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The Solar System

The Solar System. AST 111 Lecture 14. The Planets. My rough definition: The 8 large, unique celestial bodies orbiting the Sun. The Planets. The Planets. The Planets. The Planets. The Planets. The Planets. The Planets. Formal Definition. A planet is a celestial body that is:

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The Solar System

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  1. The Solar System AST 111 Lecture 14

  2. The Planets • My rough definition: • The 8 large, unique celestial bodies orbiting the Sun

  3. The Planets

  4. The Planets

  5. The Planets

  6. The Planets

  7. The Planets

  8. The Planets

  9. The Planets

  10. Formal Definition • A planet is a celestial body that is: • In orbit around the Sun • Has sufficient mass for its self-gravity to overcome its rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape • Not massive enough to produce fusion reactions at the core • Has cleared the neighborhood around its orbit

  11. Pluto fails! • Which of the above criteria does Pluto fail to meet?

  12. Planetary Motion • All planetary orbits are nearly circular and lie nearly in the same plane

  13. Planetary Motion • ALL planets orbit the Sun counterclockwise when viewed looking at Earth’s north pole

  14. Planetary Motion • Most planets rotate in the same direction as they orbit • Small axis tilts • The Sun rotates in the same direction

  15. Planetary Motion • Large moons tend to orbit in planets’ equatorial planes • Same direction as planet rotates

  16. Moons • Terrestrial planets have few moons • Look “out of place” • Jovian planets have many moons • Larger Jovian moons follow parent planet’s rotation

  17. Exceptions • Uranus rotates practically on its side • Its axis of rotation is nearly in the plane of its orbit

  18. Exceptions • Venus rotates clockwise about its own axis (viewed from high above Earth’s north pole)

  19. Exceptions • Terrestrial planets besides Earth either have no moons or small moons • Earth’s Moon is very large • Almost a quarter of the mass of Mercury

  20. Exceptions • Neptune’s moon Triton orbits Neptune clockwise • Neptune spins counter-clockwise

  21. Inner and Outer Planets What differentiates the “inner planets” from the “outer planets”?

  22. Two Types of Planets

  23. Two Types of Planets • Terrestrial Planets: • Mercury, Venus, Earth, Mars • Small and dense • Rocky exteriors, metal interiors • Solid surface • Few moons • No rings • Jovian Planets: • Jupiter, Saturn, Uranus, Neptune • Large! • Lower average density (“gas giants”) • No solid surface • Rings • Many moons • H, He, and H compounds

  24. Solar System Geometry • Conjunction: Planet lines up with the Sun as seen from Earth, looking toward the Sun!

  25. Solar System Geometry • Opposition: Planet lines up with the Sun as seen from Earth, looking away from the Sun!

  26. Solar System Geometry • Greatest Eastern Elongation: An inner planet is as far east of the Sun as it can get as viewed from Earth

  27. Solar System Geometry • Perihelion: point along orbit where closest to the Sun • Aphelion: the point along orbit where farthest from the Sun

  28. The Mars Hoax! • In 2003, Mars had a better-than-usual opposition (in terms of being close) • Consider elliptical orbits • It was claimed that Mars would be the size of the full moon • Would Mars be at conjunction or opposition for it to be as large as possible in the sky? • Was only 25% closer than usual closest-approach

  29. Exploring the Solar System • We have sent spacecraft to: • All terrestrial and jovian planets • Moons • Asteroids • Comets

  30. Pioneer 10 • Launched in 1972 • First encounter with Jupiter • Lost contact with it in 2003 (7 billion miles from Earth) • Could reach Aldebaran in 2 million years

  31. Pioneer 11 • Launched in April 1973 • Passed by Jupiter in December 1974 • Passed Saturn in September 1979

  32. Voyager 1 • Launched in 1977 • Visited Jupiter and Saturn • Opted to study Titan instead of Pluto; flung out of Solar System • Voyager 1 is 0.002 light years (11 billion miles) from the Sun • Will pass by a star in 40,000 years • 10 miles per second

  33. Voyager 2 • Launched in 1979 • Voyager 2 visited all four Jovian planets • Used a chain of gravitational slingshots • Planets were lined up just right for the “Grand Tour”

  34. Exploring the Solar System • Notable Orbiters • Galileo • Orbited and probed Jupiter and its moons • Cassini • Orbits and probes Saturn and its moons

  35. Exploring the Solar System • Mars Rovers (Sojourner, Spirit, and Opportunity) • Analyze chemistry and geology of Mars • Searching for past and present signs of water

  36. Home, sweet home…

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