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The Solar System The Solar System The Sun and all of the objects that travel around it •the Sun is a star •planets •natural satellites of the planets; rings •the asteroid belt (rocks between Mars and Jupiter: meteoroids) •comets (frozen masses of gas and dust) Solar System Terrestrial Planets: Mercury Venus Earth Mars Solid rocky surfaces Gas Giants: Jupiter Saturn Uranus Neptune Pluto solid, very icy surface What is Pluto? • Pluto, 1930 • Charon,1978. • Eclipses allowed measurements of diameters, separation, masses. • Pluto 2300 km across, Charon 1200 km, separation 20,000 km. • Combined mass only 1/6 of Earth's Moon. • Density ~ 2g / cm3, implies rock/ice composition •Dwarf planet 1 Planetary Orbits Center of Mass http://pluto.jhuapl.edu/science/everything_pluto/10_binary_planet.html Asteroid belt http://www.mistupid.com/astronomy/orbits.htm The AU and the size of Solar System • Defined as the average distance between the Earth and the Sun • It is approximately 150 million km (93 million miles) (how many meters?) • Mercury - 1/3 of an AU from the Sun Ceres 1000 km across • Pluto - about 40 AU from the Sun – the size of the Solar system 2 Measuring distances • Measuring distances on Earth Measuring distances in Astronomy • Light Year – Inch, foot, yard, mile – Centimeter, meter, kilometer • Measuring distances within the Solar System – Astronomical unit (AU)= 150, 000, 000 km • Measuring distances outside the Solar System – Light Year (LY) = 9,500,000,000,000 km – Parsec (pc) = 3.262 LY – Kpc = 1000 pc, Mpc = 1,000,000 pc – – – – a unit of distance the distance that light can travel in one year speed of light ~ 300,000 kkm/second So in one year, it can travel 9,500,000,000,000 kilometers (10 trillion km) The nearest star to the Sun is Proxima Centauri 40 000 000 000 000 km The Kuiper Belt Trans-Neptunian Objects (TNO) Any object in the solar system that orbits the Sun at a greater distance on average than Neptune The Kuiper belt, Scattered disk, and Oort cloud are names for three divisions of this volume of space. Courtesy of Windows to the Universe, http://www.windows.ucar.edu Courtesy of Windows to the Universe, http://www.windo ws.ucar.edu 3 Kuiper belt objects: Planet candidates in the Solar System The scattered disk objects and the Oort Cloud 50,000 to 100,000 AU from the Sun The largest: Pluto, Charon, 2003 UB313 Courtesy of Windows to the Universe, http://www.windows.uc ar.edu The new Solar System? 12 planets? How to define the lower end of the planet scale? http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601a.html 4 How should the word “planet” be defined? Three new planets? 26th GENERAL ASSEMBLY of the International Astronomical Union 14-25 August 2006 IAU Resolution (August 24, 2006) Definition of a “Planet” in the Solar System • (1) A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. • Mass larger than 5x1020 kg • Diameter larger than 800 km The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. • Pluto (Kuiper belt), 2003 UB313 (Kuiper belt), Ceres (Asteroid belt) are not planets http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html IAU Resolution (August 24, 2006) Definition of a “Planet” in the Solar System • (2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. • A process will be established to assign borderline objects into either “dwarf planet” and other categories. Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of Trans-Neptunian objects.1This category is to be called “plutonian objects”. http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html 5 List of dwarf planets • The IAU has officially identified three celestial bodies that have immediately received "dwarf planet" classification: • Dwarf planets Name Ceres Pluto 2003 UB313 ("Xena") • Region of Solar system Asteroid belt Kuiper belt Scattered disc • Category Asteroid Plutino • Diameter 975×909 km 2306±20 km 2400±100 km IAU Resolution (August 24, 2006) Definition of a “Planet” in the Solar System • (3) All other objects , except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”. These currently include most of the Solar System asteroids, most TransNeptunian Objects (TNOs), comets, and other small bodies Scattered disc object http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html 6