Solar system

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Photo by: Orlando Florin Rosu

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.

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

A schematic of the present-day solar system, showing the position of the planets' orbits around the Sun, comparative sizes (to each other, not the Sun), the direction in which they rotate, and the tilt of their axes. (Reproduced by permission of

The Gale Group

.)

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

An artist's conception of the formation of the solar system. The terrestrial planets received the same combination of gases present in the solar nebula (gas cloud), but once formed, each planet's atmosphere evolved as a result of its relative gravity and distance from the Sun. (Reproduced by permission of

Photo Researchers, Inc.

)

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 ]