Mercury
Mercury, the innermost planet of the solar system and the eighth in size and mass. Its closeness to the Sun and its smallness make it the most elusive of the planets visible to the unaided eye. Because its rising or setting is always within about two hours of the Sun’s, it is never observable when the sky is fully dark. Mercury is designated by the symbol ☿. Mercury Mosaic view of Mercury, showing about half the hemisphere of the planet that was illuminated when Mariner 10 departed the planet during its first flyby in March 1974. The landscape is dominated by large impact basins and craters with extensive intercrater plains. Half of the enormous Caloris impact basin is discernible as a slightly darker region near the terminator (shadow line) just above centre. The difficulty in seeing it notwithstanding, Mercury was known at least by Sumerian times, some 5,000 years ago. In Classical Greece it was called Apollo when it appeared as a morning star just before sunrise and Hermes, the Greek equivalent of the Roman god Mercury, when it appeared as an evening star just after sunset. Hermes was the swift messenger of the gods, and the planet’s name is thus likely a reference to its rapid motions relative to other objects in the sky. Even in more recent eras, many sky observers passed their entire lifetimes without ever seeing Mercury. It is reputed that Nicolaus Copernicus, whose heliocentric model of the heavens in the 16th century explained why Mercury and Venus always appear in close proximity to the Sun, expressed a deathbed regret that he had never set eyes on the planet Mercury himself.
Along with Venus, Earth, and Mars, Mercury is one of the rocky planets. It has a solid surface that is covered with craters. Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking meteoroids. Mercury's exosphere is composed mostly of oxygen, sodium, hydrogen, helium, and potassium. Mercury doesn’t have any moons.
Venus
Venus, second planet from the Sun and sixth in the solar system in size and mass. No planet approaches closer to Earth than Venus; at its nearest it is the closest large body to Earth other than the Moon. Because Venus’s orbit is nearer the Sun than Earth’s, the planet is always roughly in the same direction in the sky as the Sun and can be seen only in the hours near sunrise or sunset. When it is visible, it is the most brilliant planet in the sky. Venus is designated by the symbol ♀. colour-coded global image of the topography of Venus Colour-coded global image of the topography of Venus below its obscuring clouds, based on radar data from the Magellan spacecraft with supplemental data from Venera and Pioneer Venus missions and Earth-based radar studies. Violet hues mark the lowest elevations; red and pink hues, the highest ones. The hemisphere shown is centred on 0° longitude; north is at the top. The prominent red and pink region in the far north is the planet's highest terrain, Maxwell Montes. Venus was one of the five planets—along with Mercury, Mars, Jupiter, and Saturn—known in ancient times, and its motions were observed and studied for centuries prior to the invention of advanced astronomical instruments. Its appearances were recorded by the Babylonians, who equated it with the goddess Ishtar, about 3000 BCE, and it also is mentioned prominently in the astronomical records of other ancient civilizations, including those of China, Central America, Egypt, and Greece. Like the planet Mercury, Venus was known in ancient Greece by two different names—Phosphorus (see Lucifer) when it appeared as a morning star and Hesperus when it appeared as an evening star. Its modern name comes from the Roman goddess of love and beauty (the Greek equivalent being Aphrodite), perhaps because of the planet’s luminous jewel-like appearance.
Venus has been called Earth’s twin because of the similarities in their masses, sizes, and densities and their similar relative locations in the solar system. Because they presumably formed in the solar nebula from the same kind of rocky planetary building blocks, they also likely have similar overall chemical compositions. Early telescopic observations of the planet revealed a perpetual veil of clouds, suggestive of a substantial atmosphere and leading to popular speculation that Venus was a warm, wet world, perhaps similar to Earth during its prehistoric age of swampy carboniferous forests and abundant life. Scientists now know, however, that Venus and Earth have evolved surface conditions that could hardly be more different. Venus is extremely hot, dry, and in other ways so forbidding that it is improbable that life as it is understood on Earth could have developed there. One of scientists’ major goals in studying Venus is to understand how its harsh conditions came about, which may hold important lessons about the causes of environmental change on Earth.
Earth
Earth, third planet from the Sun and the fifth largest planet in the solar system in terms of size and mass. Its single most outstanding feature is that its near-surface environments are the only places in the universe known to harbour life. It is designated by the symbol ♁. Earth’s name in English, the international language of astronomy, derives from Old English and Germanic words for ground and earth, and it is the only name for a planet of the solar system that does not come from Greco-Roman mythology. Earth is part of the "observable universe," the region of space that humans can actually or theoretically observe with the aid of technology. Unlike the observable universe, the universe is possibly infinite. Examine the observable universe's place within the whole universe Examine the observable universe's place within the whole universe Learn about defining and measuring the observable universe within the “whole” universe. See all videos for this article Since the Copernican revolution of the 16th century, at which time the Polish astronomer Nicolaus Copernicus proposed a Sun-centred model of the universe (see heliocentric system), enlightened thinkers have regarded Earth as a planet like the others of the solar system. Concurrent sea voyages provided practical proof that Earth is a globe, just as Galileo’s use of his newly invented telescope in the early 17th century soon showed various other planets to be globes as well. It was only after the dawn of the space age, however, when photographs from rockets and orbiting spacecraft first captured the dramatic curvature of Earth’s horizon, that the conception of Earth as a roughly spherical planet rather than as a flat entity was verified by direct human observation. Humans first witnessed Earth as a complete orb floating in the inky blackness of space in December 1968 when Apollo 8 carried astronauts around the Moon. Robotic space probes on their way to destinations beyond Earth, such as the Galileo and the Near Earth Asteroid Rendezvous (NEAR) spacecraft in the 1990s, also looked back with their cameras to provide other unique portraits of the planet. Viewed from another planet in the solar system, Earth would appear bright and bluish in colour. Easiest to see through a large telescope would be its atmospheric features, chiefly the swirling white cloud patterns of midlatitude and tropical storms, ranged in roughly latitudinal belts around the planet. The polar regions also would appear a brilliant white, because of the clouds above and the snow and ice below. Beneath the changing patterns of clouds would appear the much darker blue-black oceans, interrupted by occasional tawny patches of desert lands. The green landscapes that harbour most human life would not be easily seen from space. Not only do they constitute a modest fraction of the land area, which itself is less than one-third of Earth’s surface, but they are often obscured by clouds. Over the course of the seasons, some changes in the storm patterns and cloud belts on Earth would be observed. Also prominent would be the growth and recession of the winter snowcap across land areas of the Northern Hemisphere.
Scientists have applied the full battery of modern instrumentation to studying Earth in ways that have not yet been possible for the other planets; thus, much more is known about its structure and composition. This detailed knowledge, in turn, provides deeper insight into the mechanisms by which planets in general cool down, by which their magnetic fields are generated, and by which the separation of lighter elements from heavier ones as planets develop their internal structure releases additional energy for geologic processes and alters crustal compositions. The mean distance of Earth from the Sun is about 149,600,000 km (92,960,000 miles). The planet orbits the Sun in a path that is presently more nearly a circle (less eccentric) than are the orbits of all but two of the other planets, Venus and Neptune. Earth makes one revolution, or one complete orbit of the Sun, in about 365.25 days. The direction of revolution—counterclockwise as viewed down from the north—is in the same sense, or direction, as the rotation of the Sun; Earth’s spin, or rotation about its axis, is also in the same sense, which is called direct or prograde. The rotation period, or length of a sidereal day (see day; sidereal time)—23 hours, 56 minutes, and 4 seconds—is similar to that of Mars. Jupiter and most asteroids have days less than half as long, while Mercury and Venus have days more nearly comparable to their orbital periods. The 23.44° tilt, or inclination, of Earth’s axis to its orbital plane, also typical, results in greater heating and more hours of daylight in one hemisphere or the other over the course of a year and so is responsible for the cyclic change of seasons. With an equatorial radius of 6,378 km (3,963 miles), Earth is the largest of the four inner, terrestrial (rocky) planets, but it is considerably smaller than the gas giants of the outer solar system. Earth has a single natural satellite, the Moon, which orbits the planet at a mean distance of about 384,400 km (238,900 miles).
Mars
Mars, fourth planet in the solar system in order of distance from the Sun and seventh in size and mass. It is a periodically conspicuous reddish object in the night sky. Mars is designated by the symbol ♂. Sometimes called the Red Planet, Mars has long been associated with warfare and slaughter. It is named for the Roman god of war. As long as 3,000 years ago, Babylonian astronomer-astrologers called the planet Nergal for their god of death and pestilence. The planet’s two moons, Phobos (Greek: “Fear”) and Deimos (“Terror”), were named for two of the sons of Ares and Aphrodite (the counterparts of Mars and Venus, respectively, in Greek mythology).In recent times Mars has intrigued people for more-substantial reasons than its baleful appearance. The planet is the second closest to Earth, after Venus, and it is usually easy to observe in the night sky because its orbit lies outside Earth’s. It is also the only planet whose solid surface and atmospheric phenomena can be seen in telescopes from Earth. Centuries of assiduous studies by earthbound observers, extended by spacecraft observations since the 1960s, have revealed that Mars is similar to Earth in many ways. Like Earth, Mars has clouds, winds, a roughly 24-hour day, seasonal weather patterns, polar ice caps, volcanoes, canyons, and other familiar features. There are intriguing clues that billions of years ago Mars was even more Earth-like than today, with a denser, warmer atmosphere and much more water—rivers, lakes, flood channels, and perhaps oceans. By all indications Mars is now a sterile frozen desert. However, close-up images of dark streaks on the slopes of some craters during Martian spring and summer suggest that at least small amounts of water may flow seasonally on the planet’s surface, and radar reflections from a possible lake under the south polar cap suggest that water may still exist as a liquid in protected areas below the surface. The presence of water on Mars is considered a critical issue because life as it is presently understood cannot exist without water. If microscopic life-forms ever did originate on Mars, there remains a chance, albeit a remote one, that they may yet survive in these hidden watery niches. In 1996 a team of scientists reported what they concluded to be evidence for ancient microbial life in a piece of meteorite that had come from Mars, but most scientists have disputed their interpretation. Since at least the end of the 19th century, Mars has been considered the most hospitable place in the solar system beyond Earth both for indigenous life and for human exploration and habitation. At that time, speculation was rife that the so-called canals of Mars—complex systems of long, straight surface lines that very few astronomers had claimed to see in telescopic observations—were the creations of intelligent beings. Seasonal changes in the planet’s appearance, attributed to the spread and retreat of vegetation, added further to the purported evidence for biological activity. Although the canals later proved to be illusory and the seasonal changes geologic rather than biological, scientific and public interest in the possibility of Martian life and in exploration of the planet has not faded. Mars is the fourth planet out from the Sun. It moves around the Sun at a mean distance of 228 million km (140 million miles), or about 1.5 times the distance of Earth from the Sun. Because of Mars’s relatively elongated orbit, the distance between Mars and the Sun varies from 206.6 million to 249.2 million km (128.4 million to 154.8 million miles). Mars orbits the Sun once in 687 Earth days, which means that its year is nearly twice as long as Earth’s. At its closest approach, Mars is less than 56 million km (35 million miles) from Earth, but it recedes to almost 400 million km (250 million miles) when the two planets are on opposite sides of the solar system. Mars is easiest to observe when it and the Sun are in opposite directions in the sky—i.e., at opposition—because it is then high in the sky and shows a fully lighted face. Successive oppositions occur about every 26 months. Oppositions can take place at different points in the Martian orbit. Those best for viewing occur when the planet is closest to the Sun, and so also to Earth, because Mars is then at its brightest and largest. Close oppositions occur roughly every 15 years.
Until the last part of the 20th century, Mercury was one of the least-understood planets, and even now the shortage of information about it leaves many basic questions unsettled. Indeed, the length of its day was not determined until the 1960s, and Mercury’s nearness to the Sun gave scientists bound to Earth many observational hurdles, which were overcome only by the Messenger (Mercury Surface, Space Environment, Geochemistry, and Ranging) probe. Messenger was launched in 2004, flew past the planet twice in 2008 and once in 2009, and settled into orbit in 2011. It mapped the entire surface of Mercury before crashing into the planet in 2015. Mercury’s proximity to the Sun has also been exploited to confirm predictions made by relativity theory about the way gravity affects space and time.
Jupiter
Jupiter, the most massive planet of the solar system and the fifth in distance from the Sun. It is one of the brightest objects in the night sky; only the Moon, Venus, and sometimes Mars are more brilliant. Jupiter is designated by the symbol ♃. When ancient astronomers named the planet Jupiter for the Roman ruler of the gods and heavens (also known as Jove), they had no idea of the planet’s true dimensions, but the name is appropriate, for Jupiter is larger than all the other planets combined. It takes nearly 12 Earth years to orbit the Sun, and it rotates once about every 10 hours, more than twice as fast as Earth; its colourful cloud bands can be seen with even a small telescope. It has a narrow system of rings and 92 known moons, one larger than the planet Mercury and three larger than Earth’s Moon. Some astronomers speculate that Jupiter’s moon Europa may be hiding an ocean of warm water—and possibly even some kind of life—beneath an icy crust. Jupiter has an internal heat source; it emits more energy than it receives from the Sun. The pressure in its deep interior is so high that the hydrogen there exists in a fluid metallic state. This giant has the strongest magnetic field of any planet, with a magnetosphere so large that, if it could be seen from Earth, its apparent diameter would exceed that of the Moon. Jupiter’s system is also the source of intense bursts of radio noise, at some frequencies occasionally radiating more energy than the Sun. Despite all its superlatives, however, Jupiter is made almost entirely of only two elements, hydrogen and helium, and its mean density is not much more than the density of water. Knowledge about the Jovian system grew dramatically after the mid-1970s as a result of explorations by three spacecraft missions—Pioneers 10 and 11 in 1973–74, Voyager 1 and 2 in 1979, and the Galileo orbiter and probe, which arrived at Jupiter in December 1995. The Pioneer spacecraft served as scouts for the Voyagers, showing that the radiation environment of Jupiter was tolerable and mapping out the main characteristics of the planet and its environment. The greater number and increased sophistication of the Voyager instruments provided so much new information that it was still being analyzed when the Galileo mission began. The previous missions had all been flybys, but Galileo released a probe into Jupiter’s atmosphere and then went into orbit about the planet for intensive investigations of the entire system until September 2003. In July 2016, the Juno orbiter arrived at Jupiter for a mission expected to last two years. Other looks at the Jovian system were provided in late 2000 and early 2001 by the flyby of the Cassini spacecraft on its way to Saturn and in 2007 by the flyby of the New Horizons spacecraft on its way to Pluto. Observations of the impacts of the fragmented nucleus of Comet Shoemaker-Levy 9 with Jupiter’s atmosphere in 1994 also yielded information about its composition and structure.
Jupiter has an equatorial diameter of about 143,000 km (88,900 miles) and orbits the Sun at a mean distance of 778 million km (483 million miles). The table shows additional physical and orbital data for Jupiter. Of special interest are the planet’s low mean density of 1.33 grams per cubic cm—in contrast with Earth’s 5.52 grams per cubic cm—coupled with its large dimensions and mass and short rotation period. The low density and large mass indicate that Jupiter’s composition and structure are quite unlike those of Earth and the other inner planets, a deduction that is supported by detailed investigations of the giant planet’s atmosphere and interior. Three rotation periods, all within a few minutes of each other, have been established. The two periods called System I (9 hours 50 minutes 30 seconds) and System II (9 hours 55 minutes 41 seconds) are mean values and refer to the speed of rotation at the equator and at higher latitudes, respectively, as exhibited by features observed in the planet’s visible cloud layers. Jupiter has no solid surface; the transition from the gaseous atmosphere to the fluid interior occurs gradually at great depths. Thus the variation in rotation period at different latitudes does not imply that the planet itself rotates with either of these mean velocities. In fact, the true rotation period of Jupiter is System III (9 hours 55 minutes 29 seconds). This is the period of rotation of Jupiter’s magnetic field, first deduced from Earth-based observations at radio wavelengths (see below Radio emission) and confirmed by direct spacecraft measurements. This period, which has been constant for 30 years of observation, applies to the massive interior of the planet, where the magnetic field is generated. Even a modest telescope can show much detail on Jupiter. The region of the planet’s atmosphere that is visible from Earth contains several different types of clouds that are separated both vertically and horizontally. Changes in these cloud systems can occur over periods of a few hours, but an underlying pattern of latitudinal currents has maintained its stability for decades. It has become traditional to describe the appearance of the planet in terms of a standard nomenclature for its alternating dark bands, called belts, and bright bands, called zones. The underlying currents, however, seem to have a greater persistence than this pattern. For example, the south equatorial belt has faded away several times and has even totally disappeared (most recently in 2010), only to reappear months or years later.Saturn
Saturn, second largest planet of the solar system in mass and size and the sixth nearest planet in distance to the Sun. In the night sky Saturn is easily visible to the unaided eye as a non-twinkling point of light. When viewed through even a small telescope, the planet encircled by its magnificent rings is arguably the most sublime object in the solar system. Saturn is designated by the symbol ♄. Saturn’s name comes from the Roman god of agriculture, who is equated with the Greek deity Cronus, one of the Titans and the father of Zeus (the Roman god Jupiter). As the farthest of the planets known to ancient observers, Saturn also was noted to be the slowest-moving. At a distance from the Sun that is 9.5 times as far as Earth’s, Saturn takes approximately 29.5 Earth years to make one solar revolution. The Italian astronomer Galileo in 1610 was the first to observe Saturn with a telescope. Although he saw a strangeness in Saturn’s appearance, the low resolution of his instrument did not allow him to discern the true nature of the planet’s rings. Saturn occupies almost 60 percent of Jupiter’s volume but has only about one-third of its mass and the lowest mean density—about 70 percent that of water—of any known object in the solar system. Hypothetically, Saturn would float in an ocean large enough to hold it. Both Saturn and Jupiter resemble stars in that their bulk chemical composition is dominated by hydrogen. Also, as is the case for Jupiter, the tremendous pressure in Saturn’s deep interior maintains the hydrogen there in a fluid metallic state. Saturn’s structure and evolutionary history, however, differ significantly from those of its larger counterpart. Like the other giant, or Jovian, planets—Jupiter, Uranus, and Neptune—Saturn has extensive systems of moons (natural satellites) and rings, which may provide clues to its origin and evolution as well as to those of the solar system. Saturn’s moon Titan is distinguished from all other moons in the solar system by the presence of a significant atmosphere, one that is denser than that of any of the terrestrial planets except Venus. The greatest advances in knowledge of Saturn, as well as of most of the other planets, have come from deep-space probes. Four spacecraft have visited the Saturnian system: Pioneer 11 in 1979, Voyagers 1 and 2 in the two years following, and, after almost a quarter-century, Cassini-Huygens, which arrived in 2004. The first three missions were short-term flybys, but Cassini went into orbit around Saturn for years of investigations, while its Huygens probe parachuted through the atmosphere of Titan and reached its surface, becoming the first spacecraft to land on a moon other than Earth’s. Saturn orbits the Sun at a mean distance of 1,427,000,000 km (887 million miles). Its closest distance to Earth is about 1.2 billion km (746 million miles), and its phase angle—the angle that it makes with the Sun and Earth—never exceeds about 6°. Saturn seen from the vicinity of Earth thus always appears nearly fully illuminated. Only deep space probes can provide sidelit and backlit views. Like Jupiter and most of the other planets, Saturn has a regular orbit—that is, its motion around the Sun is prograde (in the same direction that the Sun rotates) and has a small eccentricity (noncircularity) and inclination to the ecliptic, the plane of Earth’s orbit. Unlike Jupiter, however, Saturn’s rotational axis is tilted substantially—by 26.7°—to its orbital plane. The tilt gives Saturn seasons, as on Earth, but each season lasts more than seven years. Another result is that Saturn’s rings, which lie in the plane of its equator, are presented to observers on Earth at opening angles ranging from 0° (edge on) to nearly 30°. The view of Saturn’s rings cycles over a 30-year period. Earth-based observers can see the rings’ sunlit northern side for about 15 years and then, in an analogous view, the sunlit southern side for the next 15 years. In the short intervals when Earth crosses the ring plane, the rings are all but invisible.
Saturn’s rotation period was very difficult to determine. Cloud motions in its massive upper atmosphere trace out a variety of periods, which are as short as about 10 hours 10 minutes near the equator and increase with some oscillation to about 30 minutes longer at latitudes higher than 40°. Scientists attempted to determine the rotation period of Saturn’s deep interior from that of its magnetic field, which is presumed to be rooted in the planet’s metallic-hydrogen outer core. However, direct measurement of the field’s rotation was difficult because the field is highly symmetrical around the rotational axis. At the time of the Voyager encounters, radio outbursts from Saturn, apparently related to small irregularities in the magnetic field, showed a period of 10 hours 39.4 minutes; this value was taken to be the magnetic field rotation period. Measurements made 25 years later by the Cassini spacecraft indicated that the field was rotating with a period 6–7 minutes longer. It was believed that the solar wind is responsible for some of the difference between these two measurements of the rotational period. Not until Cassini flew inside Saturn’s rings on its final orbits was the rotation period accurately measured. By relating waves observed in the rings to slight variations in Saturn’s gravitational field, the rotation period of the planet was determined to be 10 hours 33 minutes 38 seconds. The time differences between the rotation periods of Saturn’s clouds and of its interior have been used to estimate wind velocities (see below The atmosphere). Because the four giant planets have no solid surface in their outer layers, by convention the values for the radius and gravity of these planets are calculated at the level at which one bar of atmospheric pressure is exerted. By this measure, Saturn’s equatorial diameter is 120,536 km (74,898 miles). In comparison, its polar diameter is only 108,728 km (67,560 miles), or 10 percent smaller, which makes Saturn the most oblate (flattened at the poles) of all the planets in the solar system. Its oblate shape is apparent even in a small telescope. Even though Saturn rotates slightly slower than Jupiter, it is more oblate because its rotational acceleration cancels a larger fraction of the planet’s gravity at the equator. The equatorial gravity of the planet, 896 cm (29.4 feet) per second per second, is only 74 percent of its polar gravity. Saturn is 95 times as massive as Earth but occupies a volume 766 times greater. Its mean density of 0.69 gram per cubic cm is thus only some 12 percent of Earth’s. Saturn’s equatorial escape velocity—the velocity needed for an object, which includes individual atoms and molecules, to escape the planet’s gravitational attraction at the equator without having to be further accelerated—is nearly 36 km per second (80,000 miles per hour) at the one-bar level, compared with 11.2 km per second (25,000 miles per hour) for Earth. This high value indicates that there has been no significant loss of atmosphere from Saturn since its formation.
Uranus
Uranus, seventh planet in distance from the Sun and the least massive of the solar system’s four giant, or Jovian, planets, which also include Jupiter, Saturn, and Neptune. At its brightest, Uranus is just visible to the unaided eye as a blue-green point of light. It is designated by the symbol ♅. Hubble Space Telescope: Uranus Image of Uranus captured by the Hubble Space Telescope, 1998. Visible are four of its major rings and 10 of its satellites. Uranus is named for the personification of heaven and the son and husband of Gaea in Greek mythology. It was discovered in 1781 with the aid of a telescope, the first planet to be found that had not been recognized in prehistoric times. Uranus actually had been seen through the telescope several times over the previous century but dismissed as another star. Its mean distance from the Sun is nearly 2.9 billion km (1.8 billion miles), more than 19 times as far as is Earth, and it never approaches Earth more closely than about 2.7 billion km (1.7 billion miles). Its relatively low density (only about 1.3 times that of water) and large size (four times the radius of Earth) indicate that, like the other giant planets, Uranus is composed primarily of hydrogen, helium, water, and other volatile compounds; also like its kin, Uranus has no solid surface. Methane in the Uranian atmosphere absorbs the red wavelengths of sunlight, giving the planet its blue-green colour. Most of the planets rotate on an axis that is more or less perpendicular to the plane of their respective orbits around the Sun. But Uranus’s axis lies almost parallel to its orbital plane, which means that the planet spins nearly on its side, its poles taking turns pointing toward the Sun as the planet travels in its orbit. In addition, the axis of the planet’s magnetic field is substantially tipped relative to the rotation axis and offset from the planet’s centre. Uranus has more than two dozen moons (natural satellites), five of which are relatively large, and a system of narrow rings. Uranus has been visited by a spacecraft only once—by the U.S. Voyager 2 probe in 1986. Before then, astronomers had known little about the planet, since its distance from Earth makes the study of its visible surface difficult even with the most powerful telescopes available. Earth-based attempts to measure a property as basic as the planetary rotation period had produced widely differing values, ranging from 24 to 13 hours, until Voyager 2 finally established a 17.24-hour rotation period for the Uranian interior. Since Voyager’s encounter, advances in Earth-based observational technology have added to knowledge of the Uranian system.
At Uranus’s distance from the Sun, the planet takes slightly more than 84 Earth years, essentially an entire human life span, to complete one orbit. The eccentricity of its orbit is low—that is, its orbit deviates little from a perfect circle—and the inclination of the orbit to the ecliptic—the plane of Earth’s orbit and nearly the plane of the solar system in general—is less than 1°. Low orbital eccentricity and inclination are characteristic of the planets of the solar system, with the notable exceptions of Mercury and Pluto. Scientists believe that collisions and gaseous drag removed energy from the orbits while the planets were forming and so reduced the eccentricities and inclinations to their present values. Thus, Uranus formed with the other planets soon after the birth of the Sun nearly 4.6 billion years ago (see solar system: Origin of the solar system). Uranus and its neighbour Neptune, the next planet outward from the Sun, are nearly twins in size. Measured at the level of the atmosphere at which the pressure is one bar (equivalent to Earth’s sea-level pressure), Uranus’s equatorial radius of 25,559 km (15,882 miles) is 3.2 percent greater than that of Neptune. But Uranus has only 85 percent the mass of Neptune and thus is significantly less dense. The difference in their bulk densities—1.285 and 1.64 grams per cubic cm, respectively—reveals a fundamental difference in composition and internal structure. Although Uranus and Neptune are significantly larger than the terrestrial planets, their radii are less than half those of the largest planets, Jupiter and Saturn. Because Uranus’s spin axis is not perfectly parallel to the ecliptic, one of its poles is directed above the ecliptic and the other below it. (The terms above and below refer to the same sides of the ecliptic as Earth’s North and South poles, respectively.) According to international convention, the north pole of a planet is defined as the pole that is above the ecliptic regardless of the direction in which the planet is spinning. In terms of this definition, Uranus spins clockwise, or in a retrograde fashion, about its north pole, which is opposite to the prograde spin of Earth and most of the other planets. When Voyager 2 flew by Uranus in 1986, the north pole was in darkness, and the Sun was almost directly overhead at the south pole. In 42 years, or one-half the Uranian year, the Sun will have moved to a position nearly overhead at the north pole. The prevailing theory is that the severe tilt arose during the final stages of planetary accretion when bodies comparable in size to the present planets collided in a series of violent events that knocked Uranus on its side. An alternate theory is that a Mars-sized moon, orbiting Uranus in a direction opposite to the planet’s spin, eventually crashed into the planet and knocked it on its side.
Neptune
Uranus, seventh planet in distance from the Sun and the least massive of the solar system’s four giant, or Jovian, planets, which also include Jupiter, Saturn, and Neptune. At its brightest, Uranus is just visible to the unaided eye as a blue-green point of light. It is designated by the symbol ♅. Hubble Space Telescope: Uranus Image of Uranus captured by the Hubble Space Telescope, 1998. Visible are four of its major rings and 10 of its satellites. Uranus is named for the personification of heaven and the son and husband of Gaea in Greek mythology. It was discovered in 1781 with the aid of a telescope, the first planet to be found that had not been recognized in prehistoric times. Uranus actually had been seen through the telescope several times over the previous century but dismissed as another star. Its mean distance from the Sun is nearly 2.9 billion km (1.8 billion miles), more than 19 times as far as is Earth, and it never approaches Earth more closely than about 2.7 billion km (1.7 billion miles). Its relatively low density (only about 1.3 times that of water) and large size (four times the radius of Earth) indicate that, like the other giant planets, Uranus is composed primarily of hydrogen, helium, water, and other volatile compounds; also like its kin, Uranus has no solid surface. Methane in the Uranian atmosphere absorbs the red wavelengths of sunlight, giving the planet its blue-green colour. Most of the planets rotate on an axis that is more or less perpendicular to the plane of their respective orbits around the Sun. But Uranus’s axis lies almost parallel to its orbital plane, which means that the planet spins nearly on its side, its poles taking turns pointing toward the Sun as the planet travels in its orbit. In addition, the axis of the planet’s magnetic field is substantially tipped relative to the rotation axis and offset from the planet’s centre. Uranus has more than two dozen moons (natural satellites), five of which are relatively large, and a system of narrow rings. Uranus has been visited by a spacecraft only once—by the U.S. Voyager 2 probe in 1986. Before then, astronomers had known little about the planet, since its distance from Earth makes the study of its visible surface difficult even with the most powerful telescopes available. Earth-based attempts to measure a property as basic as the planetary rotation period had produced widely differing values, ranging from 24 to 13 hours, until Voyager 2 finally established a 17.24-hour rotation period for the Uranian interior. Since Voyager’s encounter, advances in Earth-based observational technology have added to knowledge of the Uranian system.
At Uranus’s distance from the Sun, the planet takes slightly more than 84 Earth years, essentially an entire human life span, to complete one orbit. The eccentricity of its orbit is low—that is, its orbit deviates little from a perfect circle—and the inclination of the orbit to the ecliptic—the plane of Earth’s orbit and nearly the plane of the solar system in general—is less than 1°. Low orbital eccentricity and inclination are characteristic of the planets of the solar system, with the notable exceptions of Mercury and Pluto. Scientists believe that collisions and gaseous drag removed energy from the orbits while the planets were forming and so reduced the eccentricities and inclinations to their present values. Thus, Uranus formed with the other planets soon after the birth of the Sun nearly 4.6 billion years ago (see solar system: Origin of the solar system). Uranus and its neighbour Neptune, the next planet outward from the Sun, are nearly twins in size. Measured at the level of the atmosphere at which the pressure is one bar (equivalent to Earth’s sea-level pressure), Uranus’s equatorial radius of 25,559 km (15,882 miles) is 3.2 percent greater than that of Neptune. But Uranus has only 85 percent the mass of Neptune and thus is significantly less dense. The difference in their bulk densities—1.285 and 1.64 grams per cubic cm, respectively—reveals a fundamental difference in composition and internal structure. Although Uranus and Neptune are significantly larger than the terrestrial planets, their radii are less than half those of the largest planets, Jupiter and Saturn. Because Uranus’s spin axis is not perfectly parallel to the ecliptic, one of its poles is directed above the ecliptic and the other below it. (The terms above and below refer to the same sides of the ecliptic as Earth’s North and South poles, respectively.) According to international convention, the north pole of a planet is defined as the pole that is above the ecliptic regardless of the direction in which the planet is spinning. In terms of this definition, Uranus spins clockwise, or in a retrograde fashion, about its north pole, which is opposite to the prograde spin of Earth and most of the other planets. When Voyager 2 flew by Uranus in 1986, the north pole was in darkness, and the Sun was almost directly overhead at the south pole. In 42 years, or one-half the Uranian year, the Sun will have moved to a position nearly overhead at the north pole. The prevailing theory is that the severe tilt arose during the final stages of planetary accretion when bodies comparable in size to the present planets collided in a series of violent events that knocked Uranus on its side. An alternate theory is that a Mars-sized moon, orbiting Uranus in a direction opposite to the planet’s spin, eventually crashed into the planet and knocked it on its side.
