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| The Solar System | |
| The Solar System formed approximately 4.6 billion years ago from a giant molecular | |
| cloud. It consists of the Sun and all celestial objects gravitationally bound to it. | |
| The Sun contains 99.86% of the Solar System's mass. It is a G-type main-sequence | |
| star and is composed primarily of hydrogen (73%) and helium (25%). | |
| The eight planets of the Solar System in order from the Sun: | |
| 1. Mercury - Smallest planet, closest to Sun, no atmosphere | |
| 2. Venus - Hottest planet, thick atmosphere, rotates backwards | |
| 3. Earth - Only known planet with life, has one moon | |
| 4. Mars - Red planet, has two moons (Phobos and Deimos) | |
| 5. Jupiter - Largest planet, has Great Red Spot, 95 known moons | |
| 6. Saturn - Known for prominent rings, 146 known moons | |
| 7. Uranus - Ice giant, rotates on its side, 28 known moons | |
| 8. Neptune - Windiest planet, has Great Dark Spot, 16 known moons | |
| Pluto was reclassified as a dwarf planet by the International Astronomical Union | |
| (IAU) in 2006. Other dwarf planets include Eris, Haumea, Makemake, and Ceres. | |
| The asteroid belt is located between Mars and Jupiter. It contains millions of | |
| rocky objects, with Ceres being the largest at about 940 km in diameter. | |
| The Kuiper Belt extends beyond Neptune's orbit and contains icy bodies including | |
| Pluto. The Oort Cloud is a theoretical spherical shell of icy objects at the | |
| outer edge of the Solar System. | |
| Notable spacecraft missions: | |
| - Voyager 1 launched in 1977, entered interstellar space in 2012 | |
| - Mars rovers: Spirit (2004), Opportunity (2004), Curiosity (2012), Perseverance (2021) | |
| - New Horizons flew by Pluto in 2015 | |
| - Cassini orbited Saturn from 2004 to 2017 | |
| The habitable zone, also called the Goldilocks zone, is the region around a star | |
| where liquid water could exist on a planet's surface. Earth is within the Sun's | |
| habitable zone. | |