Solar System

Solar System, the Sun and everything that orbits the Sun, including the planets and their satellites; the dwarf planets, asteroids, Kuiper Belt Objects, and comets; and interplanetary dust and gas. The term may also refer to a group of celestial bodies orbiting another star (see Extrasolar Planets). In this article, solar system refers to the system that includes Earth and the Sun.

The dimensions of the solar system are specified in terms of the mean distance from Earth to the Sun, called the astronomical unit (AU). One AU is 150 million km (about 93 million mi). Estimates for the boundary where the Sun’s magnetic field ends and interstellar space begins—called the heliopause—range from 86 to 100 AU from the Sun.

The most distant known body orbiting the Sun is the dwarf planet Eris, whose discovery was reported in July 2005. Eris is currently about 97 AU from the Sun. Another planetlike object in the outer solar system named Sedna is currently at 90 AU but will reach about 900 AU at the farthest point in its orbit thousands of years from now. Comets known as long-period comets, however, achieve the greatest distance from the Sun; they have highly eccentric orbits ranging out to 50,000 AU or more. (A comet’s period is how long it takes it to complete one revolution about the Sun.) They are members of the Oort cloud, a spherical shell of comet nuclei that surrounds the flat plane of planetary orbits at this enormous distance.

The solar system was the only planetary system known to exist around a star similar to the Sun until 1995, when astronomers discovered a planet about 0.6 times the mass of Jupiter orbiting the star 51 Pegasi. Jupiter is the most massive planet in our solar system. Soon after, astronomers found a planet about 8.1 times the mass of Jupiter orbiting the star 70 Virginis, and a planet about 3.5 times the mass of Jupiter orbiting the star 47 Ursa Majoris. Since then, astronomers have found planets and disks of dust in the process of forming planets around many other stars. Most astronomers think it likely that solar systems of some sort are numerous throughout the universe. See Astronomy; Galaxy; Star.

II

THE SUN AND THE SOLAR WIND

The Sun is a typical star of intermediate size and luminosity. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the Sun’s hot, dense interior (see Nuclear Energy). Although this nuclear fusion is transforming 600 million metric tons of hydrogen each second, the Sun is so massive (2 × 10³⁰ kg, or 4.4 × 10³⁰ lb) that it can continue to shine at its present brightness for 6 billion years. This stability has allowed life to develop and survive on Earth.

For all the Sun’s steadiness, it is an extremely active star. On its surface, dark sunspots bounded by intense magnetic fields come and go in 11-year cycles and sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on Earth. A continuous stream of protons, electrons, and ions also leaves the Sun and moves out through the solar system. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil, samples of which were brought back from the Moon’s surface by piloted United States Apollo spacecraft (see Space Exploration; Apollo program).

The Sun’s activity also influences the heliopause, a region of space that astronomers believe marks the boundary between the solar system and interstellar space. The heliopause is a dynamic region that expands and contracts due to the constantly changing speed and pressure of the solar wind. In November 2003 a team of astronomers reported that the Voyager 1 spacecraft appeared to have encountered the outskirts of the heliopause at about 86 AU from the Sun. They based their report on data that indicated the solar wind had slowed from 1.1 million km/h (700,000 mph) to 160,000 km/h (100,000 mph). This finding is consistent with the theory that when the solar wind meets interstellar space at a turbulent zone known as the termination shock boundary, it will slow abruptly. However, another team of astronomers disputed the finding, saying that the spacecraft had neared but had not yet reached the heliopause.

III

THE MAJOR PLANETS

Eight major or classical planets are currently recognized by the International Astronomical Union (IAU), the body that gives official names to objects in the solar system. The planets are commonly divided into two groups: the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, and Neptune). The inner planets are small and are composed primarily of rock and iron. The outer planets are much larger and consist mainly of hydrogen, helium, and ice. Pluto, historically counted as the ninth planet, does not belong to either group, and was reclassified as a dwarf planet by the IAU in 2006. Some astronomers have objected to the reclassification of Pluto and the new IAU definition of a planet. Others feel strongly that classifying Pluto as one of the largest members of a family of similar icy bodies orbiting the Sun beyond Neptune provides a much better explanation of Pluto’s existence.

The IAU has defined a classical planet as a body that orbits the Sun, that has a rounded shape from the effects of its own gravity, and that is the dominant object in its region of space and has cleared the neighborhood of its orbit of other objects. A dwarf planet is also a rounded body orbiting the Sun—but one that is not massive enough to have cleared its region of space of other objects.

Mercury is surprisingly dense, apparently because it has an unusually large iron core. With only a transient atmosphere, Mercury has a surface that still bears the record of bombardment by asteroidal bodies early in its history. Venus has a carbon dioxide atmosphere 90 times thicker than that of Earth, causing an efficient greenhouse effect by which the Venusian atmosphere is heated. The resulting surface temperature is the hottest of any planet—about 477°C (about 890°F).

Earth is the only planet known to have abundant liquid water and life. However, in 2004 astronomers with the National Aeronautics and Space Administration’s Mars Exploration Rover mission confirmed that Mars once had liquid water on its surface. Scientists had previously concluded that liquid water once existed on Mars due to the numerous surface features on the planet that resemble water erosion found on Earth. Mars’s carbon dioxide atmosphere is now so thin that the planet is dry and cold, with polar caps of frozen water and solid carbon dioxide, or dry ice. However, small jets of subcrustal water may still erupt on the surface in some places.

Jupiter is the largest of the planets. Its hydrogen and helium atmosphere contains pastel-colored clouds, and its immense magnetosphere, rings, and satellites make it a planetary system unto itself. One of Jupiter’s largest moons, Io, has volcanoes that produce the hottest surface temperatures in the solar system. At least four of Jupiter’s moons have atmospheres, and at least three may contain liquid or partially frozen water. Jupiter’s moon Europa may have a global ocean of liquid water beneath its icy crust.

Saturn rivals Jupiter, with a much more intricate ring structure and a similar number of satellites. One of Saturn’s moons, Titan, has an atmosphere thicker than that of any other satellite in the solar system. Another moon of Saturn, Enceladus, has liquid-water geysers. Uranus and Neptune are deficient in hydrogen compared with Jupiter and Saturn; Uranus, also ringed, has the distinction of rotating at 98° to the plane of its orbit. See also Planetary Science.

IV

OTHER ORBITING BODIES

Three dwarf planets are currently recognized by the IAU: Pluto, Eris, and Ceres. Astronomers may designate more dwarf planets in the near future. Pluto and Eris are dwarf planets according to the IAU because they have rounded shapes from their own gravity but have not cleared their neighborhoods in space of other objects—both orbit through the Kuiper Belt, a region beyond Neptune containing thousands of small icy bodies. Pluto and Eris are composed of layers of ice around a rocky core. Ceres qualifies as a dwarf planet because it is spherical but is found in the asteroid belt, a zone between the orbits of Mars and Jupiter that contains thousands of small rocky bodies. Ceres is likely made up of a rocky core surrounded by a mantle containing a mix of rock and ice. Like asteroids, dwarf planets are listed by the IAU as minor planets with numbers and names.

The asteroids are small, rocky bodies that move in orbits primarily between the orbits of Mars and Jupiter. Numbering in the thousands, asteroids range in size from around 530 km (329 mi)—about half the size of the dwarf planet Ceres—to microscopic grains. Some asteroids are perturbed, or pulled by forces other than their attraction to the Sun, into eccentric orbits that can bring them closer to the Sun. If the orbits of such bodies intersect that of Earth, they are called meteoroids. When they appear in the night sky as streaks of light, they are known as meteors, and recovered fragments are termed meteorites. Laboratory studies of meteorites have revealed much information about primitive conditions in our solar system. The surfaces of Mercury, Mars, and several satellites of the planets (including Earth’s moon) show the effects of an intense bombardment by asteroidal and cometary objects early in the history of the solar system. On Earth that record has eroded away, except for a few recently found impact craters.

Some meteors and interplanetary dust may also come from comets, which are basically aggregates of dust and frozen gases typically 5 to 10 km (about 3 to 6 mi) in diameter. Comets orbit the Sun at distances so great that they can be perturbed by stars into orbits that bring them into the inner solar system. As comets approach the Sun, they release their dust and gases to form a spectacular coma and tail. Under the influence of Jupiter’s strong gravitational field, comets can sometimes adopt much smaller orbits. The most famous of these is Halley’s Comet, which returns to the inner solar system at 75-year periods. Its most recent return was in 1986. In July 1994 fragments of Comet Shoemaker-Levy 9 bombarded Jupiter’s dense atmosphere at speeds of about 210,000 km/h (130,000 mph). Upon impact, the tremendous kinetic energy of the fragments was released through massive explosions, some resulting in fireballs larger than Earth.

Comets circle the Sun in two main groups, within the Kuiper Belt or within the Oort cloud. The Kuiper Belt is a disk of icy debris that orbits the Sun beyond the planet Neptune. The population of the Kuiper Belt is made up of Kuiper Belt Objects (KBOs). KBOs range in size from clumps of ice mixed with rock dust (“dirty snowballs”) up to dwarf planets such as Pluto and Eris. Any of these icy objects could be considered a comet nucleus that would give off gas and dust to produce a coma and a tail if its orbit were to bring it close enough to the Sun. Most of the comets with periods of less than 500 years come from the Kuiper Belt.

The Oort cloud is a hypothetical region about halfway between the Sun and the heliopause. Astronomers believe that the existence of the Oort cloud, named for Dutch astronomer Jan Oort, explains why some comets have very long periods. A chunk of dust and ice may stay in the Oort cloud for thousands of years. Nearby stars sometimes pass close enough to the solar system that their gravitational force will push an object in the Oort cloud into an orbit that takes it close to the Sun.

The first detection of the long-hypothesized Oort cloud came in March 2004 when astronomers reported the discovery of a planetoid about 1,700 km (about 1,000 mi) in diameter. They named it Sedna, after a sea goddess in Inuit mythology. Sedna was found about 13 billion km (about 8 billion mi) from the Sun. At its farthest point from the Sun, Sedna is the most distant object in the solar system and is about 130 billion km (about 84 billion mi) from the Sun.

Many of the objects that do not fall into the asteroid belts, the Kuiper Belt, or the Oort cloud may be comets that will never make it back to the Sun. The surfaces of the icy satellites of the outer planets are scarred by impacts from such bodies. The asteroid-like object Chiron, with an orbit between Saturn and Uranus, may itself be an extremely large inactive comet. Similarly, some of the asteroids that cross the path of Earth’s orbit may be the rocky remains of burned-out comets. Chiron and similar objects called the Centaurs probably escaped from the Kuiper Belt and were drawn into their irregular orbits by the gravitational pull of the giant outer planets, Jupiter, Saturn, Neptune, and Uranus.

The Sun was also found to be encircled by rings of interplanetary dust. One of them, between Jupiter and Mars, has long been known as the cause of zodiacal light, a faint glow that appears in the east before dawn and in the west after dusk. Another ring, lying only two solar widths away from the Sun, was discovered in 1983.

V

MOVEMENTS OF THE PLANETS AND THEIR SATELLITES

If one could look down on the solar system from far above the North Pole of Earth, the planets would appear to move around the Sun in a counterclockwise direction. All of the planets except Venus and Uranus, and the dwarf planet Pluto, rotate on their axes in this same direction. The entire system is remarkably flat—only Mercury among the major planets has an obviously inclined orbit. However, the dwarf planets Pluto and Eris have orbits that are strongly tilted out of the main plane of the solar system, Pluto at 17.2° and Eris at 44°. Both objects also have highly elliptical orbits. Pluto’s orbit sometimes takes it closer than Neptune to the Sun. At its nearest point to the Sun, Eris passes inside the orbit of Pluto, though well beyond the orbit of Neptune.

The satellite systems mimic the behavior of their parent planets and move in a counterclockwise direction, but many exceptions are found. Jupiter, Saturn, Uranus, and Neptune each have a number of satellites that move around the planet in a retrograde orbit (clockwise instead of counterclockwise), and several satellite orbits are highly elliptical. Uranus has some satellites that follow its clockwise direction and others that move in counterclockwise orbits. Jupiter, moreover, has trapped two clusters of planetesimals or small rocky bodies (the so-called Trojan asteroids) leading and following the planet by 60° in its orbit around the Sun. Neptune also has groups of planetesimals that share its orbit. (Some satellites of Saturn have done the same with smaller bodies that occupy different parts of the same orbits as the satellites.) The long-period comets exhibit a roughly spherical distribution of orbits around the Sun, while most of the short-period comets appear to originate from the disklike distribution of Kuiper Belt Objects.

Within this maze of motions, some remarkable patterns exist: Mercury rotates on its axis three times for every two revolutions about the Sun; no asteroids exist with periods 1/2, 1/3, … 1/n (where n is an integer) the period of Jupiter; the three inner Galilean satellites of Jupiter have periods in the ratio 4:2:1. Some Kuiper Belt Objects (including Pluto) orbit the Sun in a 2:3 ratio to Neptune’s orbit. These and other examples demonstrate the subtle balance of forces that is established in a gravitational system composed of many bodies.

VI

THEORIES OF ORIGIN

Despite their differences, the members of the solar system probably form a common family. They seem to have originated at the same time; few indications exist of bodies joining the solar system, captured later from other stars or interstellar space.

Early attempts to explain the origin of this system include the nebular hypothesis of the German philosopher Immanuel Kant and the French astronomer and mathematician Pierre Simon de Laplace, according to which a cloud of gas broke into rings that condensed to form planets. Doubts about the stability of such rings led some scientists to consider various catastrophic hypotheses, such as a close encounter of the Sun with another star. Such encounters are extremely rare, and the hot, tidally disrupted gases would dissipate rather than condense to form planets.

Current theories connect the formation of the solar system with the formation of the Sun itself, about 4.6 billion years ago. The fragmentation and gravitational collapse of an interstellar cloud of gas and dust, triggered perhaps by nearby supernova explosions, may have led to the formation of a primordial solar nebula. The Sun would then form in the densest, central region. It is so hot close to the Sun that even silicates, which are relatively dense, have difficulty forming there. This phenomenon may account for the presence near the Sun of a planet such as Mercury, having a relatively small silicate crust and a larger than usual, dense iron core. (It is easier for iron dust and vapor to coalesce near the central region of a solar nebula than it is for lighter silicates to do so.) At larger distances from the center of the solar nebula, gases condense into solids such as are found today from Jupiter outward. Evidence of a possible preformation supernova explosion appears as traces of anomalous isotopes in tiny inclusions in some meteorites. This association of planet formation with star formation suggests that billions of other stars in our galaxy may also have planets. The high frequency of binary and multiple stars, as well as the large satellite systems around Jupiter and Saturn, attest to the tendency of collapsing gas clouds to fragment into multibody systems.

The formation of our solar system may have been an even more complex process than once thought. Recent studies of the chemistry of comets based on NASA’s Deep Impact and Stardust missions indicate that such primitive objects contain a surprising mix of materials that formed in both the hot inner regions and the cold outer regions of the early solar system. Some computer models show that the giant planets may have formed closer to the Sun, then moved outward over time, changing the orbits of other planets. Other models suggest inward migration of Jupiter and Saturn, imitating the orbital histories of some giant extrasolar planets that have been found orbiting very close to their parent stars. Our early solar system likely contained additional planets that were either destroyed in collisions with other planets or were thrown out of the solar system completely. The study of solar systems around other stars promises to provide important additional insights.

See separate articles for most of the celestial bodies mentioned in this article. See also Exobiology.