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Mercury, first planet in distance from the Sun in the solar system. The smallest of the rocky or terrestrial planets that include Venus, Earth, and Mars, Mercury has a global magnetic field, but only a trace of an atmosphere and no moons of its own. It is the second hottest planet after Venus. Mercury circles the Sun every 88 Earth days at an average distance of 58 million km (36 million mi) and takes 59 days to turns on its axis. It retains an ancient cratered surface that has changed little since the formation of the solar system, making the planet of special interest to planetary scientists. Mercury was named for the fleet-footed messenger of the gods in Roman mythology.
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Venus, second planet in distance from the Sun, but the hottest planet in the solar system (hotter than Mercury). Its hellish surface has broiling temperatures that make rocks glow red under a crushing atmosphere that shrouds the planet in thick layers of clouds. Venus is nearly the same size as Earth, but takes 243 days to rotate on its axis in the opposite direction. It also lacks a magnetic field and a moon. Why conditions on Venus and Earth are so different remains a major puzzle for planetary scientists. Venus circles the Sun at a distance of 108 million km (67 million mi) in a little over seven months (about 225 days). The planet was named for Venus, the Roman goddess of beauty.
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Earth, third planet in distance from the Sun in the solar system, the only planet known to harbor life, and the ¡°home¡± of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth¡¯s surface is covered by water, which is essential to life. The rest is land, mostly in the form of continents that rise above the oceans. Earth¡¯s surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form.
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Mars, fourth planet in distance from the Sun in the solar system. Mars is of special scientific interest because of its similarities to Earth. It has an atmosphere with seasons and changing weather, and its surface shows evidence of ancient water and volcanoes. The length of its day and the tilt of its axis are similar to those of Earth. Mars takes about two years to circle the Sun at an average distance of 228 million km (141.7 million mi). The possibility of life on Mars, now or in the distant past, is one of the major questions in astronomy. More space probes have been sent to Mars than to any other planet. Mars is named for the Roman god of war. It is sometimes called the red planet because it appears fiery red in Earth¡¯s night sky, the result of rusty, iron-oxide mineral dust that covers its surface.
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Jupiter, fifth planet from the Sun and the largest planet in the solar system. The fourth brightest object in Earth¡¯s sky, after the Sun, the Moon, and Venus, Jupiter is more than three times brighter than Sirius, the brightest star. Due to its prominence in the sky, the Romans named the planet for their chief god, Jupiter. Jupiter orbits the Sun at an average distance of 778 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.
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Saturn, sixth planet in order of distance from the Sun, and the second largest in our solar system. Saturn¡¯s most distinctive feature is a giant system of rings that surrounds the planet at its equator, stretching over twice the width of the planet itself. The first person to see the rings was the Italian scientist Galileo in 1610, using one of the earliest telescopes. Space probes have greatly increased our knowledge of Saturn, its rings, and its many moons. Flybys by the Pioneer and the Voyager probes led to the Cassini orbiter that began studying Saturn in detail in 2004. As seen from Earth, Saturn appears as a yellowish object¡ªone of the brightest in the night sky. The planet is named for Saturn, the Roman god of agriculture. Saturn takes about 29.5 years to orbit the Sun at an average distance of 1,435 billion km (891.5 million mi), or about 9.59 astronomical unit (AU). An AU is equal to the average distance between the Earth and the Sun, or 150 million km (93 million mi). Saturn rotates on its axis in about 10.5 hours and is tilted at about 27°, giving the planet distinct seasons.
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Uranus, seventh planet in distance from the Sun, third largest planet in diameter, and fourth largest in mass in the solar system. Unlike other major planets, Uranus is tipped sideways on its axis of rotation. It experiences extreme seasons, and its 13 rings and 27 known moons revolve around its equator nearly vertically to the plane of its orbit around the Sun. Because of its great size and mass, scientists classify Uranus as one of the giant or Jovian (Jupiter-like) planets¡ªalong with Jupiter, Saturn, and Neptune. Like more distant Neptune, Uranus is also classified as an ice giant planet, mainly made of the ice-forming molecules water, ammonia, and methane as a liquid mixture above what is thought to be a rocky core. Its atmosphere is mainly hydrogen and helium, along with methane gas that gives the planet a blue-green color.
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Neptune, eighth planet in distance from the Sun, fourth largest planet in diameter, and third largest in mass in the solar system. Neptune¡¯s gravity has a major influence on the Kuiper Belt, a region of icy bodies in the outer solar system that is a source of comets and includes the dwarf planet Pluto, formerly counted as the ninth planet. Because of its great size and mass, scientists classify Neptune as one of the giant or Jovian (Jupiter-like) planets¡ªalong with Jupiter, Saturn, and Uranus. Like Uranus, Neptune is also classified as an ice giant planet, mainly made of the ice-forming molecules water, ammonia, and methane as a liquid mixture above what is thought to be a rocky core. Its atmosphere is mainly hydrogen and helium, along with methane gas that gives the planet a blue-green color.
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