Solar System
Solar System
What and where is the solar system?
The solar system is defined as all celestial bodies that orbit the sun, including the sun itself. It is comprised of the sun, eight major planets, many dwarf planets, the moons that orbit planetary bodies, some 100,0000 asteroids larger than 0.6 mi (1 km) in diameter, and perhaps 1 trillion cometary nuclei. (As of 2006, Pluto was demoted to a dwarf planet by the International Astronomical Union, reducing the number of planets in the solar system to eight.) While the major planets lie within 30 astronomical units (AU) of the sun, the outermost boundary of the solar system stretches to one million AU, one third the way to the nearest star (Proxima Centauri), excluding the Sun. It is believed that the solar system was formed through the collapse of a spinning cloud of interstellar gas and dust. (The solar system can also be used to describe any star system, however, traditionally it is reserved for the sun’s system.)
What and where is the solar system?
The central, and most important object in the solar system is the sun. It is the largest and most massive object in the solar system—its diameter is 109 times that of the Earth, and it is 333,000 times more massive. The extent of the solar system is determined by the gravitational attraction of the sun. Indeed, the boundary of the solar system is defined as the surface within which the gravitational pull of the sun dominates over that of the Milky Way galaxy. Under this definition, the solar system extends outwards from the sun to a distance of about 100,000 AU. The solar system is much larger, therefore, than the distance to the remotest known (and largest) dwarf planet, Eris which orbits the sun at a furthest distance of 97.56 AU.
The sun and the solar system are situated some 26,000 light-years (the distance that light travels in one year while in a vacuum) from the center of the Milky Way galaxy. Traveling at a velocity of 137 mph (220 km/h), the sun takes about 240 million years to complete one orbit about the galactic center, and since its formation the sun has completed about 19 such trips. As it orbits the center of the galaxy, the sun also moves in an oscillatory fashion above and below the galactic plane (the sun’s motion is similar to that of a carousel fair-ground ride) with a period of about 30 million years. During their periodic sojourns above and below the plane of the galaxy, the sun and solar system suffer gravitational encounters with other stars and giant molecular clouds. These close encounters result in the loss of objects (essentially dormant cometary nuclei located in the outer Oort cloud) that are on, or near, the boundary of the solar system. These encounters also nudge some cometary nuclei toward the inner solar system, where they may be observed as long-period comets.
Solar system inventory
One of the central and age-old questions concerning the solar system is, “How did it form?’ From the very outset, cosmologists (scientists that deal with the age of the universe, galaxy, solar system, etc.) know that such a question has no simple answer, and rather than attempting to explain specific observations about the solar system, scientists have tried to build-up a general picture of how stars and planets might form. Therefore, scientists do not try to explain why there are eight major planets within the sun’s solar system, or why the second planet is 17.8 times less massive than the seventh one. Rather, they seek to explain, for example, the compositional differences that exist between the planets. Indeed, it has long been realized that it is the chemical and dynamical properties of the planets that place the most important constraints on any theory that attempts to explain the origin of the solar system.
The objects within the solar system demonstrate several essential dynamical characteristics. When viewed from above the sun’s North Pole, all of the planets orbit the sun along near-circular orbits in a counterclockwise
manner. The sun also rotates in a counterclockwise direction. With respect to the sun, therefore, the planets have prograde orbits. The major planets, asteroids and short-period comets all move along orbits only slightly inclined to one another. This is why, for example, that when viewed from Earth, the asteroids and planets all appear to move in the narrow zodiacal band of constellations. All of the major planets, with two exceptions, spin on their central axes in the same direction that they orbit the sun. That is, the planets mostly spin in a prograde motion. The planets Venus and Uranus are the three exceptions, having retrograde (backwards) spins. (In addition, the dwarf planet Pluto is an example of a dwarf planet with a retrograde orbit.)
The distances at which the planets orbit the sun increase geometrically, and it appears that each planet is roughly 64% further from the sun than its nearest inner neighbor. This observation is reflected in the so-called Titius-Bode rule, which is a mathematical relation for planetary distances. The formula for the rule is d(AU) = (4 + 3× 2n) / 10, where n = 0, 1, 2, 3,... represents the number of each planet, and d is the distance from the sun, expressed in astronomical units. The formula gives the approximate distance to Mercury when n = 0, and the other planetary distances follow in sequence. It should be pointed out here that there is no known physical explanation for the Titius-Bode rule, and it may well be just a numerical coincidence.
Certainly, the rulepredictswoefully inaccurate distances for the planet Neptune and dwarf planet Pluto.
One final point on planetary distances is that the separation between successive planets increases dramatically beyond the orbit of Mars. While the inner, or terrestrial, planets are typically separated by distances of about four-tenths of an AU, the outer, or Jovian, planets are typically separated by five to ten AU. This observation alone suggests that the planetary formation process was different somewhere beyond the orbit of Mars.
While the asteroids and short-period comets satisfy, in a general sense, the same dynamical constraints as the major planets, scientists have to remember that such objects have undergone significant orbital evolution since the solar system formed. The asteroids, for example, have undergone many mutual collisions and fragmentation events, and the cometary nuclei have suffered from numerous gravitational perturbations from the planets. Long-period comets in particular have suffered considerable dynamical evolution, first to become members of the Oort cloud, and second to become comets visible in the inner solar system.
The compositional make-up of the various solar system bodies offers several important clues about the conditions under which they formed. The four interior planets—Mercury, Venus, Earth, and Mars—are
classified as terrestrial and are composed of rocky material surrounding an iron-nickel metallic core. On the other hand, Jupiter, Saturn, Neptune, and Uranus are classified as the gas giants and are large masses of hydrogen in gaseous, liquid, and solid form surrounding Earth-size rock and metal cores. Dwarf planet Pluto fits neither of these categories, having an icy surface of frozen methane. Pluto more greatly resembles the satellites of the gas giants, which contain large fractions of icy material. This observation suggests that the initial conditions under which ices might have formed only prevailed beyond the orbit of Jupiter.
In summary, any proposed theory for the formation of the solar system must explain both the dynamical and chemical properties of the objects in the solar system. It must also be sufficient flexibility to allow for distinctive features such as retrograde spin, and the chaotic migration of cometary orbits.
The solar nebula hypothesis
Astronomers almost universally believe that the best descriptive model for the formation of the solar system is the solar nebula hypothesis. The essential idea behind the solar nebula model is that the sun and planets formed through the collapse of a rotating cloud of interstellar gas and dust. In this way, planet formation is thought to be a natural consequence of star formation.
The solar nebula hypothesis is not a new scientific proposal. Indeed, German philosopher Immanuel Kant (1724–1804) first discussed the idea in 1755. Later, French mathematician, Pierre Simon de Laplace (1749–1827) developed the model in his text, The System of the World, published in 1796. The model is still under development today.
The key idea behind the solar nebula hypothesis is that once a rotating interstellar gas cloud has commenced gravitational collapse, then the conservation of angular momentum will force the cloud to develop a massive, central condensation that is surrounded by a less massive flattened ring, or disk of material. The nebula hypothesis asserts that the sun forms from the central condensation, and that the planets accumulate from the material in the disk (Figure 1). The solar nebula model naturally explains why the sun is the most massive object in the solar system, and why the planets rotate about the sun in the same sense, along nearly circular orbits and in essentiallythesameplane(Figure2).
During the gravitational collapse of an interstellar cloud, the central regions become heated through the
release of gravitational energy. This means that the young solar nebular is hot, and that the gas and (vaporized) dust in the central regions is well mixed. By constructing models to follow the gradual cooling of the solar nebula, scientists have been able to establish a chemical condensation sequence. Near to the central proto-sun, the nebular temperature will be very high, and consequently no solid matter can exist. Everything is in a gaseous form. As one moves further away from the central proto-sun, however, the temperature of the nebula falls off. At distances beyond 0.2 AU from the proto-sun, the temperature drops below 2,000 K (3,100°F; 1,700°C). At this temperature, metals and oxides can begin to form. Still further out (at about 0.5 AU), the temperature will drop below 1,000 K (1,300°F; 730°C), and silicate rocks can begin to form. Beyond about 5 AU from the proto-sun, the temperature of the nebula will be below 200 K (-100°F; -73°C), and ices can start to condense. The temperature and distance controlled sequence of chemical condensation in the solar nebula correctly predicts the basic chemical make-up of the planets (Figure 3).
The angular momentum problem
Perhaps the most important issue to be resolved in future versions of the solar nebula model is that of the distribution of angular momentum. The problem for the solar nebula theory is that it predicts that most of the mass and angular momentum should be in the sun. In other words, the sun should spin much more rapidly than it does. A mechanism is therefore required to transport angular momentum away from the central proto-sun and redistribute it in the outer planetary disk. One proposed transport mechanism invokes the presence of magnetic field in the nebula, while another mechanism proposed the existence of viscous stresses produced by turbulence in the nebular gas.
Building the planets
Precise dating of meteorites and lunar rock samples indicate that the solar system is 4.6 billion years old. The meteorites also indicate an age spread of about 20 million years, during which time the planets themselves formed.
The standard solar nebula model suggests that the planets were created through a multi-step process. The first important step is the coagulation and sedimentation of rock and ice grains in the mid-plain of the nebula. These grains and aggregates, 0.4 in (1 cm) to 3 ft (1 m) in size, continue to accumulate in the mid-plain of the nebula to produce a swarm of some 10 trillion larger bodies, called planetesimals, that are some 0.6 mi (1 km), or so in size. Finally, the planetesimals themselves accumulate into larger, self-gravitating bodies called proto-planets. The proto-planets were probably a few hundred kilometers in size. Finally, growth of proto-planet-sized objects results in the planets.
The final stages of planetary formation were decidedly violent—it is believed that a collision with a Mars-sized proto-planet produced Earth’s moon. Likewise, it is thought that the retrograde rotations of Venus and Uranus may have been caused by glancing proto-planetary impacts. The rocky and icy planetesimals not incorporated into the proto-planets now orbit the sun as asteroids and cometary nuclei. The cometary nuclei that formed in the outer solar nebula were mostly ejected from the nebula by gravitational encounters with the large Jovian planets and now reside in the Oort cloud.
One problem that has still to be worked-out under the solar nebula paradigm concerns the formation of Jupiter. The estimated accumulation time for Jupiter is about 100 million years, but it is now known that the solar nebula itself probably only survived for between 100,000 to 10 million years. In other words, the accumulation process in the standard nebula model is too slow by a least a factor of 10 and maybe 100. Indeed, much has yet to be learned of how the solar system formed.
Active study of the solar system is ongoing. Several probes and robots—such as the Galileo spacecraft, the Cassini mission, New Horizons, and Mars Pathfinder mission—have been launched towards other planets, dwarf planets, and their moons. The information they send back will help further explain the evolution of the solar system. In fact, the Near Earth Asteroid Rendezvous-Shoemaker (NEAR-Shoemaker) spacecraft flew by the asteroid Mathilde and found it to have a surprisingly low density. Later, NEAR-Shoemaker made a soft landing (on February 12, 2001) onto the asteroid Eros, the first time a human instrument has soft-landed on an asteroid. Its mission provided a tremendous amount of information about asteroids.
Of great importance to the study of solar systems in general was the discovery in 1999 of an entire solar system around a star besides the sun. Forty-four light-years from Earth, three large planets were found
KEY TERMS
Accretion —The process by which the mass of a body increases by the gravitational attraction of smaller objects.
Angular momentum —The product of orbital distance, orbital speed, and mass. In a closed system, angular momentum is a conserved quantity—it can be transferred from one place to another, but it cannot be created or destroyed.
Oort cloud —A vast, spherical cloud of some one trillion cometary nuclei that orbit the Sun. The cloud, named after Dutch astronomer Jan Oort who first suggested its existence, extends to a distance of 105 AU from the Sun.
Planetesimal —Small, 0.6 mi (1 km) sized objects made of rock and/or ice that accrete to form proto planets.
Prograde rotation —Rotational spin in the same sense as the orbital motion. For solar system objects, the orbital motion is counterclockwise, and prograde spin results in the object revolving from east to west.
Retrograde rotation —Axial spin that is directed in the opposite sense to that of the orbital motion.
circling the star Upsilon Andromedae. Astronomers suspect the planets are similar to Jupiter and Saturn— huge spheres of gas without a solid surface. The discovery of other solar systems in the Milky Way galaxy is yielding important insight into the formation of evolution of solar systems in general.
As of 2006, astronomers estimate that at least 10% of all stars similar in size and characteristics to the sun have one or more planets orbiting them. In 2005 and 2006, some of the extrasolar planets discovered have included: Gliese 876d, around the red dwarf star Gliese 876; HD 149026 b, the largest planet core ever found; HD 188753 Ab, a planet within a triple star system; and OGLE-2005-BLG-390Lb, the most distant and coldest extrasolar planet ever discovered.
Resources
BOOKS
Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.
Bhatnager, Arvind. Fundamentals of Solar Astronomy. Hackensack, NJ: World Scientific, 2005.
Kundt, Wolfgang. Astrophysics: A New Approach. Berlin and New York: Springer, 2005.
Laureta, D.S., H.Y. McSween, Jr., eds. Meteorites and the Early Solar System. Tucson, AR: University of Arizona Press, 2006.
Lewis, John S. Physics and Chemistry of the Solar System. Amsterdam and Boston, MA: Elsevier Academic Press, 2004.
Morbidelli, Alessandro. Modern Celestial Mechanics: Aspects of Solar System Dynamics. London and New York: Taylor and Francis, 2002.
OTHER
KidsAstronomy.com. “Home page of KidsAstronomy.com.” <http://www.kidsastronomy.com/> (accessed October 1, 2006).
Martin Beech
Solar System
Solar system
The solar system comprises the Sun , nine major planets, some 100,0000 asteroids larger than 0.6 mi (1 km) in diameter, and perhaps 1 trillion cometary nuclei. While the major planets lie within 40 astronomical units (AU) of the Sun, the outermost boundary of the solar system stretches to 1 million AU, one third the way to the nearest star . It is believed that the solar system was formed through the collapse of a spinning cloud of interstellar gas and dust.
What and where is the solar system?
The central, and most important object in our solar system is the Sun. It is the largest and most massive object in the solar system—its diameter is 109 times that of Earth , and it is 333,000 times more massive. The extent of the solar system is determined by the gravitational attraction of the Sun. Indeed, the boundary of the solar system is defined as the surface within which the gravitational pull of the Sun dominates over that of the galaxy . Under this definition, the solar system extends outwards from the Sun to a distance of about 100,000 AU. The solar system is much larger, therefore, than the distance to the remotest known planet , Pluto , which orbits the Sun at a mean distance of 39.44 AU.
The Sun and the solar system are situated some 26,000 light years from the center of our galaxy. Traveling at a velocity of 137 MPH (220 km/h), the Sun takes about 240 million years to complete one orbit about the galactic center, and since its formation the Sun has completed about 19 such trips. As it orbits the center of the galaxy the Sun also moves in an oscillatory fashion above and below the galactic plane (the Sun's motion is similar to that of a carousel fair-ground ride) with a period of about 30 million years. During their periodic sojourns above and below the plane of the galaxy, the Sun and solar system suffer gravitational encounters with other stars and giant molecular clouds . These close encounters result in the loss of objects (essentially dormant cometary nuclei located in the outer Oort cloud) that are on, or near, the boundary of the solar system. These encounters also nudge some cometary nuclei toward the inner solar system, where they may be observed as long-period comets .
Solar system inventory
One of the central and age-old questions concerning the solar system is, "How did it form?" From the very outset we know that such a question has no simple answer, and rather than attempting to explain specific observations about our solar system, scientists have tried to build-up a general picture of how stars and planets might form. Therefore, scientists do not try to explain why there are nine major planets within our solar system, or why the second planet is 17.8 times less massive than the seventh one. Rather, they seek to explain, for example, the compositional differences that exist between the planets. Indeed, it has long been realized that it is the chemical and dynamical properties of the planets that place the most important constraints on any theory that attempts to explain the origin of our solar system.
The objects within our solar system demonstrate several essential dynamical characteristics. When viewed from above the Sun's north pole, all of the planets orbit the Sun along near-circular orbits in a counterclockwise manner. The Sun also rotates in a counterclockwise direction. With respect to the Sun, therefore, the planets have prograde orbits. The major planets, asteroids and short-period comets all move along orbits only slightly inclined to one another. This is why, for example, that when viewed from Earth, the asteroids and planets all appear to move in the narrow zodiacal band of constellations. All of the major planets, with three exceptions, spin on their central axes in the same direction that they orbit the Sun. That is, the planets mostly spin in a prograde motion. The planets Venus , Uranus , and Pluto are the three exceptions, having retrograde (backwards) spins.
The distances at which the planets orbit the Sun increase geometrically, and it appears that each planet is roughly 64% further from the Sun than its nearest inner neighbor. This observation is reflected in the so-called Titius-Bode rule which is a mathematical relation for planetary distances. The formula for the rule is d(AU) = (4 + 3 × 2n) / 10, where n = 0, 1, 2, 3,...,etc. represents the number of each planet, and d is the distance from the Sun, expressed in astronomical units. The formula gives the approximate distance to Mercury when n = 0, and the other planetary distances follow in sequence. It should be pointed out here that there is no known physical explanation for the Titius-Bode rule, and it may well be just a numerical coincidence. Certainly, the rule predicts woefully inaccurate distances for the planets Neptune and Pluto.
One final point on planetary distances is that the separation between successive planets increases dramatically beyond the orbit of Mars . While the inner, or terrestrial planets are typically separated by distances of about four-tenths of an AU, the outer, or Jovian planetsare typically separated by 5-10 AU. This observation alone suggests that the planetary formation process was "different" somewhere beyond the orbit of Mars.
While the asteroids and short-period comets satisfy, in a general sense, the same dynamical constraints as the major planets, we have to remember that such objects have undergone significant orbital evolution since the solar system formed. The asteroids, for example, have undergone many mutual collisions and fragmentation events, and the cometary nuclei have suffered from numerous gravitational perturbations from the planets. Long-period comets in particular have suffered considerable dynamical evolution, first to become members of the Oort cloud, and second to become comets visible in the inner solar system.
The compositional make-up of the various solar system bodies offers several important clues about the conditions under which they formed. The four interior planets—Mercury, Venus, Earth, and Mars—are classified as terrestrial and are composed of rocky material surrounding an iron-nickel metallic core. On the other hand, Jupiter , Saturn , Neptune, and Uranus are classified as the "gas giants" and are large masses of hydrogen in gaseous, liquid, and solid form surrounding Earth-size rock and metal cores. Pluto fits neither of these categories, having an icy surface of frozen methane. Pluto more greatly resembles the satellites of the gas giants, which contain large fractions of icy material. This observation suggests that the initial conditions under which ices might have formed only prevailed beyond the orbit of Jupiter.
In summary, any proposed theory for the formation of the solar system must explain both the dynamical and chemical properties of the objects in the solar system. It must also be sufficient flexibility to allow for distinctive features such as retrograde spin, and the chaotic migration of cometary orbits.
The solar nebula hypothesis
Astronomers almost universally believe that the best descriptive model for the formation of the solar system is the solar nebula hypothesis. The essential idea behind the solar nebula model is that the Sun and planets formed through the collapse of a rotating cloud of interstellar gas and dust. In this way, planet formation is thought to be a natural consequence of star formation .
The solar nebula hypothesis is not a new scientific proposal. Indeed, the German philosopher Immanuel Kant first discussed the idea in 1755. Later, the French mathematician Pierre-Simon Marquis de Laplace developed the model in his text, The System of the World, published in 1796. The model is still under development today.
The key idea behind the solar nebula hypothesis is that once a rotating interstellar gas cloud has commenced gravitational collapse, then the conservation of angular momentum will force the cloud to develop a massive, central condensation that is surrounded by a less massive flattened ring, or disk of material. The nebula hypothesis asserts that the Sun forms from the central condensation, and that the planets accumulate from the material in the disk. The solar nebula model naturally explains why the Sun is the most massive object in the solar system, and why the planets rotate about the Sun in the same sense, along nearly circular orbits and in essentially the same plane.
During the gravitational collapse of an interstellar cloud, the central regions become heated through the release of gravitational energy . This means that the young solar nebular is hot, and that the gas and (vaporized) dust in the central regions is well-mixed. By constructing models to follow the gradual cooling of the solar nebula, scientists have been able to establish a chemical condensation sequence. Near to the central proto-sun, the nebular temperature will be very high, and consequently no solid matter can exist. Everything is in a gaseous form. As one moves further away from the central proto-sun, however, the temperature of the nebula falls off. At distances beyond 0.2 AU from the proto-sun, the temperature drops below 2,000 K (3,100°F; 1,700°C). At this temperature metals and oxides can begin to form. Still further out (at about 0.5 AU), the temperature will drop below 1,000K (1,300°F; 730°C), and silicate rocks can begin to form. Beyond about 5 AU from the proto-sun, the temperature of the nebula will be below 200 K (-100°F; -73°C), and ices can start to condense. The temperature and distance controlled sequence of chemical condensation in the solar nebula correctly predicts the basic chemical make-up of the planets.
The angular momentum problem
Perhaps the most important issue to be resolved in future versions of the solar nebula model is that of the distribution of angular momentum. The problem for the solar nebula theory is that it predicts that most of the mass and angular momentum should be in the Sun. In other words, the Sun should spin much more rapidly than it does. A mechanism is therefore required to transport angular momentum away from the central proto-sun and redistribute it in the outer planetary disk. One proposed transport mechanism invokes the presence of magnetic field in the nebula, while another mechanism proposed the existence of viscous stresses produced by turbulence in the nebular gas.
Building the planets
Precise dating of meteorites and lunar rock samples indicate that the solar system is 4.6 billion years old. The meteorites also indicate an age spread of about 20 million years, during which time the planets themselves formed.
The standard solar nebula model suggests that the planets were created through a multi-step process. The first important step is the coagulation and sedimentation of rock and ice grains in the mid-plain of the nebula. These grains and aggregates, 0.4 in (1 cm) to 3 ft (1 m) in size, continue to accumulate in the mid-plain of the nebula to produce a swarm of some 10 trillion larger bodies, called planetesimals, that are some 0.6 mi (1 km), or so in size. Finally, the planetesimals themselves accumulate into larger, self-gravitating bodies called proto-planets. The proto-planets were probably a few hundred kilometers in size. Finally, growth of proto-planet-sized objects results in the planets.
The final stages of planetary formation were decidedly violent—it is believed that a collision with a Marssized proto-planet produced Earth's Moon . Likewise, it is thought that the retrograde rotations of Venus and Uranus may have been caused by glancing proto-planetary impacts. The rocky and icy planetesimals not incorporated into the proto-planets now orbit the Sun as asteroids and cometary nuclei. The cometary nuclei that formed in the outer solar nebula were mostly ejected from the nebula by gravitational encounters with the large Jovian planets and now reside in the Oort cloud.
One problem that has still to be worked-out under the solar nebula paradigm concerns the formation of Jupiter. The estimated accumulation time for Jupiter is about 100 million years, but it is now known that the solar nebula itself probably only survived for between 100,000 to 10 million years. In other words, the accumulation process in the standard nebula model is too slow by a least a factor of 10 and maybe 100. Indeed, much has yet to be learned of how our solar system formed.
Active study of our solar system is ongooing. Several probes and robots—such as the Galileo spacecraft, the Cassini mission, and the Mars Pathfinder mission—have been launched towards other planets and their moons, sending back information about their composition that may further explain the evolution of our solar system. The NEAR spacecraft flew by the asteroid Mathilde and found it to have a surprisingly low density .
Of great importance to the study of solar systems was the discovery in 1999 of an entire solar system around a star that is not our Sun. Forty-four light-years from Earth, three large planets were found circling the star Upsilon Andromedae. Astronomers suspect the planets are similar to Jupiter and Saturn—huge spheres of gas without a solid surface. The discovery of at least one other solar system in our galaxy could yield important insight into the formation of evolution of solar systems in general.
Resources
books
Introduction to Astronomy and Astrophysics. 4th ed. New York: Harcourt Brace, 1997.
Wyrun-Williams, Gareth. The Fullness of Space. Cambridge: Cambridge University Press, 1992.
periodicals
Hughes, David. "Where Planets Boldly Grow." New Scientist (December 12, 1992): 29-33.
Murray, Carl. "Is the Solar System Stable?" New Scientist (25 November 1989): 60-63.
Woolfson, M.M. "The Solar System-Its Origin and Evolution." The Quarterly Journal of the Royal Astronomical Society 34 (1993): 1-20.
Martin Beech
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Accretion
—The process by which the mass of a body increases by the gravitational attraction of smaller objects.
- Angular momentum
—The product of orbital distance, orbital speed, and mass. In a closed system, angular momentum is a conserved quantity—it can be transferred from one place to another, but it cannot be created or destroyed.
- Oort cloud
—A vast, spherical cloud of some one trillion cometary nuclei that orbit the Sun. The cloud, named after Dutch astronomer Jan Oort who first suggested its existence, extends to a distance of 105 AU from the Sun.
- Planetesimal
—Small, 0.6 mi (1 km) sized objects made of rock and/or ice that accrete to form proto planets.
- Prograde rotation
—Rotational spin in the same sense as the orbital motion. For solar system objects, the orbital motion is counterclockwise, and prograde spin results in the object revolving from east to west.
- Retrograde rotation
—Axial spin that is directed in the opposite sense to that of the orbital motion.
Solar System
Solar system
Earth's solar system is comprised of the Sun , nine major planets, some 100,000 asteroids larger than 0.6 mi (1 km) in diameter, and perhaps 1 trillion cometary nuclei. While the major planets lie within 40 Astronomical Units (AU)—the average distance of Earth to the Sun—the outermost boundary of the solar system stretches to 1 million AU, one-third the way to the nearest star. Cosmologists and Astronomers assert that the solar system was formed through the collapse of a spinning cloud of interstellar gas and dust.
The central object in the solar system is the Sun. It is the largest and most massive object in the solar system; its diameter is 109 times that of Earth, and it is 333,000 times more massive. The extent of the solar system is determined by the gravitational attraction of the Sun. Indeed, the boundary of the solar system is defined as the surface within which the gravitational pull of the Sun dominates over that of the galaxy. Under this definition, the solar system extends outwards from the Sun to a distance of about 100,000 AU. The solar system is much larger, therefore, than the distance to the remotest known planet, Pluto, which orbits the Sun at a mean distance of 39.44 AU.
The Sun and the solar system are situated some 26,000 light years from the center of our galaxy. The Sun takes about 240 million years to complete one orbit about the galactic center.
Since its formation the Sun has completed about 19 such trips. As it orbits about the center of the galaxy, the Sun also moves in an oscillatory fashion above and below the galactic plane with a period of about 30 million years. During their periodic sojourns above and below the plane of the galaxy, the Sun and solar system suffer gravitational encounters with other stars and giant molecular clouds . These close encounters result in the loss of objects (essentially dormant cometary nuclei located in the outer Oort cloud) that are on, or near, the boundary of the solar system. These encounters also nudge some cometary nuclei toward the inner solar system where they may be observed as long-period comets .
The objects within our solar system demonstrate several essential dynamical characteristics. When viewed from above the Sun's North Pole, all of the planets orbit the Sun along near-circular orbits in a counterclockwise manner. The Sun also rotates in a counterclockwise direction. With respect to the Sun, therefore, the planets have prograde orbits. The major planets, asteroids, and short-period comets all move along orbits only slightly inclined to one another. For this reason, when viewed from Earth, the asteroids and planets all appear to move in the narrow zodiacal band of constellations. All of the major planets, with three exceptions, spin on their central axes in the same direction that they orbit the Sun. That is, the planets mostly spin in a prograde motion. The planets Venus, Uranus, and Pluto are the three exceptions, having retrograde (backwards) spins.
The distances at which the planets orbit the Sun increase geometrically, and it appears that each planet is roughly 64% further from the Sun than its nearest inner neighbor. The separation between successive planets increases dramatically beyond the orbit of Mars. While the inner, or terrestrial planets are typically separated by distances of about four-tenths of an AU, the outer, or Jovian planets are typically separated by 5—10 AU.
Although the asteroids and short-period comets satisfy, in a general sense, the same dynamical constraints as the major planets, we have to remember that such objects have undergone significant orbital evolution since the solar system formed. The asteroids, for example, have undergone many mutual collisions and fragmentation events, and the cometary nuclei have suffered from numerous gravitational perturbations from the planets. Long-period comets in particular have suffered considerable dynamical evolution, first to become members of the Oort cloud, and second to become comets visible in the inner solar system.
The compositional make-up of the various solar system bodies offers several important clues about the conditions under which they formed. The four interior planets—Mercury, Venus, Earth, and Mars—are classified as terrestrial and are composed of rocky material surrounding an iron-nickel metallic core. In contrast, Jupiter, Saturn, Neptune, and Uranus are classified as the "gas giants" and are large masses of hydrogen in gaseous, liquid, and solid form surrounding Earth-size rock and metal cores. Pluto fits neither of these categories, having an icy surface of frozen methane. Pluto more greatly resembles the satellites of the gas giants, which contain large fractions of icy material. This observation suggests that the initial conditions under which such ices might have formed only prevailed beyond the orbit of Jupiter.
In summary, any proposed theory for the formation of the solar system must explain both the dynamical and chemical properties of the objects in the solar system. It must also be sufficient flexibility to allow for distinctive features such as retrograde spin, and the chaotic migration of cometary orbits.
Astronomers almost universally assert that the best descriptive model for the formation of the solar system is the solar nebula hypothesis. The essential idea behind the solar nebula model is that the Sun and planets formed through the collapse of a rotating cloud of interstellar gas and dust. In this way, planet formation is postulated to be a natural consequence of star formation.
The solar nebula hypothesis is not a new scientific proposal. Indeed, the German philosopher Immanuel Kant first discussed the idea in 1755. Later, the French mathematician, Pierre Simon de Laplace (1749–1827) developed the model in his text, The System of the World, published in 1796.
The key postulate in the solar nebula hypothesis is that once a rotating interstellar gas cloud has commenced gravitational collapse, then the conservation of angular momentum will force the cloud to develop a massive, central condensation that is surrounded by a less massive flattened ring, or disk of material. The nebula hypothesis asserts that the Sun forms from the central condensation, and that the planets accumulate from the material in the disk. The solar nebula model naturally explains why the Sun is the most massive object in the solar system, and why the planets rotate about the Sun in the same sense, along nearly circular orbits and in essentially the same plane.
During the gravitational collapse of an interstellar cloud, the central regions become heated through the release of gravitational energy. This means that the young solar nebular is hot, and that the gas and (vaporized) dust in the central regions is well mixed. By constructing models to follow the gradual cooling of the solar nebula, scientists have been able to establish a chemical condensation sequence. Near to the central proto-sun, the nebular temperature will be very high, and consequently no solid matter can exist. Everything is in a gaseous form. Farther away from the central proto-sun, however, the temperature of the nebula falls off. At distances beyond 0.2 AU from the proto-sun, the temperature drops below 3,100°F (1,700°C). At this temperature, metals and oxides can begin to form. Still further out (at about 0.5 AU), the temperature will drop below 1,300°F (730°C), and silicate rocks can begin to form. Beyond about 5 AU from the protosun, the temperature of the nebula will be below −100°F (−73°C), and ices can start to condense. The temperature and distance controlled sequence of chemical condensation in the solar nebula correctly predicts the basic chemical make-up of the planets.
Perhaps the most important issue to be resolved in future versions of the solar nebula model is that of the distribution of angular momentum. The problem for the solar nebula theory is that it predicts that most of the mass and angular momentum should be in the Sun. In other words, the Sun should spin much more rapidly than it does. A mechanism is therefore required to transport angular momentum away from the central proto-sun and redistribute it in the outer planetary disk. One proposed transport mechanism invokes the presence of a magnetic field in the nebula, while another mechanism proposed the existence of viscous stresses produced by turbulence in the nebular gas.
Precise dating of meteorites and lunar rock samples indicate that the solar system is 4.6 to 5.1 billion years old. The meteorites also indicate an age spread of about 20 million years, during which time the planets themselves formed.
The standard solar nebula model suggests that the planets were created through a multi-step process. The first important step is the coagulation and sedimentation of rock and ice grains in the mid-plain of the nebula. These grains and aggregates, 0.4 in (1 cm) to 3 ft (1 m) in size, continue to accumulate in the mid-plain of the nebula to produce a swarm of some 10 trillion larger bodies, called planetesimals, that are some 0.6 mi (1 km), or so in size. Finally, the planetesimals themselves accumulate into larger, self-gravitating bodies called proto-planets. The proto-planets were probably a few hundred kilometers in size. Finally, growth of proto-planet-sized objects results in the planets.
The final stages of planetary formation were decidedly violent—it is probable that a collision with a Mars-sized proto-planet produced Earth's Moon . Likewise, it is thought that the retrograde rotations of Venus and Uranus may have been caused by glancing proto-planetary impacts. The rocky and icy planetesimals not incorporated into the proto-planets now orbit the Sun as asteroids and cometary nuclei. The cometary nuclei that formed in the outer solar nebula were mostly ejected from the nebula by gravitational encounters with the large Jovian gas giants and now reside in the Oort cloud.
One problem that has still to be worked-out under the solar nebula hypothesis concerns the formation of Jupiter. The estimated accumulation time for Jupiter is about 100 million years, but it is now known that the solar nebula itself probably only survived for 100,000 to 10 million years. In other words, the accumulation process in the standard nebula model is too slow by at least a factor of 10 and maybe 100.
Of great importance to the study of solar systems was the discovery in 1999 of an entire solar system around another star. Although such systems should be plentiful and common in the cosmos, this was the first observation of another solar system. Forty-four light-years from Earth, three large planets were found circling the star Upsilon Andromedae. Astronomers suspect the planets are similar to Jupiter and Saturn—huge spheres of gas without a solid surface.
See also Astronomy; Big Bang theory; Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Cosmology; Dating methods; Earth (planet); Earth, interior structure; Geologic time; Revolution and rotation
Solar System
Solar system
Our solar system consists of the Sun and all of its orbiting objects. These objects include the planets with their rings and moons, asteroids, comets, meteors and meteorites, and particles of dust and debris.
The Sun, which keeps these objects in orbit with its gravitational field, alone accounts for about 99.8 percent of the mass of the solar system. Jupiter, the largest planet, represents another 0.1 percent of the mass. Everything else in the solar system together makes up the remaining 0.1 percent.
The average distance between the Sun and Pluto, the farthest planet, is about 3.66 billion miles (5.89 billion kilometers). Incorporating the entire space within the orbit of Pluto, the area encompassed by the solar system is 41.85 billion square miles (108.4 billion square kilometers). Our solar system seems quite insignificant, however, when considered in the context of the more than 100 billion stars in our galaxy, the Milky Way, and the estimated 50 billion galaxies in the universe.
Planets
A planet is defined as a body that orbits a star (in our case the Sun) and produces no light of its own, but reflects the light of its controlling star. At present, scientists know of nine planets in the solar system. They are grouped into three categories: the solid, terrestrial planets; the giant, gaseous (also known as Jovian) planets; and Pluto.
The terrestrial planets, the first group closest to the Sun, consists of Mercury, Venus, Earth, and Mars. The atmospheres of these planets contain (in varying amounts) nitrogen, carbon dioxide, oxygen, water, and argon.
Words to Know
Light-year: Distance light travels in one year in the vacuum of space, roughly 5.9 trillion miles (9.5 trillion kilometers).
Nuclear fusion: Merging of two hydrogen nuclei into one helium nucleus, releasing a tremendous amount of energy in the process.
Oort cloud: Region of space beyond the solar system that theoretically contains about one trillion inactive comets.
Planetesimals: Ancient chunks of matter that originated with the formation of the solar system but never came together to form a planet.
Protoplanet: Earliest form of a planet, plus its moons, formed by the combination of planetesimals.
Solar wind: Electrically charged subatomic particles that flow out from the Sun.
Supernova: Explosion of a massive star at the end of its lifetime, causing it to shine more brightly than the rest of the stars in the galaxy put together.
The Jovian planets, father from the Sun, consist of Jupiter, Saturn, Uranus, and Neptune. The light gases hydrogen and helium make up almost 100 percent of the thick atmospheres of these planets. Another difference between the giant planets and the terrestrial planets is the existence of ring systems. Although the rings around Saturn are the most spectacular and the only ones visible from Earth, Jupiter, Uranus, and Neptune do have rings.
On the basis of distance from the Sun, Pluto might be considered a Jovian planet, but its size places it in the terrestrial group. The major component of its thin atmosphere is probably methane, which exists in a frozen state for much of the planet's inclined orbit around the Sun.
Moons
A moon is any natural satellite (as opposed to a human-made satellite) that orbits a planet. Seven of the planets in the solar system—Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto—have moons, which total 61. Although moons do not orbit the Sun independently, they are still considered members of the solar system.
Asteroids
Asteroids are relatively small chunks of rock that orbit the Sun. Except for their small size, they are similar to planets. For this reason, they are often referred to as minor planets. Scientists believe that asteroids are
ancient pieces of matter that were created with the formation of the solar system but never came together to form a planet. An estimated one million asteroids may exist in the solar system. About 95 percent of all asteroids occupy a band of space between the orbits of Mars and Jupiter. The largest of the asteroids, named Ceres, is 580 miles (940 kilometers) in diameter, while the smallest one measured to date is only 33 feet (10 meters) in diameter.
Comets
Comets are made of dust and rocky material mixed with frozen methane, ammonia, and water. A comet speeds around the Sun on an elongated orbit. It consists of a nucleus, a head, and a gaseous tail. The tail forms when some of the comet melts as it nears the Sun and the melted material is swept back by the solar wind. Scientists believe comets originate on the edge of the solar system in an area called the Oort cloud. This space is occupied by trillions of inactive comets, which remain there until a passing gas cloud or star jolts one into orbit around the Sun.
The origin of the solar system
Over time, there have been various theories put forth as to the origin of the solar system. Most of these have since been disproved and discarded. Today the theory scientists consider most likely to be correct is a modified version of the nebular hypothesis first suggested in 1755 by German philosopher Immanuel Kant and later advanced by French mathematician Pierre-Simon Laplace.
The modern solar nebula hypothesis states that the Sun and planets formed 4.6 billion years ago from the solar nebula—a cloud of interstellar gas and dust. Due to the mutual gravitational attraction of the material in the nebula, and possibly triggered by shock waves from a nearby supernova, the nebula eventually collapsed in on itself.
As the nebula contracted, it spun increasingly rapidly, leading to frequent collisions between dust grains. These grains stuck together to form pebbles, then boulders, and then planetesimals. Solid particles as well as gas continued to stick to these planetesimals (in what's known as the accretion theory), eventually forming protoplanets, or planets in their early stages.
As the nebula continued to condense, the temperature at its core rose to the point where nuclear fusion reactions began, forming the Sun. The protoplanets spinning around the developing Sun formed the planets.
Other solar systems?
Evidence has come to light suggesting that ours may not be the only solar system in the galaxy. In late 1995 and early 1996, three new planets were found, ranging in distance from 35 to 40 light-years from Earth. The first planet, discovered by Swiss astronomers Michel Mayor and Didier Queloz, orbits a star in the constellation Pegasus. The next two planets were discovered by American astronomers Geoffrey Marcy and R. Paul Butler. One is in the constellation Virgo and the other is in Ursa Major. Other planetary discoveries soon followed, and by spring 2001, astronomers had found evidence of 63 known planets outside our solar system.
Of perhaps greater importance to the study of solar systems was the announcement in 1999 that astronomers had discovered the first planetary system outside of our own. They detected three planets circling the star Upsilon Andromedae, some forty-four light-years away. Two of the three planets are at least twice as massive as Jupiter, and astronomers suspect
they are huge spheres of gas without a solid surface. The innermost planet lies extremely close to Upsilon Andromedae—about one-eighth the distance at which Mercury circles the Sun.
The discovery of two more planetary systems in the universe was announced by astronomers in early 2001. Each is different from the other and from our solar system. In one, a star like our Sun is orbited by a massive planet and an even larger object seventeen times the size of Jupiter. According to astronomers, this large object could be a dim, failed star or an astronomical object that simply has not been seen before. In the second system, a small star is orbited by two planets of more normal size. Their orbits around the star, however, puzzle astronomers: the inner planet orbits almost twice as fast as the outer planet. With these discoveries at the beginning of the twenty-first century, astronomers may have to redefine what a normal planetary system is in the universe.
[See also Asteroid; Comet; Cosmology; Earth; Extrasolar planet; Mars; Jupiter; Mercury; Meteors and meteorites; Neptune; Orbit; Pluto; Saturn; Sun; Uranus; Venus ]