The solar system consists of the sun and everything that orbits around it: the nine planets and their moons, comets, asteroids, and various other objects. The solar system itself is a relatively tiny part of the enormous Milky Way galaxy. It orbits around the center of the galaxy once every 225 million years. The Milky Way galaxy is just one of billions of galaxies that in turn make up the universe. The sunAt the center of the solar system is the sun, which is an average star. The sun contains 99 percent of the mass of the entire solar system. It is a huge ball of hydrogen and helium gas that is 865,000 miles (1,392,000 kilometers) in diameter. It has a temperature, at its core, of more than 28,080,000° F (15,600,000° C). The intense radiation emitted by the sun provides almost all the energy to heat the planets, and, in the case of Earth, to allow life to exist. The planetsAfter the sun, the largest bodies in the solar system are the planets. The nine planets in the solar system are (in order from closest to the sun) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. They all have roughly circular orbits in the same plane, and most have at least one moon. They vary widely in size, temperature, composition, and distance from the sun. CometsThe solar system contains billions of comets, but most of them are too small or too far from the sun to ever be seen from Earth. Comets are dirty chunks of ice and rock that orbit the sun in highly elongated orbits. When their orbits bring them close to the sun (called perihelion), the sun's radiation causes them to shed particles and glow. At this point they are visible in the sky. When their orbits take them far from the sun (called aphelion), they cool down until their next perihelion. AsteroidsThe asteroids are millions of rocky fragments that orbit the sun, for the most part in a loose band between Mars and Jupiter. They are believed to be debris left over from collisions between other bodies in the solar system. The largest asteroid, Ceres, is 600 miles (965 kilometers) in diameter, but most asteroids are much smaller. Occasionally, small asteroids or other pieces of solar system debris, including interplanetary space dust called micrometeorites, fall into the Earth's atmosphere and become glowing meteors. The solar windAn important part of the solar system is the solar wind. It consists of a continuous stream of high-energy pieces of atoms that are ejected from the sun's hot surface and flow outward throughout the whole solar system. The solar wind is what causes auroras, or displays of colored light in the night sky. In the Northern Hemisphere these auroras are called the northern lights. How the solar system was formedThe solar system was formed about 4.7 billion years ago, most likely when loose clouds of dust began to collect into clumps, due to the dust's own gravitational forces. The largest cloud of dust compressed itself so much that it began to heat up, and eventually became the sun. The other clumps, over millions of years, became the planets. The planets were gradually drawn into orbits around the sun because of the strong pull of gravity coming from the sun. Exploring the solar systemIn 1957, the Soviet satellite Sputnik I became the first man-made object to orbit the Earth. Since then, scientists have sent numerous spacecraft to explore various parts of the solar system. Unmanned probes have been sent to Venus, Mars, and Jupiter, and many measurements have been made that have helped scientists understand better the sun, the planets, and their moons. Finding other solar systemsIn the late 20th century scientists began searching the universe for other stars that may have their own solar systems. Since the sun is a typical star, they believe that there may be other planets in orbit around other stars in the sky. Some may even have the right conditions to support life. Detecting the planets, however, is not easy, since they are so much dimmer than the stars that illuminate them. They are therefore virtually impossible to see with even the strongest telescopes. Nonetheless, a number of planets have been found by carefully measuring the slight changes in position and brightness of a star that are caused by a planet orbiting around it. |