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Astronomy The Planets The Planets • Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune – Pluto is now considered a dwarf planet – All revolve around the sun – All rotate about their axis • Astronomical Unit (AU) – Distance between the Sun and the Earth • Equatorial diameter – Around its center The Terrestrial Planets • The terrestrial planets are the four planets closest to the sun. (Mercury, Venus, Earth, Mars) • Terra – Latin word for Earth • Terrestrial planets are: – small, dense and have rocky surfaces. • Terrestrial planets have: – crust, mantle and iron core. • What do the number of craters on a planet’s surface tell us? – How quickly their surface changes Mercury 1. Closest to the sun 2. Smallest Planet • One day on Mercury is 59 days on Earth • One year on Mercury is 88 days on Earth • 400oC during day • -150oC at night Venus 1. Second Planet from the Sun 2. Brightest object in Earth’s night sky after the moon • Hottest planet in solar system due to density and composition of atmosphere :460oC • Volcanic Activity • Small number of impact craters • Sister planet to Earth (Similar in size) Earth 1. Third Planet from the Sun 2. One Moon 3. Only planet in solar system where liquid water exists • Water is essential for life • Only planet in the solar system with life? Mars 1. Fourth Planet from the Sun. “The Red Planet” 2. Olympus Mons – Largest volcano in the Solar System 25 km high (3 times the height of Mt. Everest) • Mars has seasons like Earth • Named for the Greek god of war • 2 moons, Phobos and Deimos • Evidence of having liquid water at one time Comparison of Inner Planets The Outer Planets • The outer planets are the four planets furthest from the sun. (Jupiter, Saturn, Uranus, Neptune) • Outer planets are called the Gas Giants • The outer planets consist mainly of liquid hydrogen and helium and may have a small cores of metal and rock • Outer planets are much larger than the terrestrial planets • Cooler than inner planets • Outer planets have more moons than inner planets Jupiter 1. Largest planet in the solar system • Composed mostly of gasses 2. 1 day on Jupiter is less than 10 hours on Earth • Storms exist on Jupiter, last for 100’s of years • The great red spot was first observed in 1664 • Jupiter has over 60 moons • Europa is completely covered by a salt water ocean covered by ice • Scientists think life may be possible in this ocean Saturn 1. Best known for its rings composed of ice and rock • Over 60 moons • Titan has a thick atmosphere and may have rivers and lakes of ethane Uranus and Neptune • Planets are similar in size and color • Uranus is unusual in that it rotates sideways compared to the other planets • Uranus has 27 known moons • Neptune has 13 known moons The Outer Planets (Number of Moons outdated…) Dwarf Planets •Only 5 known Dwarf Planets in the Solar System – could be 100’s more • Definition created in 2006 •Pluto resides in the Kuiper Belt beyond Neptune’s orbit. • Has a moon, Charon that is about half its size • Ceres is the only known dwarf planet that orbits in the asteroid belt between Mars and Jupiter Pluto Ceres Dwarf Planets What makes a Planet a “Planet” • • How did Pluto get demoted? A planet according to the International Astronomical Union must: 1. Orbit the sun 2. Not be a satellite 3. Be massive enough for its own gravity to keep it round 4. Be big enough to dominate its orbit – Scientist’s believe that Pluto fails this last test