Solar system

Earth's solar system is comprised of the Sun , nine major planets, some 100,000 asteroids larger than 0.6 mi (1 km) in diameter, and perhaps 1 trillion cometary nuclei. While the major planets lie within 40 Astronomical Units (AU)—the average distance of Earth to the Sun—the outermost boundary of the solar system stretches to 1 million AU, one-third the way to the nearest star. Cosmologists and Astronomers assert that the solar system was formed through the collapse of a spinning cloud of interstellar gas and dust.

The central object in the solar system is the Sun. It is the largest and most massive object in the solar system; its diameter is 109 times that of Earth, and it is 333,000 times more massive. The extent of the solar system is determined by the gravitational attraction of the Sun. Indeed, the boundary of the solar system is defined as the surface within which the gravitational pull of the Sun dominates over that of the galaxy. Under this definition, the solar system extends outwards from the Sun to a distance of about 100,000 AU. The solar system is much larger, therefore, than the distance to the remotest known planet, Pluto, which orbits the Sun at a mean distance of 39.44 AU.

The Sun and the solar system are situated some 26,000 light years from the center of our galaxy. The Sun takes about 240 million years to complete one orbit about the galactic center.

Since its formation the Sun has completed about 19 such trips. As it orbits about the center of the galaxy, the Sun also moves in an oscillatory fashion above and below the galactic plane with a period of about 30 million years. During their periodic sojourns above and below the plane of the galaxy, the Sun and solar system suffer gravitational encounters with other stars and giant molecular clouds . These close encounters result in the loss of objects (essentially dormant cometary nuclei located in the outer Oort cloud) that are on, or near, the boundary of the solar system. These encounters also nudge some cometary nuclei toward the inner solar system where they may be observed as long-period comets .

The objects within our solar system demonstrate several essential dynamical characteristics. When viewed from above the Sun's North Pole, all of the planets orbit the Sun along near-circular orbits in a counterclockwise manner. The Sun also rotates in a counterclockwise direction. With respect to the Sun, therefore, the planets have prograde orbits. The major planets, asteroids, and short-period comets all move along orbits only slightly inclined to one another. For this reason, when viewed from Earth, the asteroids and planets all appear to move in the narrow zodiacal band of constellations. All of the major planets, with three exceptions, spin on their central axes in the same direction that they orbit the Sun. That is, the planets mostly spin in a prograde motion. The planets Venus, Uranus, and Pluto are the three exceptions, having retrograde (backwards) spins.

The distances at which the planets orbit the Sun increase geometrically, and it appears that each planet is roughly 64% further from the Sun than its nearest inner neighbor. The separation between successive planets increases dramatically beyond the orbit of Mars. While the inner, or terrestrial planets are typically separated by distances of about four-tenths of an AU, the outer, or Jovian planets are typically separated by 5—10 AU.

Although the asteroids and short-period comets satisfy, in a general sense, the same dynamical constraints as the major planets, we have to remember that such objects have undergone significant orbital evolution since the solar system formed. The asteroids, for example, have undergone many mutual collisions and fragmentation events, and the cometary nuclei have suffered from numerous gravitational perturbations from the planets. Long-period comets in particular have suffered considerable dynamical evolution, first to become members of the Oort cloud, and second to become comets visible in the inner solar system.

The compositional make-up of the various solar system bodies offers several important clues about the conditions under which they formed. The four interior planets—Mercury, Venus, Earth, and Mars—are classified as terrestrial and are composed of rocky material surrounding an iron-nickel metallic core. In contrast, Jupiter, Saturn, Neptune, and Uranus are classified as the "gas giants" and are large masses of hydrogen in gaseous, liquid, and solid form surrounding Earth-size rock and metal cores. Pluto fits neither of these categories, having an icy surface of frozen methane. Pluto more greatly resembles the satellites of the gas giants, which contain large fractions of icy material. This observation suggests that the initial conditions under which such ices might have formed only prevailed beyond the orbit of Jupiter.

In summary, any proposed theory for the formation of the solar system must explain both the dynamical and chemical properties of the objects in the solar system. It must also be sufficient flexibility to allow for distinctive features such as retrograde spin, and the chaotic migration of cometary orbits.

Astronomers almost universally assert that the best descriptive model for the formation of the solar system is the solar nebula hypothesis. The essential idea behind the solar nebula model is that the Sun and planets formed through the collapse of a rotating cloud of interstellar gas and dust. In this way, planet formation is postulated to be a natural consequence of star formation.

The solar nebula hypothesis is not a new scientific proposal. Indeed, the German philosopher Immanuel Kant first discussed the idea in 1755. Later, the French mathematician, Pierre Simon de Laplace (1749–1827) developed the model in his text, The System of the World, published in 1796.

The key postulate in the solar nebula hypothesis is that once a rotating interstellar gas cloud has commenced gravitational collapse, then the conservation of angular momentum will force the cloud to develop a massive, central condensation that is surrounded by a less massive flattened ring, or disk of material. The nebula hypothesis asserts that the Sun forms from the central condensation, and that the planets accumulate from the material in the disk. The solar nebula model naturally explains why the Sun is the most massive object in the solar system, and why the planets rotate about the Sun in the same sense, along nearly circular orbits and in essentially the same plane.

During the gravitational collapse of an interstellar cloud, the central regions become heated through the release of gravitational energy. This means that the young solar nebular is hot, and that the gas and (vaporized) dust in the central regions is well mixed. By constructing models to follow the gradual cooling of the solar nebula, scientists have been able to establish a chemical condensation sequence. Near to the central proto-sun, the nebular temperature will be very high, and consequently no solid matter can exist. Everything is in a gaseous form. Farther away from the central proto-sun, however, the temperature of the nebula falls off. At distances beyond 0.2 AU from the proto-sun, the temperature drops below 3,100°F (1,700°C). At this temperature, metals and oxides can begin to form. Still further out (at about 0.5 AU), the temperature will drop below 1,300°F (730°C), and silicate rocks can begin to form. Beyond about 5 AU from the protosun, the temperature of the nebula will be below −100°F (−73°C), and ices can start to condense. The temperature and distance controlled sequence of chemical condensation in the solar nebula correctly predicts the basic chemical make-up of the planets.

Perhaps the most important issue to be resolved in future versions of the solar nebula model is that of the distribution of angular momentum. The problem for the solar nebula theory is that it predicts that most of the mass and angular momentum should be in the Sun. In other words, the Sun should spin much more rapidly than it does. A mechanism is therefore required to transport angular momentum away from the central proto-sun and redistribute it in the outer planetary disk. One proposed transport mechanism invokes the presence of a magnetic field in the nebula, while another mechanism proposed the existence of viscous stresses produced by turbulence in the nebular gas.

Precise dating of meteorites and lunar rock samples indicate that the solar system is 4.6 to 5.1 billion years old. The meteorites also indicate an age spread of about 20 million years, during which time the planets themselves formed.

The standard solar nebula model suggests that the planets were created through a multi-step process. The first important step is the coagulation and sedimentation of rock and ice grains in the mid-plain of the nebula. These grains and aggregates, 0.4 in (1 cm) to 3 ft (1 m) in size, continue to accumulate in the mid-plain of the nebula to produce a swarm of some 10 trillion larger bodies, called planetesimals, that are some 0.6 mi (1 km), or so in size. Finally, the planetesimals themselves accumulate into larger, self-gravitating bodies called proto-planets. The proto-planets were probably a few hundred kilometers in size. Finally, growth of proto-planet-sized objects results in the planets.

The final stages of planetary formation were decidedly violent—it is probable that a collision with a Mars-sized proto-planet produced Earth's Moon . Likewise, it is thought that the retrograde rotations of Venus and Uranus may have been caused by glancing proto-planetary impacts. The rocky and icy planetesimals not incorporated into the proto-planets now orbit the Sun as asteroids and cometary nuclei. The cometary nuclei that formed in the outer solar nebula were mostly ejected from the nebula by gravitational encounters with the large Jovian gas giants and now reside in the Oort cloud.

One problem that has still to be worked-out under the solar nebula hypothesis concerns the formation of Jupiter. The estimated accumulation time for Jupiter is about 100 million years, but it is now known that the solar nebula itself probably only survived for 100,000 to 10 million years. In other words, the accumulation process in the standard nebula model is too slow by at least a factor of 10 and maybe 100.

Of great importance to the study of solar systems was the discovery in 1999 of an entire solar system around another star. Although such systems should be plentiful and common in the cosmos, this was the first observation of another solar system. Forty-four light-years from Earth, three large planets were found circling the star Upsilon Andromedae. Astronomers suspect the planets are similar to Jupiter and Saturn—huge spheres of gas without a solid surface.

See also Astronomy; Big Bang theory; Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Cosmology; Dating methods; Earth (planet); Earth, interior structure; Geologic time; Revolution and rotation

solar system the sun and the surrounding planets, natural satellites , dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass. The principal members of the sun's retinue are the eight major planets ; other parts of the solar system are discussed in separate articles: see comet , asteroid , and meteor .

The Planets

In order of increasing average distance from the sun, the planets are Mercury , Venus , earth , Mars , Jupiter , Saturn , Uranus , and Neptune . The planets orbiting nearer the sun than the earth are termed inferior planets; those whose orbits are larger are called superior planets. The unit for measuring distance in the solar system is the astronomical unit (AU), the average distance between the earth and the sun. The mean distances of the planets from the sun range from 0.39 AU for Mercury to 30.04 AU for Nepture.

Pluto , regarded for many years after its discovery as a planet, was reclassified in 2006 as a dwarf planet, which is a planetlike celestial body that does not clear or dominate the region of its orbit. In addition, Pluto is unlike the terrestrial planets—Mercury, Venus, Earth, and Mars—which are rocky, and it is unlike the gas giants—Jupiter, Saturn, Uranus, and Neptune. Its orbit, which is tilted from the plane in which the eight planets travel about the Sun, its size, and its composition more closely resemble those of the objects residing in the Kuiper belt (which were first discovered in 1992; see under comet ) than those of a major planet, and Pluto is now recognized as a Kuiper belt, or transneptunian, object.

See the table entitled Major Planets of the Solar System .

Planetary Motion

The motion of the planets was first described accurately by Johannes Kepler at the beginning of the 17th cent.; he showed that the planets move in nearly circular elliptical orbits. Isaac Newton later showed that the laws of planetary motion discovered by Kepler apply also to all other bodies in the solar system and are based on the force of gravitation . The sun's gravitational pull is the dominant force in the solar system; the forces exerted by the other celestial bodies on one another produce small shifts and variations, called perturbations , in their orbits. The planets orbit the sun in approximately the same plane (that of the ecliptic ) and move in the same direction—counterclockwise as viewed from above the earth's North Pole. A planet's year, or sidereal period, is the time required for it to complete one full circuit around the sun. Mercury's year is 88 earth days, while Neptune's year is 165 earth years. All the planets rotate about their own axes as they revolve around the sun; their periods of rotation vary from just under 10 earth hours for Jupiter to 243 earth days for Venus. The rotation of Venus is from east to west (see retrograde motion ). The equatorial planes of the planets are tilted to various degrees with respect to their orbital planes, giving rise to yearly seasons. The smallest tilt, that of Jupiter, is 3°, whereas that of Uranus is 98°, causing its axis of rotation to lie nearly in the plane of the planet's orbit. The tilt of the earth's equatorial plane is 23 1/2 °.

Physical Properties

The planets are grouped according to their physical properties. The inner planets (Mercury, Venus, Earth, and Mars), called the terrestrial, or earthlike, planets, are dense and small in size, with solid, rocky crusts and molten metallic interiors. Except for Mercury, they possess gaseous atmospheres from which lighter elements have escaped because of the low gravitational force. The Jovian planets (Jupiter, Saturn, Uranus, and Neptune) all have great volume and mass but relatively low density. Jupiter is heavier than all the other planets combined; it is 318 times as heavy as the earth and 1,300 times as large, making its density only about one fourth that of the earth. Saturn has a mass 95 times that of the earth and a density less than that of water. The atmospheres of the Jovian planets are very thick, merging imperceptibly with the bodies of the planets, and are rich in hydrogen, hydrogen compounds, and helium. Most of the major planets have one or more moons. See satellite, natural .

Origin of the Solar System

Besides explaining the birth of the sun, planets, dwarf planets, moons, asteroids, and comets, a theory of the origin of the solar system must explain the chemical and physical differences of the planets; their orbital regularities, i.e., why they lie almost on the same plane and revolve in the same direction in nearly circular orbits; and also account for the relative angular momentum of the sun and planets arising from their rotational and orbital motions.

The Nebular Hypothesis

The nebular hypothesis, developed by Immanuel Kant and given scientific form by P. S. Laplace at the end of the 18th cent., assumed that the solar system in its first state was a nebula, a hot, slowly rotating mass of rarefied matter, which gradually cooled and contracted, the rotation becoming more rapid, in turn giving the nebula a flattened, disklike shape. In time, rings of gaseous matter became separated from the outer part of the disk, until the diminished nebula at the center was surrounded by a series of rings. Out of the material of each ring a great ball was formed, which by shrinking eventually became a planet. The mass at the center of the system condensed to form the sun. The objections to this hypothesis were based on observations of angular momentum that conflicted with the theory.

The Planetesimal and Tidal Theories

Encounter or collision theories, in which a star passes close by or actually collides with the sun, try to explain the distribution of angular momentum. According to the planetesimal theory developed by T. C. Chamberlin and F. R. Moulton in the early part of the 20th cent., a star passed close to the sun. Huge tides were raised on the surface; some of this erupted matter was torn free and, by a cross-pull from the star, was thrust into elliptical orbits around the sun. The smaller masses quickly cooled to become solid bodies, called planetesimals. As their orbits crossed, the larger bodies grew by absorbing the planetesimals, thus becoming planets.

The tidal theory, proposed by James Jeans and Harold Jeffreys in 1918, is a variation of the planetesimal concept: it suggests that a huge tidal wave, raised on the sun by a passing star, was drawn into a long filament and became detached from the principal mass. As the stream of gaseous material condensed, it separated into masses of various sizes, which, by further condensation, took the form of the planets. Serious objections against the encounter theories remain; the angular momentum problem is not fully explained.

Contemporary Theories

Contemporary theories return to a form of the nebular hypothesis to explain the transfer of momentum from the central mass to the outer material. The nebula is seen as a dense nucleus, or protosun, surrounded by a thin shell of gaseous matter extending to the edges of the solar system. According to the theory of the protoplanets proposed by Gerard P. Kuiper, the nebula ceased to rotate uniformly and, under the influence of turbulence and tidal action, broke into whirlpools of gas, called protoplanets, within the rotating mass. In time the protoplanets condensed to form the planets. Although Kuiper's theory allows for the distribution of angular momentum, it does not explain adequately the chemical and physical differences of the planets.

Using a chemical approach, H. C. Urey has given evidence that the terrestrial planets were formed at low temperatures, less than 2,200°F (1,200°C). He proposed that the temperatures were high enough to drive off most of the lighter substances, e.g., hydrogen and helium, but low enough to allow for the condensation of heavier substances, e.g., iron and silica, into solid particles, or planetesimals. Eventually, the planetesimals pulled together into protoplanets, the temperature increased, and the metals formed a molten core. At the distances of the Jovian planets the methane, water, and ammonia were frozen, preventing the earthy materials from condensing into small solids and resulting in the different composition of these planets and their great size and low density.

The discovery of extrasolar planetary systems , beginning with 51 Pegasi in 1995, have given planetary scientists pause. Because it was the only one known, all models of planetary systems were based on the characteristics of the solar system—several small planets close to the star, several large planets at greater distances, and nearly circular planetary orbits. However, all of the extrasolar planets are large, many much larger than Jupiter, the largest of the solar planets; many orbit their star at distances less than that of Mercury, the solar planet closest to the sun; and many have highly elliptical orbits. All of this has caused planetary scientists to revisit the contemporary theories of planetary formation.

Bibliography

See N. Booth, Exploring the Solar System (1996); P. R. Weissman et al., ed., Encyclopedia of the Solar System (1998); J. K. Beatty et al., ed., The New Solar System (4th ed. 1999); B. W. Jones, Discovering the Solar System (1999).

Solar system

Our solar system consists of the Sun and all of its orbiting objects. These objects include the planets with their rings and moons, asteroids, comets, meteors and meteorites, and particles of dust and debris.

The Sun, which keeps these objects in orbit with its gravitational field, alone accounts for about 99.8 percent of the mass of the solar system. Jupiter, the largest planet, represents another 0.1 percent of the mass. Everything else in the solar system together makes up the remaining 0.1 percent.

The average distance between the Sun and Pluto, the farthest planet, is about 3.66 billion miles (5.89 billion kilometers). Incorporating the entire space within the orbit of Pluto, the area encompassed by the solar system is 41.85 billion square miles (108.4 billion square kilometers). Our solar system seems quite insignificant, however, when considered in the context of the more than 100 billion stars in our galaxy, the Milky Way, and the estimated 50 billion galaxies in the universe.

Planets

A planet is defined as a body that orbits a star (in our case the Sun) and produces no light of its own, but reflects the light of its controlling star. At present, scientists know of nine planets in the solar system. They are grouped into three categories: the solid, terrestrial planets; the giant, gaseous (also known as Jovian) planets; and Pluto.

The terrestrial planets, the first group closest to the Sun, consists of Mercury, Venus, Earth, and Mars. The atmospheres of these planets contain (in varying amounts) nitrogen, carbon dioxide, oxygen, water, and argon.

Words to Know

Light-year: Distance light travels in one year in the vacuum of space, roughly 5.9 trillion miles (9.5 trillion kilometers).

Nuclear fusion: Merging of two hydrogen nuclei into one helium nucleus, releasing a tremendous amount of energy in the process.

Oort cloud: Region of space beyond the solar system that theoretically contains about one trillion inactive comets.

Planetesimals: Ancient chunks of matter that originated with the formation of the solar system but never came together to form a planet.

Protoplanet: Earliest form of a planet, plus its moons, formed by the combination of planetesimals.

Solar wind: Electrically charged subatomic particles that flow out from the Sun.

Supernova: Explosion of a massive star at the end of its lifetime, causing it to shine more brightly than the rest of the stars in the galaxy put together.

The Jovian planets, father from the Sun, consist of Jupiter, Saturn, Uranus, and Neptune. The light gases hydrogen and helium make up almost 100 percent of the thick atmospheres of these planets. Another difference between the giant planets and the terrestrial planets is the existence of ring systems. Although the rings around Saturn are the most spectacular and the only ones visible from Earth, Jupiter, Uranus, and Neptune do have rings.

On the basis of distance from the Sun, Pluto might be considered a Jovian planet, but its size places it in the terrestrial group. The major component of its thin atmosphere is probably methane, which exists in a frozen state for much of the planet's inclined orbit around the Sun.

Moons

A moon is any natural satellite (as opposed to a human-made satellite) that orbits a planet. Seven of the planets in the solar system—Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto—have moons, which total 61. Although moons do not orbit the Sun independently, they are still considered members of the solar system.

Asteroids

Asteroids are relatively small chunks of rock that orbit the Sun. Except for their small size, they are similar to planets. For this reason, they are often referred to as minor planets. Scientists believe that asteroids are

ancient pieces of matter that were created with the formation of the solar system but never came together to form a planet. An estimated one million asteroids may exist in the solar system. About 95 percent of all asteroids occupy a band of space between the orbits of Mars and Jupiter. The largest of the asteroids, named Ceres, is 580 miles (940 kilometers) in diameter, while the smallest one measured to date is only 33 feet (10 meters) in diameter.

Comets

Comets are made of dust and rocky material mixed with frozen methane, ammonia, and water. A comet speeds around the Sun on an elongated orbit. It consists of a nucleus, a head, and a gaseous tail. The tail forms when some of the comet melts as it nears the Sun and the melted material is swept back by the solar wind. Scientists believe comets originate on the edge of the solar system in an area called the Oort cloud. This space is occupied by trillions of inactive comets, which remain there until a passing gas cloud or star jolts one into orbit around the Sun.

The origin of the solar system

Over time, there have been various theories put forth as to the origin of the solar system. Most of these have since been disproved and discarded. Today the theory scientists consider most likely to be correct is a modified version of the nebular hypothesis first suggested in 1755 by German philosopher Immanuel Kant and later advanced by French mathematician Pierre-Simon Laplace.

The modern solar nebula hypothesis states that the Sun and planets formed 4.6 billion years ago from the solar nebula—a cloud of interstellar gas and dust. Due to the mutual gravitational attraction of the material in the nebula, and possibly triggered by shock waves from a nearby supernova, the nebula eventually collapsed in on itself.

As the nebula contracted, it spun increasingly rapidly, leading to frequent collisions between dust grains. These grains stuck together to form pebbles, then boulders, and then planetesimals. Solid particles as well as gas continued to stick to these planetesimals (in what's known as the accretion theory), eventually forming protoplanets, or planets in their early stages.

As the nebula continued to condense, the temperature at its core rose to the point where nuclear fusion reactions began, forming the Sun. The protoplanets spinning around the developing Sun formed the planets.

Other solar systems?

Evidence has come to light suggesting that ours may not be the only solar system in the galaxy. In late 1995 and early 1996, three new planets were found, ranging in distance from 35 to 40 light-years from Earth. The first planet, discovered by Swiss astronomers Michel Mayor and Didier Queloz, orbits a star in the constellation Pegasus. The next two planets were discovered by American astronomers Geoffrey Marcy and R. Paul Butler. One is in the constellation Virgo and the other is in Ursa Major. Other planetary discoveries soon followed, and by spring 2001, astronomers had found evidence of 63 known planets outside our solar system.

Of perhaps greater importance to the study of solar systems was the announcement in 1999 that astronomers had discovered the first planetary system outside of our own. They detected three planets circling the star Upsilon Andromedae, some forty-four light-years away. Two of the three planets are at least twice as massive as Jupiter, and astronomers suspect

they are huge spheres of gas without a solid surface. The innermost planet lies extremely close to Upsilon Andromedae—about one-eighth the distance at which Mercury circles the Sun.

The discovery of two more planetary systems in the universe was announced by astronomers in early 2001. Each is different from the other and from our solar system. In one, a star like our Sun is orbited by a massive planet and an even larger object seventeen times the size of Jupiter. According to astronomers, this large object could be a dim, failed star or an astronomical object that simply has not been seen before. In the second system, a small star is orbited by two planets of more normal size. Their orbits around the star, however, puzzle astronomers: the inner planet orbits almost twice as fast as the outer planet. With these discoveries at the beginning of the twenty-first century, astronomers may have to redefine what a normal planetary system is in the universe.

[See also Asteroid; Comet; Cosmology; Earth; Extrasolar planet; Mars; Jupiter; Mercury; Meteors and meteorites; Neptune; Orbit; Pluto; Saturn; Sun; Uranus; Venus ]

Solar System Sun and all the celestial bodies that revolve around it: the nine planets, together with their satellites and ring systems, the thousands of asteroids and comets, meteoroids, and other interplanetary material. The boundaries of the Solar System lie beyond the orbit of Pluto to include the Kuiper Belt and the Oort Cloud of comets. The Solar System came into being nearly 5000 million years ago, probably as the end-product of a contracting cloud of interstellar gas and dust. See also Big Bang

solar system The system that consists of the central Sun (G spectral type star), around which orbit nine planets, about 60 satellites, about 3000 discovered asteroids, and probably 10¹² comets. Most bodies lie close to the plane of the ecliptic. The age of the solar system, 4.56 billion years obtained from meteorites, marks the formation of the system from a rotating cloud of dust and gas (the solar nebula).

so·lar sys·tem • n. Astron. the collection of nine planets and their moons in orbit around the sun, together with smaller bodies in the form of asteroids, meteoroids, and comets.

solar system see age and early evolution of the Earth and Solar System