AST 111 Lecture 14

 My rough definition:  The 8 large, unique celestial bodies orbiting the Sun

 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

 Which of the above criteria does Pluto fail to meet?

 All planetary orbits are nearly circular and lie nearly in the same plane

 ALL planets orbit the Sun counterclockwise when viewed looking at Earth’s north pole

 Most planets rotate in the same direction as they orbit  Small axis tilts  The Sun rotates in the same direction

 Large moons tend to orbit in planets’ equatorial planes  Same direction as planet rotates

 Terrestrial planets have few moons  Look “out of place”  Jovian planets have many moons  Larger Jovian moons follow parent planet’s rotation

 Uranus rotates practically on its side  Its axis of rotation is nearly in the plane of its orbit

 Venus rotates clockwise about its own axis (viewed from high above Earth’s north pole)

 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

 Neptune’s moon Triton orbits Neptune clockwise  Neptune spins counter-clockwise

What differentiates the “inner planets” from the “outer 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

 Conjunction: Planet lines up with the Sun as seen from Earth, looking toward the Sun!

 Opposition: Planet lines up with the Sun as seen from Earth, looking away from the Sun!

 Greatest Eastern Elongation: An inner planet is as far east of the Sun as it can get as viewed from Earth

 Perihelion: point along orbit where closest to the Sun  Aphelion: the point along orbit where farthest from the Sun

 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

 We have sent spacecraft to:  All terrestrial and jovian planets  Moons  Asteroids  Comets

 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

 Launched in April 1973  Passed by Jupiter in December 1974  Passed Saturn in September 1979

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

 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”

 Notable Orbiters  Galileo  Orbited and probed Jupiter and its moons  Cassini  Orbits and probes Saturn and its moons

 Mars Rovers (Sojourner, Spirit, and Opportunity)  Analyze chemistry and geology of Mars  Searching for past and present signs of water