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The inner planets are relatively small and made primarily of rock and iron. The asteroids orbit the sun in a belt beyond the orbit of Mars, tumbling and sometimes colliding with one another. Made mostly of rock and iron, the asteroids may be the remnants of a planet that never formed. The outer planets, with the exception of Pluto, are much larger and made mainly of hydrogen, helium, and ice. Many astronomers believe that Pluto was and interstellar wanderer that was captured by the Sun's gravity and was not an original part of the solar system.
The orbits of the planets are ellipses with the sun at one focus, though all except mercury and Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane that is called the ecliptic. The ecliptic is inclined only seven degrees from the plane of the ecliptic with and inclination of seventeen degrees. Again with the exception of Pluto, the planets all orbit the sun in almost the same plane.
The average distance of the earth to the sun is used as a standard for measuring distances in the solar system and is called an astronomical unit (AU). One AU is about 93 million miles or 150 million kilometers. Mercury the planet closest to the sun is at about 0.387 AU. Pluto is the outermost planet, and it is 39.44 AU from the sun. The heilopause is the boundary between the solar system and interstellar space, and it is about 100 AU from the sun. The comets, however, achieve the greatest distance from the Sun; they have highly eccentric orbits ranging out to 50,000 AU or more.
The Sun is a regular star of intermediate size and luminosity. It is one of more than 100 billion stars in our galaxy. The Sun is by far the largest object in our solar system. The Sun is personified in many mythologies, the Greeks called it Helios and the Romans called it Sol. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the Sun's hot, dense interior. The Sun is, at present, about 75% hydrogen and 25% helium by mass; everything else amounts to only 0.1%. This changes slowly over time as the Sun converts hydrogen to helium in its core. The Sun's outer layers exhibit different rotation, at the equator the surface rotates once every 25.4 days: near the poles it's as much as 36 days. This weird behavior is caused by the fact that the Sun is not a solid body like the Earth. The different rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body. The Sun's core conditions are extreme. The pressure is 250 billion and the temperature is 15.6 million Kelvin. At the center of the core the Sun's density is more than 150 times that of water. The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. For the Sun's entire steadiness, it is an extremely active star. On its surface dark sunspots bounded by intense magnetic fields come and go in 11-year cycles. Sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on Earth; and a continuous stream of protons, electrons and ions leave the Sun and move out through the solar system, spiraling with the Sun's rotation. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil. The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so. But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards will result in the total destruction of the Earth and probably
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solar system, planet, the solar system, Jupiter, Venus, Pluto, average distance, inner solar system, outer planets, orbit, Uranus, Saturn, outer solar system, Mars, carbon dioxide, Neptune, inner planets, asteroids, surface temperature, helium, comets, Great Red Spot, interplanetary dust, satellites, solar flares, solar wind, solid body, Gas Giants, greenhouse effect, stars, major planets, outer core, Southern Hemisphere, seventh planet, fifth planet, sixth planet, inner core, nine planets, mass, hydrogen fuel, Great Dark Spot, ecliptic, temperature range, planetary system, gases, natural satellites, intermediate size, nitrogen, methane, water vapor,