The inventions of men: Apr 30, 2013

Tuesday, April 30, 2013

Radio Astronomy

HISTORY

Robert W. Wilson
American physicist and radio astronomer Robert W. Wilson won the 1978 Nobel Prize in physics. While attempting to measure the intensity of radiation from a single, specific point in the sky, he discovered cosmic microwave background radiation.
Unsuccessful attempts to detect celestial radio emission were made during the latter part of the 19th century. The American radio engineer Karl G. Jansky, while working at Bell Telephone Laboratories, in 1932, was the first to detect radio noise from the region near the center of the Milky Way, during an experiment to locate distant sources of terrestrial radio interference. The distribution of this galactic radio emission was mapped by the American engineer Grote Reber, using a 9.5-m (31-ft) paraboloid that he built in his backyard in Wheaton, Illinois. In 1943 Reber also discovered the long-sought-after radio emission from the Sun. It was later realized, however, that solar radio emission had been detected a few years earlier, when strong solar bursts had interfered with the operation of British, American, and German radar systems designed to detect aircraft.
As a result of the great improvements made during World War II in radio antennas and sensitive receivers, radio astronomy flourished in the 1950s. Radio scientists adapted their wartime radar techniques to the construction of a variety of radio telescopes in Australia, the United Kingdom, the Netherlands, the United States, and the USSR, and the interest of professional astronomers was soon aroused by a series of remarkable discoveries.
Discrete sources of radio emission were cataloged in increasing numbers, and beginning in the 1950s many radio sources were identified with distant visible galaxies. In 1963 the continuing investigation of very small radio sources led to the discovery of quasi-stellar radio sources, called quasars (see Quasar), which, because of redshift of unprecedented magnitude, could be placed at enormous distances from the Earth. Soon afterward, in 1965, the American radio astronomers Arno Penzias and Robert W. Wilson announced the discovery of a 3 K (-454° F) cosmic background radio emission, which has many implications for theories of the origin and evolution of the universe (see Cosmology). An entirely new type of radio source, the pulsar, was discovered in 1968 and was quickly identified as a rapidly rotating neutron star (see Star).
For many years radio astronomers concentrated on studying relatively long wavelengths near 1 m (about 3.3 ft), for which large antenna structures and sensitive receivers were easy to build. As techniques were developed to build larger and more precise structures, and as sensitive short-wavelength receiving equipment was perfected, the wavelength bands down to 1 mm (about 0.04 in) received increased attention. At the same time, the development of space technology (see Space Exploration) allowed observations to be made at very long wavelengths from above the ionosphere, which is normally opaque to radiation longer than about 20 m (about 66 ft).

III

PRINCIPLES OF RADIO ASTRONOMY

Cosmic radio emission, insofar as is known, comes entirely from natural processes, although from time to time radio telescopes are also used to search (so far unsuccessfully) for possible sources of radio emission from extraterrestrial intelligence (see Exobiology). Several physical mechanisms are recognized that produce the observed radio emission.

Types of Emission

Because of the random motions of electrons, all bodies emit thermal, or heat, radiation characteristic of their temperature. Careful measurements of the intensity and spectrum of emissions are used to calculate the temperature of distant celestial bodies, such as the planets in the Earth’s solar system, as well as of hot clouds of ionized gas located throughout the Galaxy.
Radio astronomy measurements, however, are often concerned with the much more intense nonthermal emission arising from charged particles such as electrons and positrons moving through weak galactic and intergalactic magnetic fields. When the particle energy is so high that its velocity is close to the speed of light, the radio emission from these “ultra-relativistic” particles is referred to as synchrotron radiation, a term borrowed from the high-energy physics laboratory, where this type of radiation was first discovered.
Both the synchrotron (nonthermal) and thermal radio sources radiate over a wide range of wavelengths. By contrast, a third category of matter—excited atoms, ions, and molecules—radiate at discrete wavelengths characteristic of the atom or molecule and the state of excitation. Wide-range radio emission is referred to as continuum emission, and discrete radio emission as line emission.

Radio Telescopes

Arecibo Radio Telescope
The Arecibo Observatory in Puerto Rico contains the largest single stationary radio telescope in the world. Because it remains stationary, the Arecibo telescope uses Earth’s rotation to turn its field of view across the sky. Radio waves bounce off the bowl of the telescope and into the detecting platform suspended above the bowl.
Radio wavelengths are relatively long, extending from about 1 mm (about 0.04 in) to more than 1 km (about 0.6 mi), and radio telescopes must be extremely large in order to focus the incoming signals to produce a sharp radio image. The world’s largest stationary radio telescope, Arecibo Observatory in Puerto Rico, is a bowl-shaped dish 305 m (1000 ft) in diameter. The largest fully steerable parabolic dish-type antennas are 50 to 100 m (about 165 to 330 ft) in diameter, and they have a resolution of about 1 arc minute, equivalent to that of the unaided human eye at optical wavelengths. Incoming radio waves are focused by the parabolic surface onto a small horn antenna that leads to an extremely sensitive radio receiver. These receivers, although similar in principle to the home radio, are able to detect signals as weak as 10-17 W. The critical parts of the receiver are often cooled to temperatures close to absolute zero in order to obtain the best possible performance. For spectral line observations, specialized receivers are used that can be tuned to as many as 1000 frequencies simultaneously.

Radio Telescopes
The Very Large Array is a collection of parabolic dish antennas, located near Socorro, New Mexico. The 27 antennas are attached to a system of Y-shaped tracks; each track is 21 km (13 mi) in length. The individual signals from each telescope are combined into one high-resolution image, making the array the world's largest radio telescope.
In order to obtain higher resolution, arrays of antennas are used as interferometers (see Interferometer) giving resolutions of approximately 1 arc second, equivalent to that of large optical telescopes under ideal viewing conditions. The largest radio telescope of this type is the Very Large Array, or VLA, located on an isolated plain near Socorro, New Mexico. The VLA contains a total of 27 parabolic dishes, each 25 m (82 ft) in diameter, located along three 21-km (13-mi) arms in a Y configuration. Each antenna element contains its own receiver, and the signals from each receiver are sent to a central building where they are combined to form the high-resolution image by a technique that is known as aperture synthesis. Other interferometers may use antennas like huge television antennas. One such installation at Cambridge, England, uses 60 antennas to detect radiation at wavelengths of 2 m (6.6 ft).
Even higher resolutions may be achieved if individual antenna elements are spaced thousands of kilometers apart. With these spacings it becomes impractical to send the signals from each antenna directly to a common point. Instead, separate broadband tape recordings are made at each antenna, and the individual tapes are then shipped to a central processing facility. This technique of very long baseline interferometry (VLBI) involves using atomic clocks at each telescope to synchronize the individual recordings to an accuracy of better than one-millionth of a second. In this way, angular resolutions of one-thousandth of an arc second are achieved, equivalent to the apparent angular dimensions of a basketball at the distance of the moon. In 1984, the U.S. government appropriated funds for the construction of an installation called the very long baseline array (VLBA), a network of 10 radio antennas spread from the U.S.-Canadian border to Puerto Rico and from Hawaii to the U.S. Atlantic coast. The VLBA is expected to provide angular resolutions in the range of 200-millionths of an arc second. Canada and Australia are both planning similar programs.

CLASSES OF RADIO SOURCES

Many discrete radio sources have been discovered and studied in our solar system, in our galaxy, and in the wide extent of the universe beyond our galaxy.

Solar System Radio Astronomy

The sun is the brightest radio source in the sky. Its radio emission is much more intense than would be expected from the thermal emission of its visible surface, which has a temperature near 6000 K (about 10,300° F). This is because most of the radio emission observed at longer radio wavelengths comes from the much hotter, but optically invisible, outer atmosphere, which has temperatures near 1,000,000 K (near 1,800,000° F). In addition to the thermal emission, numerous nonthermal storms and bursts occur, particularly during periods of high sunspot activity when the intensity of radio emission may dramatically increase by a factor of 1 million or more for brief periods of about an hour.
The only other source of natural nonthermal radio emission in the solar system is the planet Jupiter. At wavelengths near 15 m (about 49 ft), Jupiter emits strong bursts of radiation that come from relatively small regions, near the cloud surface, that rotate with the planet. The intensity of these bursts appears to be greatly influenced by the location of the satellite Io. In addition, Jupiter is surrounded by extensive radiation belts that radiate in the microwave band at wavelengths that are shorter than about 1 m (about 3.3 ft).
Thermal radiation has been observed to emanate from the surface or atmosphere of all of the planets except Pluto. These emissions have been used by instruments aboard spacecraft to derive information on planetary meteorological conditions and other phenomena.

Galactic Radio Sources

Radio Telescope Image
The Parkes 64-meter (210-foot) radio telescope in Australia produced this radio map of the Large Magellanic Cloud. This small, irregular galaxy is visible from the southern hemisphere. Regions of ionized hydrogen give off thermal emissions, shown as bright spots on this false-color image.
The Galaxy, or the Milky Way, emits radio waves as a result of synchrotron radiation from cosmic ray electrons moving through the weak galactic magnetic field. The 21-cm line emission from neutral hydrogen is also observed throughout the Galaxy. Small changes in the observed wavelength of the 21-cm line are caused by the motion of the hydrogen clouds toward or away from an observer. These changes are an example of the phenomenon known as the Doppler effect, or redshift. Clouds that are most distant from the center of the Galaxy revolve around the center with the greatest velocity, and observations of the Doppler effect are used to measure the velocity and locate the position of hydrogen clouds. In this way it has been possible to trace the shapes of the Milky Way’s spiral arms, which are not readily observed at optical wavelengths.
In addition to the diffuse background radiation, numerous discrete sources of radio emission exist in the Galaxy. These discrete sources include the following: supernova remnants, radio stars, emission nebulas, molecular clouds, and pulsars.
Supernova remnants are the clouds of debris remaining from stars that have exploded (see Supernova). Relativistic electrons produced in a supernova explosion are captured by the magnetic field surrounding the site of the explosion. As these electrons spiral around the magnetic field lines, they continue to radiate for thousands of years. In some cases the star itself continues to be a source of radio emission and is referred to as a radio star. Another important class of radio star comprises the binary (double) star systems that emit radio waves when mass is transferred from one component to the other. Radio stars are often X-ray sources as well.
Thermal radio emission is observed from clouds of ionized hydrogen (termed H II regions) located along the spiral arms of the Galaxy. When free electrons recombine with ions of hydrogen or other light elements, radio energy is released that can be observed as recombination lines in the radio portion of the spectrum.
Spectral lines also result from vibrational and rotational transitions of such interstellar molecules as water vapor (H2O), ammonia (NH3), formaldehyde (H2CO), and carbon monoxide (CO). More than 50 interstellar molecules are now known, including many complex and organic molecules. In some interstellar clouds, the radio molecular lines are unusually intense due to the maser (microwave amplification by the stimulated emission of radiation) effect (see Laser; Maser).
The intensity of most cosmic radio sources is steady, or only varies slowly with time. The pulsars, however, emit short periodic bursts or pulses of radiation about once per second. Although first discovered because of their intense pulsed radio emission, some were later found to emit optical and X-ray pulses as well. Pulsars are thought to form when stars like the sun collapse under their own gravity to dimensions of about 10 km (about 6 mi). The density then becomes extremely great, and electrons are stripped from their atoms, leaving a so-called neutron star.

Radio Galaxies

Most galaxies probably emit radio waves and do so at energies comparable to that of our own galaxy—about 1032 W. In the cases of the so-called radio galaxies, however, the radio emission is up to 100 million times stronger. Most of this energy originates not in the galaxies themselves but in clouds of superheated, ionized gases, or plasma, located hundreds of thousands or even millions of light-years away from the parent galaxy. These giant radio clouds may be 100 times the size of the galaxy itself and are among the largest known objects in the universe.
A great deal of energy is required to generate the powerful radio emissions from radio galaxies, and it may amount to a significant fraction of the total energy that would result from the nuclear burning of a whole galaxy. The origin of this energy and the manner in which it is converted to radio emissions have been major problems of astrophysics since the discovery of radio galaxies more than two decades ago.
Recent detailed pictures of radio galaxies, obtained with high resolution radio telescopes such as the VLA, often show a prominent jet of material connecting a bright, compact radio source at the galactic nucleus to the more extended radio lobes (clouds). It is widely speculated that these jets or beams transport energy away from the galactic nucleus to the radio-emitting plasma and that the source of energy lies in a massive object, possibly a black hole located at the galactic center. Frequently, a compact radio source is found at the center of radio galaxies. In one unusual radio galaxy observed in the mid-1980s, two bright clusters of stars near its center are emitting jets apparently braided together.

Quasars

Quasars (see Quasar) appear to radiate with the luminosity of hundreds of galaxies, but each quasar is smaller than a typical galaxy by a factor of nearly a million. Quasars have very large redshifts, and they are therefore believed to lie at great distances from the Milky Way. Because quasars appear to be so powerful, and because their radiation often varies rapidly, it was once thought they might be relatively nearby weak objects rather than distant powerful ones. However, evidence has accumulated supporting the cosmological interpretation of the redshifts. Radio galaxies, quasars, and bright objects called BL Lacertae objects are probably closely related phenomena.
Like the radio galaxies, some quasars are also surrounded by extended lobes of powerful radio emissions, but most of the radio emission from quasars usually comes from a bright core only a few light-years or less in diameter and coincident with the optically visible quasar.
When observed with very high resolution radio interferometers, this radio core is often found to consist of two or more smaller regions, which may appear to be moving away from each other with velocities considerably greater than the speed of light. Although these remarkably high velocities may seem at first to violate Albert Einstein’s special theory of relativity (see Relativity), they in fact can be explained as a result of motion just under the speed of light, which is directed almost toward the observer. Because the moving radio source is nearly catching up with the emitted radiation, the observed time interval between successive positions of relativistic jets of material appears shortened, and the velocity appears to be increased by a large factor over the true velocity. This phenomenon is termed apparent superluminosity.

COSMOLOGY

Because radio galaxies and quasars are such powerful radio sources, they can be detected from a great distance. Because of the long time it takes for signals to reach the Earth from distant radio sources, radio astronomers are able to see the universe as it appeared more than 10 billion years ago, or far back in time toward the origin of the universe—the so-called big bang. Unfortunately, determining the distance to a radio source is not possible from radio measurements alone, so that distinguishing between a powerful distant source and a relatively weak nearby one is impossible. The distance may be determined only if that source is optically identified with a galaxy or quasar that has a measurable redshift. Nevertheless, from studies of the statistical distribution of large numbers of radio sources, it appears that when the universe was only a few billion years old, the number of intense radio sources was much greater and their dimensions smaller.

Planetary Science

Copernican System
In the 16th century, Nicolaus Copernicus developed the heliocentric model of the solar system, in which the sun is stationary at the center, and the earth moves around it. This view of the solar system challenged Ptolemy’s geocentric model, which had been the accepted theory since the 2nd century. In Ptolemy’s model, the earth is stationary in the center of the solar system, and the other planets and the sun move in complex orbits around it. The Copernican model gradually gained acceptance, for it provided a simpler explanation of the planets' motions.
Planetary Science, study of the forces and influences that determine the composition, structure, and evolution of planets and planetary systems, including moons, dwarf planets, asteroids, and comets. Planetary scientists also study how planetary systems form around other stars. In particular, planetary science includes a study of the properties of the Earth compared to the properties of other worlds, which helps explain some of the properties of Earth through the example of other planets.
The origins of modern planetary science can be traced to the Copernican revolution of the 16th and 17th centuries, which led to overturning the old idea that Earth is unique and central in creation. Polish astronomer Nicolaus Copernicus, Italian astronomer and philosopher Galileo, and others showed that the Sun is the central body in Earth’s solar system and that Earth is only one planet among several that orbit the Sun. Continued advances in astronomy have revealed that the Sun is an average star in a universe filled with billions of stars. Recent observations indicate that a significant fraction of the stars in the universe could be encircled by planetary systems—some of which may be similar to Earth’s solar system, and many that are probably quite different. See Extrasolar Planets.
Scientists have debated what kind of object should be called a planet, but the problem can be seen as more one of terminology than of science. In 2006 the International Astronomical Union (IAU) voted on a formal definition of planet for bodies in our solar system. The term “classical planet” is used for a body that orbits the Sun, that has settled into a rounded shape from effects of its own gravitation, and that is massive enough to have cleared the neighborhood of its orbit of primordial asteroid-size bodies called planetesimals as it formed in the solar nebula. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all fit this definition. The term dwarf planet is used for an object that, like a classical planet, orbits the Sun and has a rounded shape from its own gravitation but, unlike a classical planet, is not massive enough to have cleared planetesimals from the neighborhood of its orbit. Dwarf planets orbit through regions such as the asteroid belt (a zone filled with small, rocky planetesimals) and the Kuiper Belt (a zone filled with small, icy planetesimals). Currently, Ceres in the asteroid belt and Pluto and Eris in the region of the Kuiper Belt are recognized as dwarf planets. For now, the IAU’s definition of a planet does not officially apply to extrasolar planets.

Solar System Tour
Modern planetary science draws from many fields of science, including astronomy, physics, chemistry, atmospheric science, and geology. To some degree, the study of planets also requires a biological perspective, for it is now clear that the evolution of the atmosphere and surface environment of at least one planet—Earth—has been radically influenced by the presence of life. Many scientists believe that life may not be limited to Earth and may, in fact, be fairly common throughout the universe (see Exobiology). Planetary science is therefore also concerned with life on other planets.

SOURCES OF INFORMATION USED BY PLANETARY SCIENTISTS

Planetary science evolved from the study of the solar system—Earth’s planetary system. Astronomers have observed the other planets of the solar system with telescopes and through photographic images transmitted to Earth by interplanetary spacecraft (see Space Exploration). Planetary scientists have characterized the chemical signatures of the Moon and Mars by studying the chemical compositions of rocks brought back from the Moon by astronauts and robotic spacecraft, and soils that were analyzed on the surface of Mars by robotic spacecraft. This has allowed geologists to identify the origin of a small number of meteorites—fragments of interplanetary debris that landed on Earth—as rocks that came from the Moon or from Mars. Terrestrial, or Earth-based, geologists, atmospheric scientists, oceanographers, and other scientists who study Earth have accumulated a wealth of data and have constructed a detailed picture of the composition and structure of Earth. The recent telescopic discovery of planetary systems orbiting other stars promises to expand the information base of other planetary systems. Several schemes for a large-scale systematic search for planetary systems are currently under consideration. See also Extrasolar Planets.

Telescopic Observations

Hubble Space Telescope
The Hubble Space Telescope (HST), free of the distorting effects of the earth’s atmosphere, has an unprecedented view of the heavens. Placed in orbit in 1990, it has become a vital tool for studying distant galaxies, as well as the planets of the earth’s solar system. When comet Shoemaker-Levy 9 bombarded Jupiter in 1994, the HST provided some of the best images of the event.
Astronomers have used telescopes for centuries to make exact measurements of the positions of planets and their satellites over time. Such information was critical to acceptance of the law of universal gravitation proposed by English physicist Sir Isaac Newton. Modern telescopes allow astronomers to make enlarged images of other planets and to collect the light emitted from them so that it can be analyzed by spectroscopy. The enlarged images enable scientists to study their larger surface features, and spectroscopic analysis, which separates light into its component colors, or spectra, gives information about the chemical composition of the light source. In the case of light reflected from a planet, spectroscopy reveals the compositions of its atmosphere and surface materials. Astronomers have exploited recent advances in spectroscopy and in charge-coupled devices—instruments that measure the intensity of weak light sources—to detect evidence of planets orbiting stars other than the Sun. The new techniques make use of the Doppler effect—subtle shifts in the spectra of moving light sources—to detect wobbles in the motions of stars that are orbited by smaller bodies such as planets.

The Very Large Array
Radio telescopes detect electromagnetic radiation from space in wavelengths ranging from about 1 mm (0.04 in) to more than 1 km (0.6 mi). Since radio telescopes are only sensitive to electromagnetic radiation with a relatively long wavelength, signals from a group of telescopes pointing at the same object can be combined, dramatically improving resolution. For example, the Very Large Array (VLA) in Socorro, New Mexico, has 27 dishes whose individual signals can be combined to form a single high-resolution image.
In the 1960s radio telescopes were built to gather electromagnetic radiation in the radio portion of the electromagnetic spectrum (see Radio Astronomy). Radio telescopes have provided valuable information about other stars and about the magnetic fields of other planets in our solar system, especially Jupiter’s. In 1990 the National Aeronautics and Space Administration (NASA) launched into orbit the Hubble Space Telescope (HST). The HST is an optical telescope that orbits high above the distorting effects of Earth’s atmosphere. For this reason, it can see objects that are ten times smaller than the smallest object that can be seen by any Earth-based telescope. Space telescopes designed to search for extrasolar planets include ESA’s COROT and NASA’s Kepler. Kepler can detect planets the size of Earth.

Interplanetary Space Missions

Magellan Spacecraft
In 1989 the Magellan probe became the first interplanetary spacecraft to be launched from the space shuttle. Magellan is shown here in the cargo bay of the space shuttle Atlantis in preparation for mission launch. The dish-shaped top of the spacecraft is a high-gain antenna, which Magellan used to send information about Venus back to Earth.
Interplanetary space missions allow close-up observation of other planets. On some missions, robotic landing craft actually landed on the surface of a planet to measure seismic activity and to chemically analyze soil, rock, and atmospheric samples (see Seismology). On other missions, spacecraft orbiting distant planets and their moons have taken photographs, measured magnetic fields, taken samples of atmospheres for chemical analysis, sampled solar winds and other forms of radiation in space, and spectroscopically analyzed light transmitted through planetary atmospheres.

Rock Samples

Asteroid 243 Ida
Asteroids are chunks of rock and metal too small to be considered planets. They orbit the Sun and are situated primarily between the orbits of Mars and Jupiter. NASA’s Galileo spacecraft photographed the asteroid 243 Ida in 1993. The space probe detected a tiny moon, Dactyl, orbiting Ida.
Planetary geologists analyze rock samples from Earth and other worlds to determine the chemical compositions of planets in the solar system, which gives important clues regarding the origins and evolution of planetary bodies. Astronauts from the Apollo Moon missions brought back rock samples from six different sites on the moon, and robot landing craft sent to the Moon by the former Soviet Union brought back soil samples from three other sites. Geologists have also collected thousands of meteorites, which are fragments of interplanetary debris that have landed on Earth. Since the lunar and Martian landing missions of the 1970s, planetary scientists have determined by chemical analysis that about a dozen of the known meteorites originated on the Moon and about a dozen more came from Mars. These planetary fragments appear to have been blasted from the surfaces of these worlds by the impact of large asteroids that originated in the asteroid belt between Mars and Jupiter. The remaining meteorites appear to be asteroid fragments that came directly to the Earth from the asteroid belt after being shattered and knocked out of orbit by collision with other asteroids.

III

ORIGINS AND COMPOSITIONS OF PLANETS

Life of a Star
A star begins life as a large, relatively cool mass of gas in a nebula, such as the Orion Nebula (left). As gravity causes the gas to contract, the nebula’s temperature rises, eventually becoming hot enough to trigger nuclear reactions in its atoms and form a star. A main sequence star (middle) shines because of the massive, fairly steady output of energy from the fusion of hydrogen nuclei to form helium. The main sequence phase of a medium-sized star is believed to last as long as 10 billion years. The Sun is just over halfway through this phase. Stars eventually use up their energy supply, ending their lives as white dwarfs, which are extremely small, dense globes, or in the case of larger stars, as spectacular explosions called supernovas. A supernova is shown within the Large Magellanic Cloud at the bottom right of the rightmost photo.
Astronomers believe that planetary systems are formed of elemental materials that were created in the interiors of giant stars. Some of this material comes from giant stars that shed material into space as they age. Most of the matter to form planets, however, comes from stars that explode as supernovas and spread debris enriched with the heavier chemical elements into space. According to the currently accepted views, the most likely first stage in the evolution of a planetary system is a later supernova near the clouds of interstellar dust and gas. A shock wave from the supernova explosion may compress a nearby cloud to a sufficiently high density so that the weak attractive force of gravitation is made strong enough to cause the cloud to collapse in on itself. The gravitational attraction of particles for each other and collisions between the particles of the cloud cause the cloud to form a large central body known as a protostar, encircled by a thin disk of dust, gas, and debris known as a planetary disk. In the case of Earth’s solar system, the protostar eventually became the Sun and the planetary disk broke up into the planets of the solar system. By studying our own planetary system, planetary scientists gain insight into the general mechanisms that determine the structure of planetary systems.

Formation of Planets

Birth of a Planetary System
This infrared image of the star Beta Pictoris reveals a disk of gas and dust surrounding the star. Astronomers believe that solar systems begin as disks of gas and dust, which gradually coalesce into planets and other solar system bodies.
As an interstellar cloud begins to contract into a star, any random swirling motion in the cloud becomes more orderly and translates into a general rotation of the entire cloud. As the cloud continues to contract, its speed of rotation increases, just as figure skaters spin faster as they pull in their arms. The physical principle for this is known as the conservation of angular momentum, and it means that the total angular momentum of the cloud must remain constant. Because angular momentum depends on the distance of the mass from the center of rotation and the speed of rotation, as the distance decreases, the speed must increase to compensate and keep the momentum constant. In an interstellar cloud, this means that as distant parts of the cloud move closer to the center of rotation, the speed of the cloud’s rotation must increase.
A nonrotating cloud of interstellar gas and dust would contract into a sphere at the center of mass of the cloud, but the vast majority of objects in space rotate. Frictional drag within the cloud and other dynamic interactions cause the outer parts of the rotating cloud to flatten into a disk that surrounds the central spherical body. Planetary systems, such as our own solar system, form from material in these so-called planetary disks. Observations suggest that planetary disks surround as many as 60 percent of the new stars in young star clusters.
A planetary disk heats up as it forms. Once a star forms in the center, the rest of the disk cools by radiation. As it cools, solid mineral grains and ice crystals condense, much as snowflakes condense in cooling air. As the grains collide, they stick together to form larger grains that sweep up other grains ever more quickly, a process called accretion. The disk around a newly forming star quickly becomes a sort of factory in which dust grains and ice crystals aggregate and grow into asteroid-sized bodies called planetesimals (small planets). The planetesimals gather more material through gravitational attraction and collision until eventually only a few planet-sized bodies are left.
In Earth’s solar system, the planet-forming process apparently happened relatively quickly. The planets reached their present sizes and arrangement probably within 10 million to 50 million years after the Sun’s ignition. In this view, the giant planets formed when their cores reached 10 to 15 times the mass of Earth, sufficient to attract hydrogen-rich gas from the solar nebula. In an alternative view, another more direct process may form gas giant planets such as Jupiter. A region of dust and gas becomes gravitationally unstable and quickly collapses into a large body that retains much of the gas that might otherwise be blown off by solar radiation.

Compositions of the Solar System’s Planets

The compositions of the planets of Earth’s solar system follow directly from the materials that condensed at different distances from the protostar that became the Sun. Near the Sun, condensed mineral grains were made of rocky material, and they formed four rocky planets: Mercury, Venus, Earth, and Mars. These planets are collectively known as the terrestrial planets. The name is derived from terra, the Latin name for Earth, and it refers to the inner planets’ similarity to Earth.
Between Mars and the next most distant planet, Jupiter, is a belt of rocky and carbon-rich planetesimals that never coalesced into a planet. This is called the asteroid belt, and the bodies composing it are known as asteroids. Gravitational disturbances caused by the massive, nearby planet Jupiter probably kept the asteroids from forming a planet. The asteroid belt vividly shows the transition in composition from the terrestrial planets to the outer, more carbon-rich planets. The asteroids nearest Mars, closest to the Sun, are composed primarily of the rocks, minerals, and metals of the terrestrial planets, but asteroids beyond the middle of the belt, closer to Jupiter, are colored black by sooty, carbon-containing material. All interplanetary bodies beyond this point show this dark coloration.

Uranus and Its Rings
The planet Uranus rotates on an axis that is tilted nearly horizontal. Other planets in the solar system have axes that are more vertical. This infrared image taken by the Hubble Space Telescope shows Uranus's rings orbiting in the plane of the planet's tipped equator. The colors are not real and are used to bring out details such as clouds in the atmosphere and the shape of the rings. The white disks are moons.
Beyond the asteroid belt, icy grains were added to rocky and carbon-rich materials. Out of this material formed the four major gaseous planets—Jupiter, Saturn, Neptune, and Uranus— collectively known as the Jovian planets because of their similarity to the planet Jupiter. The Jovian planets, also called gas giants, are all huge gaseous spheres of hydrogen and helium that surround relatively small cores of metallic and rocky material. The atmosphere of Jupiter is almost three-quarters hydrogen and one-quarter helium by weight, with traces of carbon dioxide (CO2) and the more common hydrogen-rich compounds—for example water (H2O), methane (CH4), and ammonia (NH3). This is very similar to the composition of the sun. To planetary scientists, this indicates that the outer disk was cold enough that the ices of carbon dioxide, methane, and ammonia could form. These ice compounds, which are far more common in the solar system and in the universe than the silicates and metals of the terrestrial planets, condensed into crystals that stuck together and rapidly formed very large bodies. When Jupiter and Saturn reached masses of about 15 Earth masses, their gravitational fields simply swept up the remaining dust and gases still floating free in the solar system, including the remaining hydrogen and helium. Uranus and Neptune are sometimes classified as ice giants, apart from the “gas giants” Jupiter and Saturn. Unlike Jupiter and Saturn, the two outermost planets are mainly made of water in a hot, compressed, slushy state that scientists refer to as “ice.”
The differences in composition of the planets show up directly in their mean densities, which can be determined by studying the motions of their own satellites and by spacecraft sent to them from Earth. The inner planets have densities characteristic of metal-bearing rock (about 3 to 5 g/cu cm), while the Jovian planets and their satellites have lower densities, characteristic of ices or ice/soil mixtures (about 1 to 3 g/cu cm). Saturn’s mean density is lower than water. If there were a large enough pool of water to place it in, Saturn would float.
Beyond the orbit of Neptune is the dwarf planet Pluto, once counted as the ninth planet in the solar system. Pluto almost certainly formed in the region of icy bodies called the Kuiper Belt, named for the astronomer who predicted its existence. Not much is known about Pluto because it is smaller than our Moon and far from Earth. Measurements of the motion of one of its moons, Charon, indicate that Pluto’s density is higher than the density of the Jovian planets and suggests that it is composed primarily of rock and a mixture of ices. Pluto also orbits the Sun in a plane that is about 17° off from the plane in which all the other planets orbit. Other icy bodies that appear similar to Pluto have been discovered in its vicinity. These objects are sometimes called Kuiper Belt Objects (KBOs). Many astronomers now classify Pluto as merely one of the largest members of the Kuiper Belt. If astronomers had known about KBOs, Pluto almost certainly would not have been called a planet after it was discovered in the 1930s. The International Astronomical Union reclassified Pluto as a dwarf planet in 2006 on the grounds that it had not cleared the neighborhood of its orbit of other bodies. Slightly larger and about 27 percent more massive than Pluto is the dwarf planet Eris, a KBO that has a more distant and steeply inclined orbit than Pluto.

STRUCTURES AND FEATURES OF THE TERRESTRIAL PLANETS

Surface of Mars
The Viking orbiters took more than 50,000 pictures of the surface of Mars. This animation was created by making a mosaic of Viking orbiter images and enhancing the natural color to make features more apparent. The path of the animation is along Valles Marineris, a system of Martian canyons over 4000 km (over 2400 mi) long and over 7 km (over 4 mi) deep in some places.
NASA
The terrestrial planets and the larger satellites are in a constant state of change and evolution. Worlds that have atmospheres show evidence of wind erosion and wind-driven transport of material, and still other worlds exhibit volcanism and other signs of motion and activity beneath their surfaces. Motion of material deep within a planet often creates a strong magnetic field. Even the geologically inactive worlds occasionally experience collisions with interplanetary debris that leave large impact craters as evidence.
It is impossible to “see” the interior of a planet, so planetary scientists must use indirect means of determining the processes that are at work on a planet. The presence or absence of craters on a planet’s surface is one of the most important clues available to planetary scientists. As a rule, surfaces showing sparse numbers of craters are half a billion years old or more, those with a moderate concentration of craters are a billion years old or older, and surfaces crowded with craters can be nearly as old as the solar system itself. Surfaces devoid of craters only exist on worlds that have active volcanoes, geologically active atmospheres, or other internal mechanisms for renewing the surface at intervals of a half billion years or less.

Interior Structures

The early planetary bodies were heated by various mechanisms such as radioactivity trapped in the minerals, energy delivered by impacting meteorites, and compression as the planets increased in mass. The planets, their satellites, and even some of the larger asteroids grew hot enough to melt their interiors. In the liquid, or molten state, dense materials such as metals flowed to the centers of the planets—theircores—and lower-density materials such as minerals and gases floated to the outer layers. Thus, the terrestrial planets have iron-nickel cores at their centers, surrounded by thick, dense, mineral-rich rock layers known as mantles, topped with surface crusts of low-density rock. The process of separating molten materials into distinct layers, or strata, by density is known as density stratification.
Scientists find direct proof of density stratification in meteorites, which tend to be either rocky or metallic. The rocky fragments appear to be derived from the outer shells of planetesimals that have stratified, while the metallic fragments, composed chiefly of iron and nickel, appear to be derived from planetesimal cores, where dense metals sank to the center. A few meteorites have both types of material in distinct layers, clearly showing the results of stratification.
Seismic studies of Earth and the Moon also reveal evidence of stratification. Using instruments left by the Apollo astronauts, geologists studying the Moon have found that seismic waves caused by slight tremors known as moonquakes are reflected at various depths, indicating that the Moon’s interior is stratified. Earth-based geologists studying the intense seismic waves following an earthquake have deduced that the lightweight basaltic and granitic rocks of Earth’s surface crust are underlain by a mantle of dense mineral-rich rock and a core of even denser metallic material. The seismic data, as well as Earth’s magnetic field indicate that the metallic core at the center of Earth is partially molten.

Volcanism and Tectonic Activity

Neptune’s Moon Triton
Triton is largest of Neptune’s moons and the seventh known moon from the planet. Its surface reveals relatively few craters, but is crisscrossed by ridges and valleys. Triton’s surface is probably shaped by the freezing and thawing of nitrogen and methane ice.
NASA.
The existence of a volcano on a planet is the most obvious sign that the planet has a layer of molten material beneath its surface. Earth and some of the satellites of the Jovian planets have active volcanoes and thus certainly have molten interiors. Venus has huge volcanic mountains and extensive crater-free, lava-covered plains, indicating that its volcanoes have been active within the past 500 million to 800 million years, but it is not clear whether Venus is still volcanically active. Mars presents a transitional case. One hemisphere of Mars is mostly ancient, heavily cratered terrain that shows little evidence of volcanic activity, yet the other hemisphere is dominated by huge volcanoes that rise more than 20 km (more than 12 mi) above sparsely cratered plains. In contrast, Mercury’s surface is heavily cratered, indicating that it has mostly been geologically inactive for billions of years.
In worlds with molten cores, temperature differences from the hot interior regions of the fluid cores to the cooler outer regions drive massive glacierlike motions known as convection currents. These currents create stresses in the surface rocks that eventually lead to massive fractures. On Earth, these fracturing events are experienced as earthquakes. Worlds that have surface fractures caused by internal motions are said to be tectonically, or structurally, active. On Earth, tectonic motions have broken the surface rock layer into large plates that drift over Earth’s surface in a process called plate tectonics. Along the borders between neighboring plates, earthquakes are common, and volcanoes pour molten material out onto Earth’s surface. The surface of Venus shows intense folding that also indicates tectonic stresses, but the stresses on Venus were apparently not strong enough to create individual plates, as on Earth.
Planetary scientists generally agree that the primary current source of heat for tectonic and volcanic activity on the terrestrial planets is heat released by radioactive minerals in the rocky material. Because Earth is the largest of the terrestrial planets, its mass has insulated it best against heat loss through radiation. Consequently, Earth remains the most tectonically active of the terrestrial planets. In contrast, Mercury is the smallest terrestrial planet and cooled the most rapidly—it has been tectonically inactive for most of its existence. Mars is the second smallest terrestrial planet and has been only partially active within the last billion years. Venus is the second largest terrestrial planet and has clearly been tectonically active within the past 500 million to 800 million years.

Geysers on Enceladus
Plumes of icy material, left, extend from the south polar region of Enceladus, a moon of Saturn, in an image taken February 17, 2005, by the Cassini spacecraft. A color-coded image, right, shows a much more extended plume, reaching as far as 418 km (260 mi) into space. Planetary scientists concluded that the plumes represent liquid-water geysers, and they theorized that this active volcanism was caused by tidal forces that created friction and heat within the interior of Enceladus. The detection of carbon molecules on the moon’s surface, along with the presence of heat and liquid water, means that Enceladus might be able to support life.
Reasoning by analogy from the example of the terrestrial planets, most mid-20th century planetary scientists assumed that the icy satellites orbiting the Jovian planets were too small and too cold to be tectonically active. However, just prior to the flight of the United States spacecraft Voyager 1 past Jupiter, a group of planetary geologists predicted that Jupiter’s innermost large satellite, Io, might show volcanic activity due to heat released by forces called tidal forces. Voyager 1’s photographs of erupting volcanoes on Io were one of planetary science’s greatest predictive successes. The Voyager probes later showed volcanic activity on Neptune’s satellite Triton and revealed evidence of intense tectonic fracturing on Jupiter’s satellite Ganymede, Saturn’s satellite Enceladus, and Uranus’s satellite Miranda. Jupiter’s satellite Europa was revealed to have a white ice surface with few impact craters, suggesting that it has been repeatedly recoated by volcanic eruptions of hot water that smoothed the surface before freezing.
Scientists predicted volcanic activity on Io based on the fact that Io is subject to intense tidal forces—flexing forces that arise from fluctuations in the gravitational force that holds the satellite in orbit. When a satellite orbits a planet, the gravitational force of the planet stretches the satellite slightly. If the satellite’s orbit is somewhat elliptical, or perhaps occasionally altered by the gravitational pull of neighboring satellites, then the gravitational force acting on the satellite changes and the satellite flexes. Repeated flexing of a satellite due to tidal forces causes frictional heat to build up within it. The satellites closest to the larger planets experience the strongest tidal forces and most intense flexing, and only these show tectonic activity. The more distant satellites do not undergo the intense flexing that generates heat capable of melting rock.
In 2006 scientists reported that the Cassini spacecraft in orbit around Saturn had detected geysers on Enceladus. Planetary scientists theorized that tidal forces like those on Io heated pockets of liquid water just below the surface. The heated water erupted as geysers through cracks in the icy surface.

Atmospheres

Divisions of the Atmosphere
Without our atmosphere, there would be no life on Earth. A relatively thin envelope, the atmosphere consists of layers of gases that support life and provide protection from harmful radiation.
The atmospheres of the terrestrial planets are remarkably diverse. For example, the atmospheres of Venus and Mars are dominated by carbon dioxide. In contrast, nitrogen, oxygen, and water vapor, which are rare in the atmospheres of the other planets, dominate the atmosphere of Earth. There is also great diversity in surface pressure. For example, the atmospheric pressure at the surface of Venus is 90 times greater than that at the surface of Earth, yet the atmospheric pressure at the surface of Mars is 150 times less than the atmospheric pressure at the surface of Earth. Mercury, the smallest and innermost terrestrial planet, has almost no atmosphere, and most of its thin, gaseous envelope is made of solar wind particles that are temporarily trapped by its weak gravitational field.
Planetary scientists have determined that the atmospheres of the terrestrial planets share a common history. The volatile, or gaseous, materials composing these atmospheres were probably derived partly from the original material of the planets and partly from planetesimals that arrived after the planets had already formed. The atmospheres of Venus, Earth, and Mars evolved as volcanoes emitted gases (probably primarily water vapor and carbon dioxide) from the interior. Atmospheric evolution, however, produced a different result on each planet.
Venus is 30 percent closer to the Sun than Earth, which makes the sunlight falling on it about twice as strong. The temperature on Venus has therefore always been higher than on Earth, with the result that any water released by volcanoes or delivered by planetesimal impacts remained primarily in the vapor form, rather than the liquid form as it is on Earth or the solid form as it is on Mars. When water vapor is exposed to ultraviolet radiation, it breaks down into oxygen and hydrogen gas in a process known as ultraviolet dissociation. Hydrogen gas is too light to be held by the gravitational field of any of the terrestrial planets, and so the hydrogen in Venus’s water slowly escaped to space. The oxygen left behind combined with other atmospheric chemicals and with surface rocks so that Venus’s atmosphere now contains very little water or oxygen, yet it retains all or nearly all of its original carbon dioxide in gaseous form. Thus, Venus has a dense atmosphere composed of about 98 percent carbon dioxide. Despite this dense atmosphere, winds at the surface appear to be too slow to influence the landforms of Venus to the same degree that winds on Earth and Mars influence their landforms.
As Earth cooled, its water condensed to form liquid oceans. Most of the carbon dioxide in Earth’s atmosphere dissolved into the oceans and combined with calcium and magnesium—two of the most abundant elements in Earth’s crust—to form minerals such as calcite and magnesite, materials of common rocks such as limestone. Plants and other living organisms also converted the gaseous carbon dioxide to solid matter, much of which is now buried beneath Earth’s surface as petroleum deposits. In this way, Earth did not produce the massive carbon dioxide atmosphere of Venus, but rather buried its carbon dioxide in its rocky crust.
With the water vapor and carbon dioxide removed from Earth's atmosphere, the residual nitrogen became the dominant component of the air. The atmosphere has evolved, however. As described below, oxygen was added by plants. Also, the geologically recent exploitation of fossil fuels by humans has caused the concentration of carbon dioxide in Earth’s atmosphere to increase significantly over the last century. Carbon dioxide in the atmosphere absorbs infrared radiation from the Sun in a process known as the greenhouse effect. Atmospheric scientists are concerned that the increased carbon dioxide in the atmosphere is causing a general warming of the climate of Earth’s surface that could have negative consequences for agricultural production and human economies. (Venus’ still more massive carbon dioxide atmosphere has produced a much stronger greenhouse effect that heats that planet to around 900° F.)
For most of Earth’s history, a layer of ozone high in its atmosphere has absorbed the Sun’s ultraviolet light and protected its water vapor from ultraviolet dissociation. Equally important for living organisms, the ozone layer protects the fragile chemical bonds of genetic material from damage caused by ultraviolet light. Scientists in the 1970s and 1980s realized that certain human-made chemicals are damaging the ozone layer, and international agreements have been made to phase out production of these chemicals. The ozone is formed from Earth’s oxygen.
Earth’s oxygen and its ozone layer are direct results of the emergence of living organisms on Earth. Ozone is derived from oxygen, which is released by plants as they convert water and carbon dioxide into the carbohydrate molecules that form leaves, stems, roots, and other structural parts by photosynthesis. The oxygen in Earth’s atmosphere is so reactive that it would soon combine with other materials on Earth and disappear from Earth’s atmosphere if there were no mechanisms to renew it. However, the plant life of Earth’s land surface and oceans keeps the concentration of oxygen in Earth’s atmosphere at almost 20 percent.
Earth’s atmosphere plays a major role in shaping its surface. Erosion and transport of soils and rock by wind creates distinctive landforms and patterns, but water is the most important sculptor of Earth’s landscape. Water is continuously evaporated from the oceans, transported by winds in the form of clouds, and deposited over land, which it carves into coastlines and river valleys. In addition to its direct geological activity, water is essential to many of the living organisms that also affect Earth’s surface by anchoring soil and decreasing erosion is some places, while breaking down rock and increasing erosion in other places.
The gravitational field at the surface of Mars is less than half that of Earth or Venus. Therefore, although the volcanic gases on Mars in its early days were probably in the same proportions as on the early Earth and early Venus, Mars’s weak gravitational field could not hold these gases like Earth and Venus could. The gases on Mars thus escaped more rapidly than on Earth or Venus. Because Mars is farther from the Sun than Earth, it receives less solar radiation. Mars therefore is colder than Earth. At present, nearly all of Mars’s water is trapped as ice below the surface and in the form of water-bearing minerals called hydrated minerals. Some of Mars’s carbon dioxide is also frozen in its polar ice caps. The temperature on Mars is too cold to allow much of its water ice to thaw. However, the temperature in Mars’s summer hemisphere is warm enough to thaw the frozen carbon dioxide in the polar ice cap to its gaseous state. The gas then flows to the winter hemisphere’s pole, where it freezes again.
The bulk movement of carbon dioxide gas from pole to pole creates surface winds that drive dust storms covering the entire Martian surface for months at a time. Photographs from the United States spacecraft Mariner 9 in 1972 first showed that Mars has the largest sand dune fields in the solar system. The strong winds blowing dust across the Martian surface created these dunes and other landforms characteristic of extreme wind erosion. The Mariner photographs also showed the surface of Mars laced with what appear to be dry riverbeds, indicating that liquid water once flowed on its surface. Because Mars is currently too cold and its atmosphere is too thin for water to exist in the liquid state on its surface, the cause of climate variations that allowed rivers to flow is a mystery. The time scale for this atmospheric change is not known.
Titan’s dense atmosphere may be similar to Earth’s primitive atmosphere. Nitrogen makes up about 94 percent of Titan’s atmosphere, followed by methane at about 5 percent, with small amounts of other gases. Methane may fall as a constant drizzle. Radar and photographic images from the European-American Cassini-Huygens spacecraft indicate that methane gas also forms clouds and precipitates as rain, shaping the landscape of Titan in patterns similar to water erosion on Earth. The Cassini-Huygens probe also appears to have detected fluid lakes, possibly of liquid methane, at Titan’s north pole.
Both Neptune’s moon Triton and the dwarf planet Pluto have thin atmospheres containing nitrogen and methane. The distant dwarf planet Eris likely also has an atmosphere that is currently frozen on its surface. When Eris makes its nearest approach to the Sun, its atmosphere may turn to gas.

Magnetic Fields

Earth’s Magnetic Field
Magnetic fields surrounding planets are caused by the motion of electrically charged particles inside the planets. This motion occurs in rotating planets with molten, conductive interiors, where currents of charged particles flowing inside the planets can generate large magnetic fields. Of the terrestrial planets, only Earth has both the fluid core and high rate of rotation required to create a strong magnetic field. Mercury has a weak magnetic field about 1 percent as strong as Earth’s. Studies of its slow rotation indicate that Mercury’s core is also partly molten. Venus has a fluid core, but it rotates too slowly—nearly 273 days per rotation—to generate a large field. The Moon and Mars appear to lack the required fluid core. However, Mars apparently had a magnetic field until an estimated 3.8 billion years ago. Earth’s magnetic field is one of the surest indicators that it has a fluid interior.

Aurora Borealis, or Northern Lights
Luminous displays called auroras often occur in phase with sunspot cycles. Auroras typically occur above the earth’s polar regions when charged particles from the sun interact with gases in the earth’s atmosphere. Excited gas molecules give off visible radiation, or light, often in the red and green part of the spectrum. This display of multiple auroral bands was photographed in Fairbanks, Alaska.
The region in space in which a planet’s magnetic field interacts with charged particles from the Sun and with cosmic rays is called a magnetosphere. When electrically charged particles from space encounter a planet’s magnetic field, they either pass through the field, distorting and dragging it in their wake or, if the field is strong, they are trapped by it and deflected toward the north and south magnetic poles. The Sun emits a steady stream of charged particles known as the solar wind, which increases in intensity during solar flare activity. The interaction of the magnetically trapped particles with Earth’s upper atmosphere also causes displays of light in the sky known as auroras, or the northern and southern lights.

STRUCTURES AND FEATURES OF THE JOVIAN PLANETS

Great Red Spot
Jupiter’s atmosphere is composed mostly of hydrogen and helium with lesser amounts of minor gases. White clouds of frozen ammonia crystals and other colored clouds, including the Great Red Spot, swirl around in atmospheric currents as the planet rotates. The Great Red Spot was photographed by Voyager 1 in 1979.
The Jovian planets have dense cores composed of terrestrial-like material, but these cores are completely engulfed in dense envelopes of hydrogen, helium, and trace gases. For example, Jupiter’s terrestrial core appears to be 15 times more massive than the entire Earth, and its hydrogen/helium envelope is some 200 times more massive than its terrestrial core. The intense gravitational field of a Jovian planet compresses its gaseous envelope so densely that the inner layers of these giants behave more like fluids than gases. The temperatures within the fluid-gas envelopes of Jupiter and Saturn are high enough at deep levels to ionize the hydrogen—that is, to cause the negatively-charged electrons to be stripped away from the positively-charged nuclei of the hydrogen atoms. The temperature differentials within the fluid-gas envelopes drive massive convection currents. Currents of the ionized hydrogen affect the planets’ magnetic fields, while shallower currents produce incredibly rich cloud formations and other atmospheric phenomena—for example, Jupiter’s Great Red Spot—that can be seen through telescopes on Earth.

Io, One of Jupiter's Moons
The Voyager 1 spacecraft launched by the United States National Aeronautics and Space Administration (NASA) photographed both hemispheres of Io, the innermost moon of Jupiter, in 1979. The hemisphere shown at left always faces Jupiter because Io’s period of revolution around the planet is equal to its rotation around its own axis. The moon’s colors depict its many volcanoes and the large lava flows and sulphur-dioxide snow resulting from Io’s tremendous volcanic activity. During the three months between the photos of Io taken by Voyager 1 and Voyager 2, the surface of the moon changed dramatically—some volcanos stopped erupting while previously dormant volcanos became active.
Jupiter, Saturn, Neptune, and Uranus all have the requisite fluid interiors and high rates of rotation to create large magnetic fields. These fields are similar to the magnetic field of Earth, although the magnetic fields of the Jovian planets are much stronger and span a much greater space than Earth’s field. Through cameras carried into space on spacecraft missions, astronomers have observed spectacular auroras near Jupiter’s magnetic poles. These Jovian auroras resemble Earth’s auroras, but because Jupiter’s magnetic field is much stronger than Earth’s, its auroras are much larger and more brilliant.

STRUCTURE OF ICY MOONS AND DWARF PLANETS

Many of the same processes and forces that shaped the inner terrestrial planets into solid bodies also acted to form out of ice and rock the large moons around the outer planets and the larger objects found in the Kuiper Belt. Most of these bodies have settled into a rounded shape from effects of their own gravitation and have differentiated interiors with a rocky core surrounded by a mantle of icy material topped by an outer icy crust. Jupiter’s moons Europa, Ganymede, and Callisto, and Saturn’s largest moon Titan are made of water ice around a rocky core. Some of the largest moons may have subsurface oceans somewhere below their crusts. The dwarf planets Pluto and Eris, and probably Neptune’s moon Triton, all formed in the Kuiper Belt out of material found in comets and also likely have rocky cores surrounded by ice. The heating that has melted ice and produced eruptions on several moons is thought to be due to complex tidal effects from gravitation as the moons orbit their planet and interact with other moons.

VII

DIRECTIONS FOR FUTURE RESEARCH

Study of Earth’s planetary system has revealed much about the origin, evolution, and essential processes of planetary systems in general. In turn, knowledge of the general principles governing planetary systems has shed new light on Earth. The immediate practical applications of planetary science concern the preservation of Earth’s environment in a state that supports life. The long-term applications of planetary science focus on the evolution of the physical structures of planetary systems and on the search for planets surrounding stars other than the Sun. Scientific agencies in several countries are currently considering proposals for several projects designed to send more robotic probes to other planets, satellites, asteroids, and comets, and to detect planetary systems orbiting stars other than the Sun, particularly planets like Earth.

Characteristics of the Planets

Characteristic

Mercury

Venus

Earth

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto*

Equatorial radius (Earth radii†)	0.3825	0.9488	1	0.5325	11.21	9.449	4.007	3.883	0.1874
Equatorial inclination (degrees)	0.01	2.64	23.5	25.2	3.13	26.7	82.2	28.3	57.4
Mass (Earth masses‡)	0.0553	0.8150	1	0.1074	317.8	95.16	14.54	17.15	0.0023
Average density (g/cm3)	5.4	5.2	5.5	3.9	1.3	0.69	1.3	1.6	1.8
Rotational period (days)	58.6	-243	1	1.03	0.414	0.444	-0.718	0.671	-6.4
Orbital period (years)	0.2408	0.6152	1	1.881	11.86	29.46	84.01	164.8	247.9
Average distance from the Sun (AUs)	0.3871	0.7233	1	1.524	5.203	9.59	19.10	30	39.30
Orbital eccentricity (ratio)	0.206	0.00674	0.0167	0.0935	0.0489	0.0576	0.0497	0.00995	0.248
Orbital inclination (degrees)	7	3.39	0.0003	1.85	1.30	2.49	0.772	1.77	17.2
Moons (number)	0	0	1	2	63	60	27	13	3
*Reclassified as a dwarf planet by the International Astronomical Union in 2006 †Planet's radius expressed as a multiple of Earth's radius (6,378 km) ‡Planet's mass expressed as a multiple of Earth's mass (5.974×1024 kg)

Orbit astronomy and physics

Orbit (astronomy and physics), path or trajectory of a body through space. A force of attraction or repulsion from a second body usually causes the path to be curved. A familiar type of orbit occurs when one body revolves around a second, strongly attracting body. In the solar system the force of gravity causes the moon to orbit about the earth and the planets to orbit about the sun, whereas in an atom electrical forces cause electrons to orbit about the nucleus. In astronomy, the orbits resulting from gravitational forces, which are discussed in this article, are the subject of the scientific field of celestial mechanics.

An orbit has the shape of a conic section—a circle, ellipse, parabola, or hyperbola—with the central body at one focus of the curve. When a satellite traces out an orbit about the center of the earth, its most distant point is called the apogee and its closest point the perigee. The perigee or apogee height of the satellite above the earth's surface is often given, instead of the perigee or apogee distance from the earth's center. The ending -gee refers to orbits about the earth; perihelion and aphelion refer to orbits about the sun; the ending -astron is used for orbits about a star; and the ending -apsis is used when the central body is not specified. The so-called line of apsides is a straight line connecting the periapsis and the apoapsis.

LAWS OF MOTION

Early in the 17th century, the German astronomer and natural philosopher Johannes Kepler deduced three laws that first described the motions of the planets about the sun: (1) The orbit of a planet around the sun is an ellipse. (2) A straight line from the planet to the center of the sun sweeps out equal areas in equal time intervals as it goes around the orbit; the planet moves faster when closer to the sun and slower when distant. (3) The square of the period (in years) for one revolution about the sun equals the cube of the mean distance from the sun's center, measured in astronomical units.

The physical causes of Kepler's three laws were later explained by the English mathematician and physicist Isaac Newton as consequences of Newton's laws of motion (see Mechanics) and of the inverse square law of gravity. Kepler's second law, in fact, expresses the conservation of angular momentum. Moreover, Kepler's third law, in generalized form, can be stated as follows: The square of the period (in years) times the total mass (measured in solar masses) equals the cube of the mean distance (in astronomical units). This last law permits the masses of the planets to be calculated by measuring the size and period of satellite orbits.

III

ORBITAL ELEMENTS

Elements of Orbits

Orbits of objects going around the sun are discussed in terms of their orientation with respect to three different planes. These are the plane of the orbit in question, the plane of the earth’s orbit (also known as the plane of the ecliptic), and the plane of the celestial equator. The elliptical orbit has center C and focus S. Six elements may be used to describe an orbit: size (periapsis distance SP), elongation (eccentricity e, which is the ratio CS/CP), longitude of the ascending node (angle Ω), argument of periapsis (angle ω), inclination (angle i), and the time when the orbiting body is at the periapsis.

Six orbital elements describe an orbit. The first two elements are size and elongation. The size of the orbit is given by the periapsis distance (SP) and the elongation of the orbit is given by the eccentricity ( e). For the ellipse in the accompanying figure, the eccentricity is the ratio CS/CP, where S is the focus and C the center of the ellipse. For elliptical orbits, e is greater than 0, but less than 1; for circular orbits, e is exactly 0; and for parabolic orbits, e is exactly 1. A body in a hyperbolic orbit—that is, when e is greater than 1—makes a single close passage to a central body and escapes along a so-called open orbit, never to return.

The next three orbital elements are concerned with the orbit's orientation. For this discussion, however, several parameters need to be defined: The reference plane for objects orbiting around the sun is the plane of the earth's orbit, also known as the plane of the ecliptic; the equinox (g) is the northbound intersection of the earth's orbit and the plane of the celestial equator; and the ascending node (N) is the northbound intersection of the orbit in question and the reference plane (see Coordinate System).

The three orbital elements that describe an orbit's orientation are the inclination (i), the longitude of the ascending node (Ω), and the argument of the periapsis (ω). The inclination is the angle between the reference plane and the orbit's plane. The longitude of the ascending node is the angle in the reference plane between the equinox and the ascending node. The argument of the periapsis measures the angular displacement in the plane of the orbit between the ascending node and the line that passes through the center of the orbit (C) and the periapsis (P). Finally, the sixth orbital element is the time when the celestial body in question is at the periapsis.

An orbit can also be described in terms of its semimajor axis (AC, CP, or a). This axis is half the long axis (AP) of the ellipse, or half the distance between the points of periapsis (P) and the apoapsis (A). The semimajor axis is longer than the periapsis distance (SP) and shorter than the apoapsis distance (AS), by an amount (CS) that is equal to the product of the semimajor axis and the eccentricity: CS = e(AC) = e(CP) = ea

PERTURBATIONS

An orbit is perturbed when the forces are more complex than those between two spherical bodies. (Kepler's laws are exact only for unperturbed orbits.) The attraction between planets causes their elliptical orbits to change with time. The sun, for example, perturbs the lunar orbit by several thousand kilometers. Atmospheric drag causes the orbit of an earth satellite to shrink, and the oblate shape of the earth causes the direction of its node and perigee to change. The theory of relativity developed by German-born American physicist Albert Einstein explains an observed perturbation in the perihelion of the planet Mercury.

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