "In the solar system, there are eight official planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune" 8 Planets
Dennis L. Mammana "Astronomy". Science Supplement 2001. "In other discoveries concerning extrasolar planets, astronomers determined that at least three such bodies are orbiting the star Upsilon Andromeda" 3 Exoplanets
How Many Extrasolar Planets Discovered? National Maritime Museum, 2007. "At the time of writing (November 2005) astronomers have discovered 157 planets around 99 stars other than the sun." 157 Exoplanets

Over the years the number of planets in and out of the solar system has been debated on. In August of 2006 there was talk about three new planets in the solar system. 2,500 astronomers gathered at the International Astronomical Union (IAU) General Assembly and defined what does and does not count as planet. In the solar system there are currently eight planets. In order of distance from the sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. From 1930 to 2006 Pluto was the ninth planet of the solar system. On August 24, 2006 is was classified as a dwarf planet along with planets Eris and Ceres.

Detection of an exoplanet started in 1855 around the star 70 Ophiuchi. However Captain W.S. Jacob stated that "orbital anomalies made it "highly probable" that there was a "planetary body" in this system". The first exoplanet that receive ensuring confirmation was the planet that orbited the star Gamma Cephei. It was discovered by astronomers Bruce Campbell, G.A.H. Walker, and S. Yang in 1988. The following years the existence of the planet was both supported and doubted but in 2003 the planet's existence was confirmed. In 1992 astronomers Aleksander Wolszczan and Dale Frail discovered planets around the pulsar PSR 1257+12. This discovery is considered to be the first definitive detection of exoplanets. Since then each year many new exoplanets have been discovered. As of 2007 there are 243 exoplanets.

David Louisia -- 2007