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A star and all of the objects that travel in orbit around it The area of space that is influenced by the gravity of a star Our solar system is just one of many others. The Sun Planets Dwarf Planets Satellites/Moons Comets Asteroids Meteoroids Distance and size NOT to scale All planets orbit the sun in almost-circular elliptical orbits on approximately the same plane (the ecliptic). Dwarf Planets, comets, asteroids, and meteoroids also orbit the sun Most Satellites/Moons orbit planets (some orbit dwarf planets or even asteroids) Almost all planets, dwarf planets, and moons rotate and revolve counter-clockwise In 2006, the IAU defined a planet as a celestial body that: ◦ Is in orbit around the sun ◦ Has sufficient mass to assume a (nearly) round shape ◦ Has cleared the neighborhood around its orbit Jovian Outer Planets Terrestrial Inner Planets “Earth-like” ◦ Mercury ◦ Venus ◦ Earth ◦ Mars “Jupiter-like” ◦ Jupiter ◦ Saturn ◦ Uranus ◦ Neptune Gas Giants Ice Giants Closer to the sun Relatively small size and mass as compared to outer planets Rocky surface Craters present to some degree on all inner planets Feb. 2015 – NASA image Further from the sun Very massive Gas Giants and Ice Giants Lower average density than terrestrial planets All have rings 2006 image from Keck Observatory In 2006, the IAU defined a dwarf planet as a celestial body that: ◦ Is in orbit around the sun ◦ Has sufficient mass that it assumes a (nearly) round shape ◦ Has NOT cleared the neighborhood around its orbit ◦ Is NOT a satellite Smaller than planets Have “company” in their orbital areas Can have moons Most are located in the Kuiper Belt, but Ceres is in the Asteroid Belt Bright spot? More to come, as Dawn mission approaches Ceres this month A satellite (AKA-moon) is an object that revolves around a planet or dwarf planet (or an asteroid) As of 2008, there are 176 confirmed moons in our solar system Most are located around the outer planets Explorers’ Guide to the Solar System 16 Many smaller objects also orbit the sun ◦ Asteroids ◦ Comets ◦ Meteoroids Asteroids are small, rocky, and irregular (up to 1000 km) Sometimes called “minor planets” Most orbit in a region between the orbits of Mars and Jupiter known as the Asteroid Belt Combined, they would be about half the size of Earth’s moon An ice and rock core (nucleus) that melts as it approaches the sun and gets blown into space by solar wind pressure (coma and tail). Highly elliptical orbits originate in deep space Kitt Peak telescope image of comet NEAT in 2004 (NASA, NOAO, NSF, T. Rector, Z. Levay and L.Frattare) On 12 November 2014, ESA's Rosetta mission soft-landed its Philae probe on comet 67P/ ChuryumovGerasimenko During the next phase of the mission, Rosetta will accompany the comet through perihelion in August 2015 Image taken by Rosetta 165 km from the center of Comet 67P, April 15, 2015 Meteoroids are small irregular grains of rock, most are asteroid pieces and comet dust (up to a few meters in diameter) ◦ If they collide with Earth’s atmosphere, they form visible streaks of light as they burn up and are called meteors (shooting stars) ◦ If a meteor reaches Earth’s surface, it is then classified as a meteorite Their location ◦ Meteoroid= space ◦ Meteor= Earth’s atmosphere ◦ Meteorite= Earth’s surface Explorers’ Guide to the Solar System 24