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The Planets

The nine planets of the solar system may be divided into two groups: the inner, or terrestrial, planets, and the outer, or Jovian, planets. This division is based not only on distance from the Sun, but also on the physical properties of the planets. The Inner Planets. The inner planets are all comparable in size, density, and other characteristics to the Earth and so are generally referred to as the terrestrial, or Earth-like, planets. Included are Mercury, Venus, Earth, and Mars. The inner planets are all comparable in size, density, and other characteristics to the Earth and so are generally referred to as the terrestrial, or Earth-like, planets. Included are Mercury, Venus, Earth, and Mars.

The Earth is the largest of the terrestrial planets. By far the most massive constituents of the Earth are the iron core and the rocky mantle and crust. The water in the oceans and the gases in the air form only a thin veneer of volatile materials surrounding the rock of the planet proper. The Sun provides the heat and light that make the Earth habitable for life as we know it. The oceans and atmosphere of the Earth absorb and redistribute the heat in a complex fashion. Various types of geological evidence show that the Earth has passed through ice ages in the past. Various processes were probably involved in their cause, including changes in the Earth's motions, but the exact mechanisms are not yet certain. The early years of the Earth were apparently rather violent, as no geological record is preserved of the first half-billion years of its existence.

The Earth-Moon system is often referred to as a "double planet" system, because the Moon is more nearly comparable in size to the Earth than the other satellites are to their primaries (except for Pluto and its moon). The Earth's Moon is 81 times less in mass than the Earth but only 4 times less in mass than the planet Mercury. It is one of a group of the six largest satellites in the solar system that have approximately comparable mass, and the only such large one in the inner solar system. Compared to the mass of its primary, the Earth, the Moon is abnormally massive. The return of samples from several lunar sites during the Apollo program, and the establishment of stations to measure seismic activity and other physical quantities at these sites, has provided more knowledge about the Moon than currently exists for any other body in the solar system except the Earth. If the Moon has a central iron core, it is unexpectedly small, compared to that of the Earth, and of surprisingly little mass. The bulk of the Moon is mantle and crust that has had an extensive history of melting and chemical differentiation. The Moon contains no atmosphere, and its surface is heavily cratered. Its topmost soil is a very fine-grained substance with little chips of rock sprinkled throughout. This is called the lunar regolith. The Moon is heavily depleted in the more volatile elements and compounds as compared to the Earth.

The next inner planet toward the Sun is Venus, long considered a mystery planet because it is shrouded in clouds that hide the details of its underlying surface. Venus is nearly as large and as massive as the Earth, contains relatively little water, and has nothing resembling the oceans of the Earth. Instead, carbon dioxide in an amount comparable to that in the carbonate rocks of the Earth fills the Venusian atmosphere, producing a pressure at the surface about 100 times higher than that at the surface of the Earth and a temperature far too high to support life of any kind as we know it. Venus has a slow retrograde rotation, so that it rotates in a direction opposite to that of most of the other objects in the solar system.

The next planet outward from the Earth away from the Sun is Mars, which is only about one-tenth of the mass of the Earth. Its tenuous atmosphere is composed principally of carbon dioxide, with a pressure at the surface more than 100 times smaller (0.7 percent) than that at the surface of the Earth. The surface of Mars can be considered to be roughly divided into two hemispheres, one a surface of ancient, heavily cratered terrain and the other a geologically younger terrain having a much lower density of cratering. Mars has long been suspected as a possible abode for some form of life, at least in the geological past. Although experiments performed by the Viking spacecraft landers produced no evidence for life, the controversial claim was made in 1996 that certain meteorites found on Earth and thought to be of Martian origin showed possible traces of bacterialike fossils. It is hoped that further probes sent to the planet may yield more information on this issue.

The planet nearest the Sun is Mercury, with a mass half that of Mars and with only a trace atmosphere consisting of such elements as helium, sodium, and hydrogen. Its surface is heavily cratered. Mercury has an interesting resonance with its orbital motion, presenting one face and then the other during its closest approaches to the Sun.

The Outer Planets. The terrestrial planets just described have in common a rocky composition whose major constituents have high boiling points. It is believed that the entire solar system was formed from the gravitational contraction of a large cloud of gas and dust composed mainly of hydrogen and helium and only a small percentage of heavier elements. The Sun's composition is believed to be essentially the same as that of the original nebula. The inner planets lost most of their lighter, volatile elements early as a result of their proximity to the hot Sun, whereas the more distant outer planets were able to retain their light gases. The result is that the outer planets became far more massive and were able to hold very extensive atmospheres of light gases such as hydrogen, as well as light, icy substances such as water (H2O), ammonia (NH3), and methane (CH4).

The most massive planet in the solar system, with about one-thousandth the mass of the Sun and more than 300 times the mass of the Earth, is Jupiter. Composed primarily of hydrogen and helium, Jupiter may have an interior composed of ice (and other frozen volatiles) and rocks, or both, exceeding several times one Earth mass of rocky material and three Earth masses of the ices. The total amount of material heavier than hydrogen and helium is probably in the range of 10-20 Earth masses. Jupiter rotates rapidly on its axis, so that its figure is significantly flattened toward its equatorial plane, and the gases in its surface show a banded structure along lines of latitude. Jupiter radiates into space about twice as much energy as it absorbs from the Sun, with the additional heat emerging from the interior of the planet. Spacecraft have also revealed that Jupiter is ringed.

The next planet outward from Jupiter is the strikingly ringed Saturn, another gas giant also thought to be composed predominantly of hydrogen and helium. Its mass is slightly less than a third that of Jupiter, but it also appears to have something approaching 20 Earth masses of heavier materials in the form, presumably, of icy or rocky materials. Saturn also rotates rapidly, is highly flattened toward its equatorial plane, and exhibits a banded structure along latitude lines.

Beyond Saturn are Uranus and Neptune, two planets of similar size. Uranus has a mass about 15 times and Neptune a mass about 17 times that of the Earth. Hydrogen and helium predominate in the atmospheres of both planets. The planetary interiors lie hidden beneath thick atmospheres, but data from Voyager 2 suggest that Uranus has a superheated water ocean, up to 10,000 km (6,000 mi) deep, surrounding an Earth-size core of molten rock materials. Neptune has an active atmosphere and, apparently, some form of internal energy source. The rotation period of Uranus is a little longer than 17 hours and that of Neptune a little longer than 16 hours. Uranus is unique among the planets in being tilted on its rotation axis by about 98 degrees with respect to the plane of the ecliptic, so that its rotation is retrograde. Uranus and Neptune both have ring systems.

Pluto is a planet whose characteristics were largely unknown until 1978. Pluto's diameter is 2,284 km (1,416 mi). The density of the planet is about the same as that of water, so Pluto may be composed of an ice-rock mixture. Pluto has a rather elliptical orbit that at times takes the planet closer to the Sun than Neptune ever reaches. Beginning in 1979, for example, Pluto lay within Neptune's orbit, and it continued to do so until it recrossed the orbit in 1999. This would ordinarily be a rather unstable state of affairs, but perturbations of the Pluto orbit caused by Neptune occur in such a way that a collision between the two planets cannot happen. The irregularities in Neptune's orbit that led to Pluto's discovery seemed too large to be caused solely by the tiny planet, which is why a few scientists have hypothesized a tenth planet (Planet X) as the cause. As noted, however, later changes in estimates of Neptune's mass may just as likely account for the irregularities. The rating of Pluto itself as a full-fledged planet has been challenged at times, since some astronomers feel that it could just as well be classified as the largest of various more distant objects, mentioned below, that occasionally cross the orbit of Neptune.

The terrestrial planets just described have in common a rocky composition whose major constituents have high boiling points. It is believed that the entire solar system was formed from the gravitational contraction of a large cloud of gas and dust composed mainly of hydrogen and helium and only a small percentage of heavier elements. The Sun's composition is believed to be essentially the same as that of the original nebula. The inner planets lost most of their lighter, volatile elements early as a result of their proximity to the hot Sun, whereas the more distant outer planets were able to retain their light gases. The result is that the outer planets became far more massive and were able to hold very extensive atmospheres of light gases such as hydrogen, as well as light, icy substances such as water (H2O), ammonia (NH3), and methane (CH4).

The Outer Planets. The terrestrial planets just described have in common a rocky composition whose major constituents have high boiling points. It is believed that the entire solar system was formed from the gravitational contraction of a large cloud of gas and dust composed mainly of hydrogen and helium and only a small percentage of heavier elements. The Sun's composition is believed to be essentially the same as that of the original nebula. The inner planets lost most of their lighter, volatile elements early as a result of their proximity to the hot Sun, whereas the more distant outer planets were able to retain their light gases. The result is that the outer planets became far more massive and were able to hold very extensive atmospheres of light gases such as hydrogen, as well as light, icy substances such as water (H2O), ammonia (NH3), and methane (CH4).

The most massive planet in the solar system, with about one-thousandth the mass of the Sun and more than 300 times the mass of the Earth, is Jupiter. Composed primarily of hydrogen and helium, Jupiter may have an interior composed of ice (and other frozen volatiles) and rocks, or both, exceeding several times one Earth mass of rocky material and three Earth masses of the ices. The total amount of material heavier than hydrogen and helium is probably in the range of 10-20 Earth masses. Jupiter rotates rapidly on its axis, so that its figure is significantly flattened toward its equatorial plane, and the gases in its surface show a banded structure along lines of latitude. Jupiter radiates into space about twice as much energy as it absorbs from the Sun, with the additional heat emerging from the interior of the planet. Spacecraft have also revealed that Jupiter is ringed.

The next planet outward from Jupiter is the strikingly ringed Saturn, another gas giant also thought to be composed predominantly of hydrogen and helium. Its mass is slightly less than a third that of Jupiter, but it also appears to have something approaching 20 Earth masses of heavier materials in the form, presumably, of icy or rocky materials. Saturn also rotates rapidly, is highly flattened toward its equatorial plane, and exhibits a banded structure along latitude lines.

Beyond Saturn are Uranus and Neptune, two planets of similar size. Uranus has a mass about 15 times and Neptune a mass about 17 times that of the Earth. Hydrogen and helium predominate in the atmospheres of both planets. The planetary interiors lie hidden beneath thick atmospheres, but data from Voyager 2 suggest that Uranus has a superheated water ocean, up to 10,000 km (6,000 mi) deep, surrounding an Earth-size core of molten rock materials. Neptune has an active atmosphere and, apparently, some form of internal energy source. The rotation period of Uranus is a little longer than 17 hours and that of Neptune a little longer than 16 hours. Uranus is unique among the planets in being tilted on its rotation axis by about 98 degrees with respect to the plane of the ecliptic, so that its rotation is retrograde. Uranus and Neptune both have ring systems.

Pluto is a planet whose characteristics were largely unknown until 1978. Pluto's diameter is 2,284 km (1,416 mi). The density of the planet is about the same as that of water, so Pluto may be composed of an ice-rock mixture. Pluto has a rather elliptical orbit that at times takes the planet closer to the Sun than Neptune ever reaches. Beginning in 1979, for example, Pluto lay within Neptune's orbit, and it continued to do so until it recrossed the orbit in 1999. This would ordinarily be a rather unstable state of affairs, but perturbations of the Pluto orbit caused by Neptune occur in such a way that a collision between the two planets cannot happen. The irregularities in Neptune's orbit that led to Pluto's discovery seemed too large to be caused solely by the tiny planet, which is why a few scientists have hypothesized a tenth planet (Planet X) as the cause. As noted, however, later changes in estimates of Neptune's mass may just as likely account for the irregularities. The rating of Pluto itself as a full-fledged planet has been challenged at times, since some astronomers feel that it could just as well be classified as the largest of various more distant objects, mentioned below, that occasionally cross the orbit of Neptune.

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