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Planets
Planet, any of the nine major celestial bodies that orbit the sun and shine by reflecting its light, or any similar celestial body orbiting a star other than the sun. Smaller bodies that also orbit the sun and are not satellites of a planet—are called asteroids or planetoids.
One of the planets in the solar system, the third in distance from the sun and the fifth largest of the planets in diameter. The mean distance of the earth from the sun is 149,503,000 km (92,897,000 mi). It is the only planet known to support life, although some of the other planets have atmospheres and contain water.
The earth is not a perfect sphere but is slightly oblate, or flattened at the poles. The diameter of the earth measured around the North Pole and the South Pole is about 42 km (26 mi) less than the diameter of the earth measured at the equator.
Fifth planet from the sun and the largest planet in the earth’s solar system. With the exception of the sun, the moon, and Venus, Jupiter is the brightest object in Earth’s sky—more than three times brighter than Sirius, the brightest star. Due to its prominence in the sky, the ancient Romans named Jupiter for Jove, the chief god of Roman mythology.
Jupiter orbits the sun at an average distance of 780 million km (484 million mi), which is about five times the distance from Earth to the sun. Jupiter’s year, or the time it takes to complete an orbit about the sun, is 11.9 Earth years, and its day, or the time it takes to rotate on its axis, is about 9.9 hours, less than half an Earth day.
Unlike the rocky inner planets of the solar system (Mercury, Venus, Earth, and Mars), Jupiter is a dense ball of gas. It has a relatively small core of molten rock and iron, but Jupiter has no solid surfaces. Jupiter’s mass is about 318 times the mass of Earth and its diameter is about 11.2 times the diameter of Earth. The force of gravity at the level of the highest clouds in Jupiter’s atmosphere is about 2.5 times the force of gravity on Earth.
Because Jupiter has such a large diameter and high rate of rotation, material at the surface must travel quickly to circle the planet. This speed gives the material a great deal of momentum, or a strong tendency to fly away from the planet and continue traveling in a straight line through space. Material at the equator has the highest speed because, in a Jovian day, it must travel the greatest distance to circle the planet. Hence, material at the equator has the greatest momentum, and the strongest tendency to fly away from the planet. Because of Jupiter’s weak, gaseous structure, the planet can not hold this material in as well as a more solid planet could, which results in Jupiter having the distorted shape of a flattened ball. The diameter of its equator is 143,000 km (89,000 mi), yet the diameter through its axis of rotation is only 133,700 km (83,000 mi).
One of the planets in the solar system, it is the fourth planet from the sun and orbits the sun at a distance of about 228 million km (about 141 million mi). Mars is named for the Roman god of war because it appears fiery red in the earth’s night sky.
Mars is a relatively small planet, with about half the diameter of Earth and about one-tenth Earth’s mass. The force of gravity on the surface of Mars is about one-third of that on Earth. Mars has twice the diameter and twice the surface gravity of Earth’s moon. The surface area of Mars is almost exactly the same as the surface area of the dry land on Earth.
The Martian day, or the time it takes Mars to rotate once on its axis, is about a half an hour longer than an Earth day. Its year, or the time it takes to revolve once around the sun, is about two Earth years long. Mars has two moons, Phobos and Deimos, which are named after the dogs of the Roman god Mars. These tiny bodies are heavily cratered dark chunks of rock and may be asteroids captured by the gravitational pull of Mars. Phobos orbits Mars once in less than one Martian day, so it appears to rise in the west and set in the east, usually twice each day. Deimos has the more ordinary habit of rising in the east and setting in the west.
In the solar system, the planet closest to the sun. Its mean distance from the sun is approximately 58 million km (about 36 million mi); its diameter is 4875 km (3030 mi); its volume and mass are about : that of Earth; and its mean density is approximately equal to that of Earth. Mercury revolves about the sun in a period of 88 days. Radar observations of the planet show that its period of rotation is 58.7 days, or two-thirds of its period of revolution. The planet, therefore, rotates one and a half times during each revolution. Because its surface consists of rough, porous, dark-colored rock, Mercury is a poor reflector of sunlight.
Fourth largest of the planets in the solar system, and eighth major planet in order of increasing distance from the sun. Neptune is, on average, about 4.5 billion km (2.8 billion mi) from the sun. It is about 49,400 km (30,700 mi) in diameter, or about 3.8 times as wide as the earth. Even though Neptune's volume is 72 times that of Earth's, its mass is only 17 times Earth's mass. Like Jupiter and Saturn, Neptune is one of the "gas giant" planets, composed of a deep atmosphere around a liquid surface and sometimes a solid core. Neptune's atmosphere consists of mostly hydrogen and helium, but up to three percent of Neptune's atmosphere is made of methane, which gives the planet its striking blue color. Its core contains more rock and metal than the cores of other gas giant planets. Neptune has a magnetic field, which is tilted more than 50� to the rotation axis.
Ninth planet from the sun and outermost known member of the solar system. Pluto was discovered as the result of a telescopic search inaugurated in 1905 by the American astronomer Percival Lowell, who postulated the existence of a distant planet beyond Neptune as the cause of slight perturbations in the motions of Uranus. Continued by members of the Lowell Observatory staff, the search ended successfully in 1930, when the American astronomer Clyde William Tombaugh found Pluto near the position Lowell had predicted.
Pluto revolves about the sun once in 247.7 Earth years at an average distance of 5.9 billion km (3.67 billion mi) from the sun. The orbit is so eccentric that at certain points along its path Pluto is closer to the sun than is Neptune. No possibility of collision exists, however, because Pluto's orbit is inclined more than 17.2� to the plane of the ecliptic and never actually crosses Neptune's path.
Sixth planet in order of distance from the sun, and the second largest in the solar system. Saturn's most distinctive feature is its ring system, which was first seen in 1610 by Italian scientist Galileo, using one of the first telescopes. He did not understand that the rings were separate from the body of the planet, so he described them as handles (ansae).The Dutch astronomer Christiaan Huygens was the first to describe the rings correctly. In 1655, desiring further time to verify his explanation without losing his claim to priority, Huygens wrote a series of letters in code, which when properly arranged formed a Latin sentence that read in translation, "It is girdled by a thin flat ring, nowhere touching, inclined to the ecliptic." The rings are named in order of their discovery, and from the planet outward they are known as the D, C, B, A, F, G, and E rings. These rings are now known to comprise more than 100,000 individual ringlets, each of which circles the planet.
Major planet in the solar system, equivalent in brightness to a sixth-magnitude star and represented by the symbols � and �. It ranks seventh in order of distance from the sun, revolving outside the orbit of Saturn and inside the orbit of Neptune . Uranus was accidentally discovered in 1781 by the British astronomer Sir William Herschel and was originally named the Georgium Sidus (Star of George) in honor of his royal patron King George III of Great Britain. The planet was later, for a time, called Herschel in honor of its discoverer. The name Uranus, which was first proposed by the German astronomer Johann Elert Bode, was in use by the late 19th century.
Uranus has a diameter of 51,120 km (31,771 mi), and its mean distance from the sun is 2.87 billion km (1.78 billion mi). Uranus takes 84 years for a single revolution, or orbit, and 17 hr 15 min for a complete rotation about its axis, which is inclined 98� to the plane of the planet's orbit around the sun. Uranus's atmosphere consists largely of hydrogen and helium, with a trace of methane. Through a telescope the planet appears as a small, bluish-green disk with a faint green periphery. Compared to the earth, Uranus has a mass 14.5 times greater, a volume 67 times greater, and a gravity 1.17 times greater. Uranus's magnetic field, however, is only a tenth as strong as earth's, with an axis tilted 55� from the rotational axis. The density of Uranus is approximately 1.2.
One of the planets in the solar system, the second in distance from the sun. Except for the sun and the moon, Venus is the brightest object in the sky. The planet is called the morning star when it appears in the east at sunrise, and the evening star when it is in the west at sunset. In ancient times the evening star was called Hesperus and the morning star Phosphorus or Lucifer. Because of the distances of the orbits of Venus and earth from the sun, Venus is never visible more than three hours before sunrise or three hours after sunset.
When viewed through a telescope, the planet exhibits phases like the moon. Full Venus appears the smallest because it is on the far side of the sun from earth. Maximum brilliance (a stellar magnitude of -4.4, or 15 times the brightest star) is seen in the crescent phase. The phases and positions of Venus in the sky repeat with the synodic period of 1.6 years. Transits across the face of the sun are rare, occurring in pairs at intervals of a little more than a century. The next two will be in 2004 and 2012.
