Relativity

Relativity, theory, developed in the early 20th century, which originally attempted to account for certain anomalies in the concept of relative motion, but which in its ramifications has developed into one of the most important basic concepts in physical science (see Physics). The theory of relativity, developed primarily by German American physicist Albert Einstein, is the basis for later demonstration by physicists of the essential unity of matter and energy, of space and time, and of the forces of gravity and acceleration (see Acceleration; Energy; Gravitation).

II

CLASSICAL PHYSICS

Physical laws generally accepted by scientists before the development of the theory of relativity, now called classical laws, were based on the principles of mechanics enunciated late in the 17th century by the English mathematician and physicist Isaac Newton. Newtonian mechanics and relativistic mechanics differ in fundamental assumptions and mathematical development, but in most cases do not differ appreciably in net results; the behavior of a billiard ball when struck by another billiard ball, for example, may be predicted by mathematical calculations based on either type of mechanics and produce approximately identical results. Inasmuch as the classical mathematics is enormously simpler than the relativistic, the former is the preferred basis for such a calculation. In cases of high speeds, however, assuming that one of the billiard balls was moving at a speed approaching that of light, the two theories would predict entirely different types of behavior, and scientists today are quite certain that the relativistic predictions would be verified and the classical predictions would be proved incorrect.

In general, the difference between two predictions on the behavior of any moving object involves a factor discovered by the Dutch physicist Hendrik Antoon Lorentz, and the Irish physicist George Francis FitzGerald late in the 19th century. This factor is generally represented by the Greek letter β (beta) and is determined by the velocity of the object in accordance with the following equation:

in which v is the velocity of the object and c is the velocity of light (see Light). The beta factor does not differ essentially from unity for any velocity that is ordinarily encountered; the highest velocity encountered in ordinary ballistics, for example, is about 1.6 km/sec (about 1 mi/sec), the highest velocity obtainable by a rocket propelled by ordinary chemicals is a few times that, and the velocity of the earth as it moves around the sun is about 29 km/sec (about 18 mi/sec); at the last-named speed, the value of beta differs from unity by only five billionths. Thus, for ordinary terrestrial phenomena, the relativistic corrections are of little importance. When velocities are very large, however, as is sometimes the case in astronomical phenomena, relativistic corrections become significant. Similarly, relativity is important in calculating very large distances or very large aggregations of matter. As the quantum theory applies to the very small, so the relativity theory applies to the very large.

Until 1887 no flaw had appeared in the rapidly developing body of classical physics. In that year, the Michelson-Morley experiment, named after the American physicist Albert Michelson and the American chemist Edward Williams Morley, was performed. It was an attempt to determine the rate of the motion of the earth through the ether, a hypothetical substance that was thought to transmit electromagnetic radiation, including light, and was assumed to permeate all space. If the sun is at absolute rest in space, then the earth must have a constant velocity of 29 km/sec (18 mi/sec), caused by its revolution about the sun; if the sun and the entire solar system are moving through space, however, the constantly changing direction of the earth's orbital velocity will cause this value of the earth's motion to be added to the velocity of the sun at certain times of the year and subtracted from it at others. The result of the experiment was entirely unexpected and inexplicable; the apparent velocity of the earth through this hypothetical ether was zero at all times of the year.

What the Michelson-Morley experiment actually measured was the velocity of light through space in two different directions. If a ray of light is moving through space at 300,000 km/sec (186,000 mi/sec), and an observer is moving in the same direction at 29 km/sec (18 mi/sec), then the light should move past the observer at the rate of 299,971 km/sec (185,982 mi/sec); if the observer is moving in the opposite direction, the light should move past the observer at 300,029 km/sec (186,018 mi/sec). It was this difference that the Michelson-Morley experiment failed to detect. This failure could not be explained on the hypothesis that the passage of light is not affected by the motion of the earth, because such an effect had been observed in the phenomenon of the aberration of light; see Interference; Interferometer; Wave Motion.

In the 1890s FitzGerald and Lorentz advanced the hypothesis that when any object moves through space, its length in the direction of its motion is altered by the factor beta. The negative result of the Michelson-Morley experiment was explained by the assumption that the light actually traversed a shorter distance in the same time (that is, moved more slowly), but that this effect was masked because the distance was measured of necessity by some mechanical device which also underwent the same shortening, just as when an object 2 m long is measured with a 3-m tape measure which has shrunk to 2 m, the object will appear to be 3 m in length. Thus, in the Michelson-Morley experiment, the distance which light traveled in 1 sec appeared to be 300,000 km (186,000 mi) regardless of how fast the light actually traveled. The Lorentz-FitzGerald contraction was considered by scientists to be an unsatisfactory hypothesis because it could not be applied to any problem in which measurements of absolute motion could be made.

III

SPECIAL THEORY OF RELATIVITY

In 1905, Einstein published the first of two important papers on the theory of relativity, in which he dismissed the problem of absolute motion by denying its existence. According to Einstein, no particular object in the universe is suitable as an absolute frame of reference that is at rest with respect to space. Any object (such as the center of the solar system) is a suitable frame of reference, and the motion of any object can be referred to that frame. Thus, it is equally correct to say that a train moves past the station, or that the station moves past the train. This example is not as unreasonable as it seems at first sight, for the station is also moving, due to the motion of the earth on its axis and its revolution around the sun. All motion is relative, according to Einstein. None of Einstein's basic assumptions was revolutionary; Newton had previously stated “absolute rest cannot be determined from the position of bodies in our regions.” Einstein stated the relative rate of motion between any observer and any ray of light is always the same, 300,000 km/sec (186,000 mi/sec), and thus two observers, moving relative to one another even at a speed of 160,000 km/sec (100,000 mi/sec), each measuring the velocity of the same ray of light, would both find it to be moving at 300,000 km/sec (186,000 mi/sec), and this apparently anomalous result was proved by the Michelson-Morley experiment. According to classical physics, one of the two observers was at rest, and the other made an error in measurement because of the Lorentz-FitzGerald contraction of his apparatus; according to Einstein, both observers had an equal right to consider themselves at rest, and neither had made any error in measurement. Each observer used a system of coordinates as the frame of reference for measurements, and these coordinates could be transformed one into the other by a mathematical manipulation. The equations for this transformation, known as the Lorentz transformation equations, were adopted by Einstein, but he gave them an entirely new interpretation. The speed of light is invariant in any such transformation.

According to the relativistic transformation, not only would lengths in the line of a moving object be altered but also time and mass. A clock in motion relative to an observer would seem to be slowed down, and any material object would seem to increase in mass, both by the beta factor. The electron, which had just been discovered, provided a means of testing the last assumption. Electrons emitted from radioactive substances have speeds close to the speed of light, so that the value of beta, for example, might be as large as 0.5, and the mass of the electron doubled. The mass of a rapidly moving electron could be easily determined by measuring the curvature produced in its path by a magnetic field; the heavier the electron, the greater its inertia and the less the curvature produced by a given strength of field (see Magnetism). Experimentation dramatically confirmed Einstein's prediction; the electron increased in mass by exactly the amount he predicted. Thus, the kinetic energy of the accelerated electron had been converted into mass in accordance with the formula E=mc² (see Atom; Nuclear Energy). Einstein's theory was also verified by experiments on the velocity of light in moving water and on magnetic forces in moving substances.

The fundamental hypothesis on which Einstein's theory was based was the nonexistence of absolute rest in the universe. Einstein postulated that two observers moving relative to one another at a constant velocity would observe identically the phenomena of nature. One of these observers, however, might record two events on distant stars as having occurred simultaneously, while the other observer would find that one had occurred before the other; this disparity is not a real objection to the theory of relativity, because according to that theory simultaneity does not exist for distant events. In other words, it is not possible to specify uniquely the time when an event happens without reference to the place where it happens. Every particle or object in the universe is described by a so-called world line that describes its position in time and space. If two or more world lines intersect, an event or occurrence takes place; if the world line of a particle does not intersect any other world line, nothing has happened to it, and it is neither important nor meaningful to determine the location of the particle at any given instant. The “distance” or “interval” between any two events can be accurately described by means of a combination of space and time, but not by either of these separately. The space-time of four dimensions (three for space and one for time) in which all events in the universe occur is called the space-time continuum.

All of the above statements are consequences of special relativity, the name given to the theory developed by Einstein in 1905 as a result of his consideration of objects moving relative to one another with constant velocity.

IV

GENERAL THEORY OF RELATIVITY

In 1915 Einstein developed the general theory of relativity in which he considered objects accelerated with respect to one another. He developed this theory to explain apparent conflicts between the laws of relativity and the law of gravity. To resolve these conflicts he developed an entirely new approach to the concept of gravity, based on the principle of equivalence.

The principle of equivalence holds that forces produced by gravity are in every way equivalent to forces produced by acceleration, so that it is theoretically impossible to distinguish between gravitational and accelerational forces by experiment. In the theory of special relativity, Einstein had stated that a person in a closed car rolling on an absolutely smooth railroad track could not determine by any conceivable experiment whether he was at rest or in uniform motion. In general relativity he stated that if the car were speeded up or slowed down or driven around a curve, the occupant could not tell whether the forces so produced were due to gravitation or whether they were acceleration forces brought into play by pressure on the accelerator or on the brake or by turning the car sharply to the right or left.

Acceleration is defined as the rate of change of velocity. Consider an astronaut standing in a stationary rocket. Because of gravity his or her feet are pressed against the floor of the rocket with a force equal to the person's weight, w. If the same rocket is in outer space, far from any other object and not influenced by gravity, the astronaut is again being pressed against the floor if the rocket is accelerating, and if the acceleration is 9.8 m/sec² (32 ft/sec²) (the acceleration of gravity at the surface of the earth), the force with which the astronaut is pressed against the floor is again equal to w. Without looking out of the window, the astronaut would have no way of telling whether the rocket was at rest on the earth or accelerating in outer space. The force due to acceleration is in no way distinguishable from the force due to gravity. According to Einstein's theory, Newton's law of gravitation is an unnecessary hypothesis; Einstein attributes all forces, both gravitational and those associated with acceleration, to the effects of acceleration. Thus, when the rocket is standing still on the surface of the earth, it is attracted toward the center of the earth. Einstein states that this phenomenon of attraction is attributable to an acceleration of the rocket. In three-dimensional space, the rocket is stationary and therefore is not accelerated; but in four-dimensional space-time, the rocket is in motion along its world line. According to Einstein, the world line is curved, because of the curvature of the continuum in the neighborhood of the earth.

Thus, Newton's hypothesis that every object attracts every other object in direct proportion to its mass is replaced by the relativistic hypothesis that the continuum is curved in the neighborhood of massive objects. Einstein's law of gravity states simply that the world line of every object is a geodesic in the continuum. A geodesic is the shortest distance between two points, but in curved space it is not generally a straight line. In the same way, geodesics on the surface of the earth are great circles, which are not straight lines on any ordinary map. See Geometry; Navigation.

V

CONFIRMATION AND MODIFICATION

As in the cases mentioned above, classical and relativistic predictions are generally virtually identical, but relativistic mathematics is more complex. The famous apocryphal statement that only ten people in the world understood Einstein's theory referred to the complex tensor algebra and Riemannian geometry of general relativity; by comparison, special relativity can be understood by any college student who has studied elementary calculus.

General relativity theory has been confirmed in a number of ways since it was introduced. For example, it predicts that the world line of a ray of light will be curved in the immediate vicinity of a massive object such as the sun. To verify this prediction, scientists first chose to observe a star appearing very close to the edge of the sun. Such observations cannot normally be made, because the brightness of the sun obscures a nearby star. During a total eclipse, however, stars can be observed and their positions accurately measured even when they appear quite close to the edge of the sun. Expeditions were sent out to observe the eclipses of 1919 and 1922 and made such observations. The apparent positions of the stars were then compared with their apparent positions some months later, when they appeared at night far from the sun. Einstein predicted an apparent shift in position of 1.745 seconds of arc for a star at the very edge of the sun, with progressively smaller shifts for more distant stars. The expeditions that were sent to study the eclipses verified these predictions. In recent years, comparable tests were made of radio-wave deflections from distant quasars, using radio-telescope interferometers (see Radio Astronomy). The tests yielded results that agreed, to within 1 percent, with the values predicted by general relativity.

Another confirmation of general relativity involves the perihelion of the planet Mercury. For many years it had been known that the perihelion (the point at which Mercury passes closest to the sun) revolves about the sun at the rate of once in 3 million years, and that part of this perihelion motion is completely inexplicable by classical theories. The theory of relativity, however, does predict this part of the motion, and recent radar measurements of Mercury's orbit have confirmed this agreement to within about 0.5 percent.

Yet another phenomenon predicted by general relativity is the time-delay effect, in which signals sent past the sun to a planet or spacecraft on the far side of the sun experience a small delay, when relayed back, compared to the time of return as indicated by classical theory. Although the time intervals involved are very small, various tests made by means of planetary probes have provided values quite close to those predicted by general relativity (see Radar Astronomy). Numerous other tests of the theory could also be described, and thus far they have served to confirm it.

The general theory of relativity predicts that a massive rotating body will drag space and time around with it as it moves. This effect, called frame dragging, is more noticeable if the object is very massive and very dense. In 1997 a group of Italian astronomers announced that they had detected frame dragging around very dense, rapidly spinning astronomical objects called neutron stars. The astronomers found evidence of frame dragging by examining radiation emitted when the gravitational pull of a dense neutron star sucks matter onto its surface. This radiation showed slight differences from the radiation that was predicted by classical physics.

In 1998 another group of astronomers from the United States and Europe announced that the orbits of some artificial satellites around the earth showed the effects of frame dragging. The earth is much lighter and less dense than a neutron star, so the effects of the earth’s frame dragging are much more subtle than those of the neutron star’s frame dragging. The astronomers found that the orbits of two Italian satellites seem to shift about 2 m (about 7 ft) in the direction of the earth’s rotation every year. The launch of the U.S. spacecraft Gravity Probe B in 2000 should provide even more evidence of frame dragging around the earth and other bodies.

VI

LATER OBSERVATIONS

Since 1915 the theory of relativity has undergone much development and expansion by Einstein and by the British astronomers James Hopwood Jeans, Arthur Stanley Eddington, and Edward Arthur Milne, the Dutch astronomer Willem de Sitter, and the German American mathematician Hermann Weyl. Much of their work has been devoted to an effort to extend the theory of relativity to include electromagnetic phenomena (see Unified Field Theory). Although some progress has been made in this area, these efforts have been marked thus far by less success. No complete development of this application of the theory has yet been generally accepted. See Elementary Particles.

The astronomers mentioned above also devoted much effort to developing the cosmological consequences of the theory of relativity. Within the framework of the axioms laid down by Einstein, many lines of development are possible. Space, for example, is curved, and its exact degree of curvature in the neighborhood of heavy bodies is known, but its curvature in empty space is not certain. Moreover, scientists disagree on whether it is a closed curve (such as a sphere) or an open curve (such as a cylinder or a bowl with sides of infinite height). The theory of relativity leads to the possibility that the universe is expanding; this is the most likely theoretical explanation of the experimentally observed fact that the spectral lines of all distant nebulae are shifted to the red; on the other hand the expanding-universe theory also supplies other possible explanations. The latter theory makes it reasonable to assume that the past history of the universe is finite, but it also leads to alternative possibilities. See Cosmology.

Much of the later work on relativity was devoted to creating a workable relativistic quantum mechanics. A relativistic electron theory was developed in 1928 by the British mathematician and physicist Paul Dirac, and subsequently a satisfactory quantized field theory, called quantum electrodynamics, was evolved, unifying the concepts of relativity and quantum theory in relation of the interaction between electrons, positrons, and electromagnetic radiation. In recent years, the work of the British physicist Stephen Hawking has been devoted to an attempted full integration of quantum mechanics with relativity theory.

 

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.

II

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.

A

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.

B

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.

C

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.

A

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.

B

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.

IV

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.

A

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.

B

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.

C

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.

D

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.

V

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.

VI

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)