The Solar System A planet by any other name ….
Dozens of moons Thousands of asteroids Trillions of comets What is the Solar System? • Answer: The system of objects in the solar neighborhood (near the Sun) • What are these objects? One Star Six Planets Nine Planets
Discovered Planets • All planets through Saturn known since the ancients – all you have to do is look up to see them • Uranus in 1781 by William Herschel • “Georgium Sidus” after George III of England • Neptune in 1846 by Johann Galle using predictions by Urbain Jean Joseph Leverrier and John Couch Adams • Pluto in 1930 by Clyde Tombaugh at Lowell Observatory
Planets • The first step to studying planets? • Compare and contrast • What are important quantities? • You have: • A stick • A tree • A car • A house
Concept Test • Which of the following is a true statement about density? • A stone and a boulder have the same density. • A bowling ball and a soccer ball have the same density. • A boulder has a higher density than a pebble. • A soccer ball has a higher density than a billiard ball. • None of the above.
Density and Mass • What is mass? • Mass is similar to weight, it measures how much stuff an object is made of • Example: A bowling ball and a soccer ball are about the same size, but have different masses • What is density? • Density is mass per volume. It helps to tell you what kind of stuff an object is made of • Example: A log and a tree have different masses (and sizes), but the same density because they are made of the same stuff
Terrestrial Planets • Close to the sun • Small • Mass • Radius • High density • Primarily rocky • Solid surface • Weak magnetic field • Few moons • No rings
Jovian Planets • Far from the sun • Large • Mass • Radius • Low density • Primarily gaseous • No solid surface • Strong magnetic fields • Many moons • Many rings
Planetary Rings • All the “Jovian planets” have rings • These are not solid, but composed of millions of tiny particles of ice and dust • Rings have structure: gaps and spokes
What About Pluto? • Pluto does not easily fit into either category • Far from the sun (jovian) • Small (terrestrial) • Neither rocky nor gaseous (icy) • One moon • No rings • It is similar in composition to some moons in the outer solar system and its orbit is similar to a group of objects called “Kuiper Belt Objects” or KBOs
Charon • Largest of any moon in relation to the planet it orbits (1/2 the size of Pluto) • Pluto and Charon are tidally locked to each other (always show the same face) • Charon discovered in 1978 by astronomers at the US Naval Observatory
Concept Test • Which of the following is NOT a reason why many astronomers think Pluto should not be classified as a planet. • It is smaller than some moons. • It has an orbit similar to many Kuiper Belt Objects (KBO). • Pluto is more similar in shape to irregular asteroids and comets. • Pluto is more similar in composition to many moons and KBOs. • Its orbit is far more elliptical and inclined to the Sun relative to the other planets.
KBOs • Belt of icy objects outside Neptune’s orbit.
Quaoar • a = 43 AU • Orbit nearly circular (e = 0.04) • P = 285 years.
Sedna • a = 479 AU • Orbit (e = 0.84) • Aphelion = 76 AU • Perihelion = 884 AU • Currently = 90 AU • P = 10,500 years
Eris Orbit • A = 68 AU • E = 0.44 • P = 560 years Eris • D = 2400 km • Density = 2300 kg/m3
Homework #6 • For Wednesday 17-Sept: Read articles describing IAU controversy over whether Pluto is a planet. • Questions: • Give three short reasons why Pluto IS a planet. • Give three short reasons why Pluto is NOT a planet. Come to class prepared to debate the question.