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SOLAR SYSTEM - a star and all of the objects that orbit around it
According to the NEBULAR THEORY, stars are born inside of immense clouds of dust and gas called NEBULAs when pockets of greater mass attract more and more mass due to their gravity. These pockets of high mass eventually flatten into a spinning disc as more mass accumulates around the central point. A star forms at the center of the disc when enough mass gathers to start the nuclear fusion process. Pockets of higher mass within the disc begin to "accrete" into planets that orbit the star.
HISTORICAL MODELS OF THE SOLAR SYSTEM
GEOCENTRIC MODEL - places Earth at the center, with all celestial objects orbiting the Earth
HELIOCENTRIC MODEL - places the Sun at the center, with all planets orbiting the Sun
Prior to the 1600's most scholars accepted the geocentric model of the solar system, seeing objects move across the sky and logically assuming they were orbiting around us. However, observations made by astronomers such as Copernicus, Brahe, and Galileo provided evidence in favor of the HELIOCENTRIC MODEL, which describes the motion of the planets (including Earth) around the Sun.
ROLE OF GRAVITY
GRAVITY is an attractive force between any two objects with mass, depending on their mass and distance apart. Gravity is responsible for the shape of objects in our solar system, including Earth. Because gravity pulls toward the center of mass from all directions, objects with sufficient mass tend to form spheres (ball shapes). This applies to stars and planets alike.
Gravity also causes the motion of the planets, as their attraction to the Sun keeps them in orbit around it. Likewise, planets with moons exert a similar gravitational force on their moons, causing the moons to orbit the planet as the plaent orbits the Sun.
Finally, gravity also influences tides on Earth, with the gravitational attraction between the Earth and Moon (and to a lesser extent, the Sun) causing the oceans to bulge.
PLANETS
Where did Earth come from?
INNER PLANETS - smaller, terrestrial, few moons, warmer temperatures, faster orbits
Mercury - smallest planet, closest to sun, little/no atmosphere
Venus - similar in size to Earth, dense atmosphere, hottest planet
Earth - only planet known to support life, covered in liquid water, moderate temperatures due to atmosphere
Mars - about half the size of Earth, red color from iron oxide (rust on surface), very thin atmosphere
OUTER PLANETS - larger, gaseous ("gas giants"), many moons, ring systems, colder temperatures, slower orbits
Jupiter - largest planet, mass more than twice the other planets combined, giant storm called "red spot"
Saturn - 2nd largest planet, known for its extensive ring system
Uranus - rotates "sideways", blue-green color due to methane
Neptune - farthest from sun, similar in size/composition to Uranus, discovered by mathematical calculations
DWARF PLANETS - spherical objects that orbit the Sun but have a "cluttered orbital path", meaning they orbit near other large objects such as asteroids. Pluto is now considered a dwarf planet because there are large asteroids within its orbit. Several other similarly sized objects were recently discovered just outside of its orbit as well, which led to a reclassification of what it means to be a "planet".
MOONS - large objects that orbit a planet rather than the Sun
ASTEROIDS AND COMETS
ASTEROIDS are small, rocky objects in our solar system that are not spherical and do not fit the criteria for a planet, dwarf planet, or moon.
COMETS are small objects made of gas, dust, and ice that orbit the Sun. They tend to have extremely elliptical orbits and reach the far edges of the solar system.