For thousands of years, stargazers have known that the planets of the solar system are different from stars because each of the six visible planets was seen to have its unique motion through the sky, whereas the stars seemed to be motionless. The word “planet,” meaning wanderer, derives from the appearance of the individual motions of the planets through the background of seemingly fixed stars. The reason for this appearance is simply that the planets are relatively close to us and they orbit the sun; they are therefore seen to change position in the sky. But the stars do not orbit the sun and are so far away that their movement, although rapid, can be detected only through many years of observation. The same phenomenon could be seen if one were to observe a bird flying past and then to look at an airplane in the sky. The plane would undoubtedly be the faster of the two, although it would actually take much longer to cross one’s field of vision because of its distance.
The other visible difference between planets and stars, known to man for centuries, is that stars twinkle, whereas planets do not. This is because planets differ greatly from stars in composition. Stars are vast globes of incandescent gas, creating their own light through nuclear energy production, whereas planets are smaller, darker bodies that reflect the light from their parent stars. If the sun were suddenly extinguished, the planets in our solar system would cease to shine.
The planets within the solar system also differ from each other: tiny, barren Mercury, with its heavily cratered surface, for example, contrasts sharply with our own water-covered planet Earth. In turn, neither of these planets bears any resemblance to the giant gaseous worlds of Jupiter and Saturn. Each member of the sun’s family has its own peculiar characteristics that distinguish it from all the other planetary bodies in our cosmic neighborhood.
The orbits of the planets
The ideas of Ptolemy in the second century involving a geocentric planetary system, with the planets moving around the earth in circular orbits, were gradually superseded by the Copernican heliocentric theory in the sixteenth century. This theory stated that the planets moved in a circular orbit around the sun. It was later improved by Johannes Kepler in the early seventeenth century, who determined that the planetary orbits are elliptical and not circular. It is now known that these orbits all lie in roughly the same plane. The inclinations of planetary orbits are measured with respect to the plane of the earth’s orbit around the sun, known as the ecliptic. The orbit of Uranus almost matches that of Earth, with an inclination of less than one degree. The inclination of Mercury’s orbit, however, is 7°, and Pluto has the most oblique planetary orbit in the solar system, tilted at 17°2′.
The rocky planets
The four innermost members of our solar system—Mercury, Venus, Earth, and Mars— are all solid, rocky bodies, a common feature that has defined them as the terrestrial planets. Each bears evidence of surface erosion processes, such as vulcanism or meteorite bombardment, which have played a significant part in shaping the surfaces we see today.
In 1974 and 1975, Mercury was scrutinized by the Mariner 10 space probe. A densely cratered surface was revealed, with mountainous regions, valleys, and mare-type areas. The surface of this tiny world is very similar to that of our moon. Venus, however, is completely different from Mercury. It was not until recently that astronomers were able to pierce the dense, yellow-white clouds that cover Venus to photograph the surface below. Radar mapping, carried out in 1978 by the American Pioneer Venus mission, demonstrated a surface that consists mainly of upland regions with two dominant highland areas—Ishtar Terra and Aphrodite Terra. The U.S. spacecraft Magellan, which began orbiting Venus in 1990, has also sent back highly detailed images of the planet’s surface.
In contrast to Mercury and Venus, most of Earth’s surface is covered by water, although the visible terrain displays many features similar to those on the other rocky planets. Volcanoes, meteorite craters, mountain ranges, and valley systems are all in evidence on Earth as they are on its rocky companions. But the prevailing weather conditions are so destructive on our planet that most of these features are relatively short-lived on the geological timescale. On planetary bodies that are almost totally devoid of atmosphere, such as Mercury, virtually the only process to change the surface appearance is bombardment by meteorites, although vulcanism may play a part.
The outermost of the terrestrial planets is Mars, which, apart from Earth, is probably the most geologically active of the four. The effects of wind and water erosion are very much in evidence on the planet’s surface. Mars is also considered by some astronomers to be still volcanically active.
The atmospheres of the four rocky planets vary greatly. Mercury and Mars both retain little atmosphere because of their low surface gravity. The atmosphere of Mercury contains traces of hydrogen and helium, and that of Mars is composed mainly of gaseous carbon dioxide with traces of oxygen and water vapor. Earth and Venus, however, with much higher gravity, can retain denser atmospheres and both have cloud layers. Earth’s atmosphere includes argon, oxygen, carbon dioxide, nitrogen, other gases, and water vapor. Venus’s atmosphere has mostly carbon dioxide, but there are also clouds of sulfuric acid. Argon and nitrogen are also present.
The gaseous giants
Beyond Mars lie the four gaseous planets—Jupiter, Saturn, Uranus, and Neptune. Each of these four worlds dwarfs the inner terrestrial members of the sun’s family.
Through a telescope, Jupiter appears as a flattened globe, striated with parallel bands of clouds that mark the outermost layers of the planet’s deep atmosphere. Saturn’s appearance is essentially the same with its cloud layers less well pronounced than those of Jupiter, but what Saturn lacks in the way of surface detail it makes up for with its magnificent ring system. Saturn’s atmosphere consists mainly of hydrogen and helium with small amounts of methane, ammonia, and phosphine. Like Jupiter, its main body is composed of liquid hydrogen and helium. Jupiter, Uranus, and Neptune also have ring systems, but that of Saturn is by far the brightest and most beautiful of the four. Earth-based telescopes reveal three major, distinct rings—Ring A, Ring B, and the fainter Ring C, with a gap between the A and B rings known as the Cassini Division. The information gathered on the Voyager 1 mission showed that these rings comprise many narrower ringlets, giving the impression of a gigantic record.
Both Uranus and Neptune are considerably smaller than Jupiter and Saturn—their masses are only six per cent that of Jupiter. The two smaller planets have similar atmospheres with hydrogen, helium, and methane being the most predominant gases. Acetylene has also been found on Neptune. Jupiter and Saturn are known to have solid rocky cores and magnetic fields; the Voyager 2 probe found evidence, in January 1986, of a magnetic field around Uranus. And in August 1989, Voyager 2s sensing instruments detected a magnetic field on Neptune that is about as strong as Earth’s.
Both Jupiter and Saturn have large families of satellites with well over 15 smaller bodies orbiting each planet. Uranus has 15 moons and Neptune has eight.
The outermost planet
Since its discovery by Clyde Tombaugh in 1930, the planet Pluto has remained very much of a mystery to astronomers. It was discovered as a result of its gravitational perturbations on Uranus and Neptune and cannot be seen with the naked eye, although estimates give its diameter as being approximately 1,430 miles (2,300 kilometers). Pluto is about 700 times fainter than Neptune at opposition and has the most elliptical orbit of all the planets, spending about one-eighth of its orbit inside that of Neptune; at its closest approach to the sun, it comes within 2,749,600,000 miles (4,425,100,000 kilometers). The planet is not known to have an atmosphere, although frozen methane has been detected on its surface, because its mean surface temperature is well below —369° F. ( — 223° C), a temperature in which most matter exists in solid form. Pluto is probably not dense enough to retain any atmosphere.
In 1978, Pluto was discovered to have a satellite that has a diameter of about one-third that of its parent planet. Charon, as the moon was named, is the largest satellite in our solar system relative to the size of its parent planet, and Pluto and Charon can therefore be said to be a twin planetary system.
The key quantities of a planet are its mass, radius, and rotation period. Using Kepler’s Third Law, a planet’s mass can be calculated from the observed distances and orbital periods of any satellites it may have. Alternatively, the mass can be determined from the planet’s gravitational effect on the trajectories of unmanned probes.
The radius of a planet can be arrived at with trigonometry, using the values for its apparent angular size and its distance from the earth. The radius enables a planet’s volume and density to be estimated, which give an indication of the internal composition of the planet. Once the mass and radius have been calculated, a wealth of other data can be computed. By equating Newton’s Second Law of Motion with the inverse-square law of gravitation, the acceleration due to gravity can be determined at any point from the planet.From a knowledge of a planet’s gravity, its escape velocity can be calculated (that is, the velocity an object requires to effectively escape the planet’s gravitational field). The escape velocity, combined with data on the temperature of gases within the atmosphere of the planet, enables estimates of the composition of the atmosphere to be made. Its rotation rate is another major characteristic of a planet. The centrifugal force on the atmosphere as the planet spins and its mass cause the planet to bulge at its equator, at a right angle to the rotation axis. The extent of this bulging can be used to estimate the relative proportions of materials of different density within the planet.