Home About us Products Services Contact us Bookmark
:: wikimiki.org ::
Solar System

Solar system

The solar system comprises our Sun and the retinue of celestial objects gravitationally bound to it. Traditionally, this is said to consist of the Sun, nine planets and their 158 currently known moons; however, a large number of other objects, including asteroids, meteoroids, planetoids, comets, and interplanetary dust, orbit the Sun as well. Although the term "solar system" is frequently applied to other star systems and the planetary systems which may comprise them, it should strictly refer to our system specifically: the word "solar" is derived from the Sun's Latin name, Sol (and the term sometimes appears as Solar System). When talking about another stellar system (or planetary system), including the star(s) and bodies associated with them through gravity, it is usual to shorten it to "the system" (e.g. "the Alpha Centauri system" or "the 51 Pegasi system").

Structure and layout of the solar system

The Sun (astronomical symbol ☉) is a main sequence G2 star that contains 99.86% of the system's known mass. Its two largest orbiting bodies, Jupiter and Saturn, account for 91% of the remainder (The Oort Cloud might hold a substantial percentage, but as yet its existence is unconfirmed). In broad terms, the charted regions of our solar system consist of the Sun and its planetary system: the eight bodies in relatively unique orbits (commonly called planets or major planets) and two belts of smaller objects (which can be called minor planets, planetoids, meteoroids, planetesimals or, in the case of Pluto, planets). Objects in orbit round the Sun all lie within the same shallow plane, called the ecliptic, and all orbit in the same direction. Many are in turn orbited by moons, and the largest are encircled by planetary rings of dust and other particles. The major planets are, in order, Mercury (☿), Venus (♀), Earth (♁), Mars (♂), Jupiter (♃), Saturn (♄), Uranus (♅/10px), Neptune (♆), and Pluto (♇), though Pluto's status has been thrown into question by the discovery of (see below). Eight of the nine planets are named after or derived from gods and goddesses from Greco-Roman mythology; Earth, a Germanic word, is known in many Romance languages as Terra, the Roman goddess of the Earth. Distances within the solar system are measured most often in astronomical units, or AU. 1 AU is the distance between the Earth and the Sun, or 149 598 000 kilometers. Pluto is roughly 38 AU from the Sun, while Jupiter lies at roughly 5.2 AU. For very large distances within the solar system, such as regions beyond Pluto or the orbital circumferences of planets, the terameter (Tm, one milliard kilometers) is sometimes used. Despite the fact that many diagrams (like the image at the top of this article), for practicality's sake, represent the solar system as having each orbit the same distance apart, in actuality the orbits are largely arranged geometrically, that is, each is roughly double the distance from the Sun as the one before it. Venus’s distance from the Sun is roughly double that of Mercury, Earth’s distance is roughly double that of Venus, Mars’s double that of Earth and so on. This relationship is roughly expressed in the Titius-Bode law, a mathematical formula for predicting the semi-major axes of planets in AU. In its simplest form, it is written : a= 0.4 + 0.3\times k where k=0,1,2,4,8,16,32,64,128. By this formulation, we would expect Mercury's orbit (k=0) to be 0.4 AU, and Mars's orbit (k=4) to be at 1.6 AU. In fact their orbits are 0.38 and 1.52 AU.Ceres, the largest asteroid, lies at k=8. This law is only a rough guide, and doesn't fit all of the planets (Neptune is far closer than predicted, though Pluto lies at Neptune's predicted orbit). As of now, there is no scientific explanation for why this law "works," and many claim it is merely a coincidence. Pluto

Origin and evolution of the solar system

The current hypothesis of solar system formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant. It states the solar system was formed from a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2-3 times the mass of the Sun. Over time, the nebula began to collapse, possiby due to disturbance by a nearby supernova. This explosion sent shock waves into space, which squeezed the nebula, pushing more and more matter inward until gravitational forces overcame its internal gas pressure and it also began to collapse. As the nebula collapsed, it decreased in size, which in turn caused it to spin faster to conserve angular momentum. And as the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning pancake shape with a bulge at the center. When the nebula further condensed, a protostar was formed in the middle. This system was heated by the friction of the rocks colliding into each other. Lighter elements such as hydrogen and helium evaporated out of the centre and migrated to the edges of the disc, thus concentrating the heavier elements to form dust and rocks in the centre. These heavier elements clumped together to form planetesimals and protoplanets. In the outer regions of this solar nebula, ice and volatile gases were able to survive, and as a result, the inner planets are rocky and the outer planets were massive enough to capture large amounts of lighter gases, such as hydrogen and helium. After 100 million years, the pressures and densities of hydrogen in the centre of the collapsed nebula became great enough for the protosun to sustain thermonuclear fusion reactions. As a result of this, hydrogen was converted to helium, and a great amount of heat was released. 4×1H → 4He + neutrinos + photons During that time, the protostar turned into the Sun and the protoplanets and planetesimals were transformed into planets. All of the planets formed in a relatively short time of a few million years.

Regions of the solar system

protostar's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind) [http://quake.stanford.edu/~wso/gifs/HCS.html]. (click to enlarge) ]] According to their location, the objects in the solar system are divided into three zones: Zone I or the inner solar system, including terrestrial planets and the Main belt of asteroids; Zone II, including the giant planets, their satellites and the centaurs, and Zone III, or the outer solar system, comprising the area of the Trans-Neptunian objects including the Kuiper Belt, the Oort cloud, and the vast region in between.

Interplanetary medium

The environment in which the solar system resides is called the interplanetary medium. The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, which forms a very tenuous "atmosphere" (the heliosphere), permeating the interplanetary medium in all directions for at least ten billion (10) miles (16 Tm or 16 km) into space. Small quantities of dust are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. Some of the dust is likely interstellar dust from outside the solar system. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the Solar System, the heliospheric current sheet.

The inner planets

The four inner or terrestrial planets are characterised by their dense, rocky makeup. They formed in the hotter regions close to the Sun, where lighter and more volatile materials evaporated, leaving only those with high melting points, such as silicates, which form the planets' solid crusts and semi-liquid mantles, and iron, which forms their cores. All have impact craters and many possess tectonic surface features, such as rift valleys and volcanoes. The four inner planets are: volcanoes
- Mercury (0.39 AU from the Sun): The closest planet to the Sun is also the smallest and most atypical of the inner planets, having no atmosphere and, to date, no observed geological activity save that produced by impacts. Its relatively large iron core suggests that it was once a much larger world whose outer mantle was sheared off in early formation by the Sun’s gravity.
- Venus (0.72 AU): The first truly terrestrial planet, Venus, like the Earth, possesses a thick silicate mantle around an iron core, as well as a substantial atmosphere and evidence of one-time internal geological activity, such as volcanoes. It is much drier than Earth, and its atmosphere is 90 times as dense as Earth’s, however, and composed overwhelmingly of carbon dioxide with traces of sulfuric acid.
- Earth/Moon (1 AU): The largest of the inner planets, Earth is also the only one to demonstrate unequivocal evidence of ongoing geological activity. Its liquid hydrosphere, unique among the terrestrials, is probably the reason why Earth is also the only planet where multi-plate tectonics has been observed, since water acts as a lubricant for subduction. Its atmosphere is radically different from the other terrestrials, having been altered by the presence of life to contain 21 percent free oxygen. Its satellite, the Moon, is sometimes considered a terrestrial planet in a co-orbit with its partner, since its orbit around the Sun never actually loops back on itself when observed from above. The Moon possesses many of the features in common with other terrestrial planets, though it lacks an iron core.
- Mars (1.5 AU): Smaller than the Earth or Venus, Mars possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically active and [http://www.universetoday.com/am/publish/mars_volcanoes_active.html recent evidence] suggests it may have continued to be so until very recently. Mars possesses two tiny moons thought to be captured asteroids.

The asteroid belt

Asteroids are objects smaller than planets that mostly occupy the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun, and are composed in significant part of non-volatile minerals. The main belt contains tens of thousands (possibly millions) over 1 km across, though they can be as small as dust. Despite their large numbers, the total mass of the main asteroid belt is unlikely to be more than a thousandth that of the Earth. Asteroids with a diameter of less than 50 m are called meteoroids. The largest asteroid, Ceres, has a diameter of roughly 1000 km; large enough to be spherical, which would make it a planet by some definitions of the word. The asteroids are thought to be the remnants of a small terrestrial planet that failed to coalesce due to the gravitational interference of Jupiter. They are subdivided into asteroid groups and families based on their specific orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. Trojan asteroids are located in either of Jupiter's L4 or L5 points, though the term is also sometimes used for asteroids in any other planetary Lagrange point as well. The inner solar system is dusted with rogue asteroids, many of which cross the orbits of the inner planets.

The outer planets

The four outer planets, or gas giants, (sometimes called Jovian planets) are so large they collectively make up 99 percent of the mass known to orbit the Sun. Their large sizes and distance from the Sun meant they could hold on to much of the hydrogen and helium too light for the smaller and hotter terrestrial planets to retain.
- Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely hydrogen and helium is not very different from that of the Sun. Three of its 63 satellites, Ganymede, Io and Europa, share elements in common with the terrestrial planets, such as volcanism and internal heating. Jupiter has a faint, smoky ring.
- Saturn (9.5 AU), famous for its extensive ring system, shares many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 49 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan is the only satellite in the solar system with a substantial atmosphere.
- Uranus (19.6 AU) and Neptune (30 AU), while having many characteristics in common with the other gas giants, are nonetheless more similar to each other than they are to Jupiter or Saturn. They are both substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of “ices”, such as water, ammonia and methane. For this reason some astronomers suggested that they belong in their own category, “Uranian planets,” or “ice giants.” Both planets possess dark, insubstantial ring systems. Neptune’s largest moon Triton is geologically active. Centaurs are icy comet-like bodies that have less-eccentric orbits so that they remain in the region between Jupiter and Neptune. The first centaur to be discovered, 2060 Chiron, has been called a comet since it has been shown to develop a tail, or coma, just as comets do when they approach the sun.

The trans-Neptunian region

The area beyond Neptune, often referred to as the outer solar system or simply the "trans-Neptunian region", is still largely unexplored.

The Kuiper belt

This region's first formation, which actually begins inside the orbit of Neptune, is the Kuiper belt, a great ring of debris, similar to the asteroid belt but composed mainly of ice and far greater in extent, which lies between 30 to 50 AU from the Sun. This region is thought to be the place of origin for short-period comets, such as Halley's comet. Though there are estimated to be over 70,000 Kuiper belt objects with a diameter greater than 100 km, the total mass of the Kuiper belt is relatively low, perhaps equalling or just exceeding the mass of the Earth. Many Kuiper belt objects have orbits that take them outside the plane of the ecliptic.
- Pluto, the solar system's smallest planet, is considered to be part of the Kuiper Belt population. Like others in the belt, it has a relatively eccentric orbit inclined 17 degrees to the ecliptic and ranging from 29.7 AU from the Sun at perihelion to 49.5 AU at aphelion. It has a large moon (the largest in the solar system relative to its own size), called Charon, and, new observations suggest, two other, much smaller moons. Like the Earth/Moon, Pluto and Charon are often considered a double planet. A member of the traditional nine planets, Pluto's tiny mass (less than 1% of Earth's) and diameter have called this status into question. Kuiper belt objects with Pluto-like orbits are called Plutinos. Other Kuiper belt objects have resonant orbits and are grouped accordingly. The remaining Kuiper belt objects, in more "classical" orbits, are classified as Cubewanos. The Kuiper Belt has a very sharply defined edge. At around 49 AU, a sharp dropoff occurs in the number of objects observed. This dropoff is known as the "Kuiper Cliff", and as yet its cause is unknown. Some speculate that something must exist beyond the belt large enough to sweep up the remaining debris, perhaps as large as Earth or Mars. This view is still controversial, however.

The scattered disc

Overlapping the Kuiper belt but extending much further outwards is the scattered disc. Scattered disc objects are believed to have been originally native to the Kuiper belt, but were ejected into erratic orbits in the outer fringes. One particular scattered disc object, originally found in 2003 but confirmed two years later by Mike Brown, has renewed the old debate about what constitutes a planet since, though its size is not yet known, it is almost certainly larger than Pluto. It currently has no name, but has been given the provisional designation , and has been nicknamed "Xena" by its discoverers, after the television character. It has many similarities with Pluto: its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and is steeply inclined to the ecliptic plane, indeed, at 44 degrees, more so than any known object in the solar system. Like Pluto, it is believed to consist largely of rock and ice, and has a [http://www.gps.caltech.edu/%7Embrown/planetlila/moon/index.html moon]. Whether it and the largest Kuiper belt objects should be considered planets or whether instead Pluto should be reclassified as a minor planet has not yet been resolved.

A new region?

Sedna, the newly discovered Pluto-like object with a gigantic, highly elliptical 10,500-year orbit that takes it from about 76 to 928 AU, has too distant a perihelion to be a scattered member of the Kuiper Belt and could be the first in an entirely new population. is also believed to be a member of this population.

Comets

Comets are composed largely of volatile ices and have highly eccentric orbits, generally having a perihelion within the orbit of the inner planets and an aphelion far beyond Pluto. Short-period comets exist with apoapses closer than this, however, and old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids. Long period comets have orbits lasting thousands of years. Some comets with hyperbolic orbits may originate outside the solar system.

And beyond

The point at which the solar system ends and interstellar space begins is not precisely defined, since its outer boundaries are delineated by two separate forces: the solar wind and the Sun's gravity. gravity The heliosphere expands outward in a great bubble to about 95 AU, or three times the orbit of Pluto. The edge of this bubble is known as the termination shock; the point at which the solar wind collides with the opposing winds of the interstellar medium. Here the wind slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath that looks and behaves very much like a comet's tail; extending outward for a further 40 AU at its stellar-windward side, but tailing many times that distance in the opposite direction. The outer boundary of the sheath, the heliopause, is the point at which the solar wind finally terminates, and one enters the environment of interstellar space. Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way. But even at this point, we could not be said to have left the solar system, for the Sun's gravity will still hold sway even up to the Oort cloud, the great mass of icy objects, currently hypothetical, believed to be the source for all long-period comets and to surround our solar system like a shell from 50,000 to 100,000 AU beyond the Sun, or almost halfway to the next star system. The vast majority of the solar system, therefore, is completely unknown.

Age of the solar system

Scientists estimate that the solar system is 4.6 billion years old. To calculate this figure, they examine an unstable element, which is subject to radioactive decay. By observing how much this element has decayed, they can calculate how old this element is. The oldest rocks on earth are approximately 3.9 billion years old, however it is hard to find these rocks as the earth has been thoroughly resurfaced. To estimate the age of the solar system, scientists must find rocks from space, such as meteorites – which are formed during the early condensation of the solar nebula. The oldest meteorite was found to have an age of 4.6 billion years, hence the solar system must be around 4.6 billion years old.

Galactic orbit of the solar system

The solar system is part of the Milky Way galaxy, a spiral galaxy with a diameter of about 100,000 light years containing approximately 200 billion stars, of which our Sun is rather large and bright. (The vast majority of stars are red dwarfs; our Sun is placed near the middle of the Hertzsprung-Russell diagram, but stars larger and hotter than it are rare, whereas stars dimmer and cooler than it are very common, although we can observe only those few other red dwarfs that are very near our Sun in space). Estimates place the solar system at between 25,000 and 28,000 light years from the galactic center in the Orion Arm. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years. At the galactic location of the solar system, the escape velocity with regard to the gravity of the Milky Way is about 1000 km/s. The solar system appears to have a very unusual orbit. It is both extremely close to being circular, and at nearly the exact distance at which the orbital speed matches the speed of the compression waves that form the spiral arms. The solar system appears to have remained between spiral arms for most of the existence of life on Earth. The radiation from supernovae in spiral arms could theoretically sterilize planetary surfaces, preventing the formation of large animal life on land. By remaining out of the spiral arms, Earth may be unusually free to form large animal life on its surface.

Planetary system formation

For many years, our solar system had the only planetary system known, and so theories of planetary formation only had to explain one system to be plausible. The discovery in recent years of many extrasolar planets has uncovered systems very different to our own, and theories have had to be revised accordingly. Exoplanets have not been seen by astronomers yet, however we know they exist because of the gravitational tug the planets induce on the star, and hence making the star ‘wobble’. Astronomers can calculate how massive the planets are by observing how much the star wobbles. Exoplanets can also be observed more directly by their occultation of the stars' discs, which dims them slightly. In October, 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of a massive planet orbiting 51 Pegasi – a Sun-like star in the constellation Pegasus. This planet is about half as massive as Jupiter, and had an orbital period of 4.2 Earth days, due to its closeness to the star (0.05 AU). Since then, over 160 more planets have been identified. Many extrasolar planetary systems contain such a “hot Jupiter”: a planet comparable to or larger than Jupiter orbiting very close to the parent star, perhaps orbiting it in a matter of days. It has been hypothesised that while the giant planets in these systems formed in the same place as the gas giants in our system did, some sort of migration took place which resulted in the giant planet spiralling in towards the parent star. Any terrestrial planets which had previously existed would presumably either be destroyed or ejected from the system. There has also been some photographic evidence to suggest that regions in the Orion Nebula, which is 1500 light years from Earth, have star systems forming.

Discovery of the solar system

The planets out to Saturn were known to ancient astronomers, who observed the wandering of these objects against the apparently fixed pattern of stars. Venus and Mercury were each identified as single objects despite the difficulty of connecting "evening" and "morning stars". It was also identified that the two non-pointlike objects, the sun and the Moon, moved across the same fixed background. However knowledge of the nature of these celestial drifters was entirely speculative and largely incorrect. The nature and structure of the solar system were long misperceived, for at least two reasons:
- The Earth was considered stationary, and the motion of objects in the sky was therefore taken at face value: the sun was thought to orbit the Earth, for example (This conception of the universe, in which the Earth is at the center, is called the Geocentric model; geos means "Earth" in Greek).
- Many solar system objects and phenomena cannot be perceived at all without technical aid. Over the last several hundred years, conceptual and technological advances have helped us understand the solar system much better. The first and most fundamental of the conceptual advances was the Copernican Revolution, which proposed that the planets orbit the sun—models of the solar system with the sun in the center are called heliocentric (helios meaning "Sun" in Greek). Despite the name, the most striking (and then-controversial) Copernican realization was not that the sun was central but that the Earth was peripheral, orbital: planets had been considered merely points in the sky, but if the Earth itself was a planet, perhaps the other planets were, like Earth, huge solid spheres. Philosophically, there were a number of objections to heliocentrism:
- If the Earth is moving, what force keeps the air from flying off into space?
- The Earth is made of heavy rock. Heavy rock moves down. Down in a sphere means the centre. The planets are ephemeral and light, so they are above. How can Earth be a planet?
- If the Earth is mobile, then why do we not observe parallax in the stars (the stars appearing to shift in relation to further objects due to the change in position)? The subsequent invention of the telescope gave the principal technological advance on discovering the solar system, with Galileo's improved version of the telescope rapidly giving benefit in terms of discovering satellites of other planets, especially Jupiter's four major satellites. This showed that all objects in the universe did not orbit the Earth. However, perhaps Galileo's most important discovery was that the planet Venus has phases like the Moon, proving that it must orbit the Sun. Then, in 1687, Isaac Newton devised his law of universal gravitation which explained the force that both kept the Earth moving through the heavens and also kept the air from flying away. Finally, in 1838, astronomer Friedrich Wilhelm Bessel successfully measured the parallax of the star 61 Cygni, proving conclusively that the Earth was in motion.

Exploration of the solar system

Since the start of the space age, a great deal of exploration has been performed by unmanned space missions that have been organized and executed by various space agencies. The first probe to land on another solar system body was the Soviet Union's Luna 2 probe, which impacted on the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on Venus in 1965, Mars in 1976, the asteroid 433 Eros in 2001, and Saturn's moon Titan in 2005. Spacecraft have also made close approaches to other planets: Mariner 10 passed Mercury in 1973. The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980–1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far beyond Pluto's orbit, and astronomers anticipate that they will encounter the heliopause which defines the outer edge of the solar system in the next few years. Pluto remains the only planet not having been visited by a man-made spacecraft, though that will change with the launching of New Horizons by NASA in January 2006. It is scheduled to fly by Pluto in July 2015 and then make an extensive study of as many Kuiper Belt objects as it can. Through these unmanned missions, we have been able to get close-up photographs of most of the planets and, in the case of landers, perform tests of their soils and atmospheres. Manned exploration, meanwhile, has only taken human beings as far as the Moon, in the Apollo program. The last manned landing on the Moon took place in 1972, but the recent discovery of ice in deep craters in the polar regions of the Moon has prompted speculation that mankind may return to the Moon in the next decade or so. Manned missions to Mars have been eagerly anticipated by generations of space enthusiasts, and it was hoped that the first manned interplanetary flights would take place in the 1980s, after the successful Apollo program. Europe (ESA and EU) now plans manned Lunar and Mars missions as part of Aurora Exploration Programme endorsed in 2001. United States followed with similar programme called Vision for Space Exploration in 2004.

Attributes of major planets

All attributes below are measured relative to the Earth: Of the other objects, Ganymede has the largest mass (0.02). Note: Although is a minor planet, it is being considered as possibly being a major planet (the tenth in the solar system). See Planet (Table) for a more comprehensive table.

Attributes of the largest minor planets

The largest minor planets are smoothly rounded, like planets, because their gravity overcomes material strength that keeps smaller bodies in non-spherical shapes. Before the discovery of 2060 Chiron and the trans-Neptunian objects, the term "minor planet" was a synonym for asteroid, but many people now prefer to restrict the use of "asteroid" to refer to rocky bodies of the inner solar system. Most trans-Neptunian objects are icy, like comets, although those we can detect at that distance are much larger than comets. Several asteroids, in the strict sense, are large enough to be spherical. The largest known trans-Neptunian objects are much larger than the large asteroids. (Natural satellites of major planets also range smoothly from small non-spherical objects to large spherical ones, and the largest are larger than 1 Ceres, the largest asteroid). All attributes below are measured relative to the Earth:

Other facts

The total surface area of the solar system's objects that have solid surfaces and a diameter greater than 1 km is ~1.7 km2 —about 11 times the area of the Earth's land masses. It has been suggested that the Sun may be part of a binary star system, with a distant companion named Nemesis. Nemesis was proposed to explain some timing regularities of the great extinctions of life on Earth. The hypothesis says that Nemesis creates periodical perturbations in the Oort cloud of comets surrounding the solar system, causing a "comet shower". Some of them hit Earth, causing destruction of life. This hypothesis is no longer taken seriously by most scientists, mostly because infrared surveys failed to spot any such object, which should have been very conspicuous at those wavelengths. The concept of the tenth planet has frequently been explored in science fiction works and conspiracy theories (see also Planet X, and hypothetical planet).

The solar system in small scales

Scaling down the size of the solar system makes it easier for students to grasp the relative distances. The enormous ratio of interplanetary distances to planetary diameters makes constructing a scale model of the solar system a challenging task. (For example, the distance between the Earth and the Sun is almost 12,000 times the diameter of the Earth.) Several places have built such models.

The solar system in astrology

See also


- Astronomical symbols
- Definition of planet
- Geological features of the Solar System
- Laws of Kepler
- :Category:Lists of Solar system objects
- Minor planet
- Numerical model of solar system
- Origin of life
- Planetary system
- Planetary nomenclature
- Solar system by size
- Stellar system
- Table of planetary attributes
- Timeline of solar system astronomy
- Titius-Bode law
- Zodiacal light

External links


- [http://solarsystem.nasa.gov/index.cfm NASA's Solar System Exploration site]
- [http://space.jpl.nasa.gov NASA's Solar System Simulator]
- [http://www.jpl.nasa.gov/solar_system NASA/JPL Solar System main page]
- [http://members.aol.com/astroequation/ Astronomical Enigma] Mathematical Order in the orbits of the solar system.
- [http://www.solarviews.com Solarviews]
- [http://celestia.sourceforge.net Celestia] Free 3D realtime space-simulation (OpenGL)
- [http://www.nineplanets.org/ The Nine Planets] Comprehensive solar system site by Bill Arnett
- [http://www.krysstal.com/solarsys_planets.html Planetary data]
- [http://www.solstation.com/habitable.htm Stars and Habitable Planets]
- [http://www.michaelschultz.de/index_en.html Solar System] An interactive planets animation (145 zoom steps and time effects)
- [http://my.execpc.com/~culp/space/timeline.html Timeline of solar system exploration]
- [http://www.anzwers.org/free/universe/index.html An Atlas of the Universe]
- mirror matter [http://uk.arxiv.org/abs/astro-ph/0104251 planets] and other [http://uk.arxiv.org/abs/astro-ph/0110161 mirror objects] in the solar system?
- [http://www.solarsystem.org.uk/ The Virtual Solar System, including a scale model of the system]
- ko:태양계 ms:Sistem suria ja:太陽系 simple:Solar system th:ระบบสุริยะ zh-min-nan:Thài-iông-hē

Astronomical object

Solar system Extrasolar objects
Simple objects Compound objects Extended objects

- Sun
- Planetary system
  - Planets
    - Mercury
    - Venus
    - Earth
      - Moon
    - Mars
      - satellites
    - Jupiter
      - satellites
    - Saturn
      - satellites
    - Uranus
      - satellites
    - Neptune
      - satellites
    - Pluto
      - satellites
  - Asteroids
    - "Vulcanoids"
    - "Apoheles"
    - Near-Earth asteroids
      - "Arjunas"
      - Atens
      - Apollos
      - Amors
    - Mars-crossers
    - Asteroid belt
      - Hungarias
      - Phocaeas
      - Nysas
      - Alindas
      - Hildas
      - Pallas
      - Marias
      - Koronis
      - Eos
      - Themis
      - Griquas
      - Cybeles
      - Thule
    - Trojan asteroids
      - Mars trojans
      - Jupiter trojans
      - Neptune trojans
    - Outer planet crossers
    - Damocloids
    - Centaurs
  - Trans-Neptunian objects
    - Kuiper Belt
      - Cubewanos
      - Plutinos
      - Twotinos
      - Scattered Disk Objects
      - Sedna
  - Comets
    - Periodic comets
    - Long-period comets
    - Lost comets
  - Oort cloud
  - Meteoroids
    - Meteors
    - Meteor showers
- Exoplanets
  - Very hot Jupiters
  - Hot Jupiters
  - Eccentric Jupiters
  - Pulsar planets
  - Hot Neptunes / Super-Earths
  - Transiting planets
  - Rogue / Interstellar planets
  - Hypothetical planet types
    - Chthonian planets
    - Oceanic planets
    - Trojan planets
- Brown dwarfs
  - Lithium dwarfs
  - Methane dwarfs
  - Sub-brown dwarfs
- Stars by spectral type
  - Blue stars
  - Blue-white stars
  - White stars
  - Yellow-white stars
  - Yellow stars
  - Orange stars
  - Red stars
  - Peculiar stars
    - Carbon stars
      - C-type stars
      - S-type stars
    - Shell stars
    - Wolf-Rayet stars
    - Peculiar A-type stars
    - Metallic A-type stars
    - Barium stars
    - P Cygni stars
    - Blue stragglers
- Stars by luminosity class
  - Subdwarf stars
  - Dwarf (Main sequence) stars
  - Subgiant stars
  - Giant stars
  - Bright giant stars
  - Supergiant stars
  - Hypergiant stars
- Stars by population
  - Population III stars
  - Population II stars
    - Halo stars
    - Thick disk stars
  - Population I stars
- Stars by stellar evolution
  - Protostars
  - Young stellar objects
  - Main sequence stars
  - Red giant stars
  - Red supergiant stars
  - Blue supergiant stars
  - Wolf-Rayet stars
  - White dwarf stars
  - Neutron stars
- Variable stars
  - Intrinsic variables
    - Pulsating variables
      - Cepheid variables
      - W Virginis variables
      - Delta Scuti variables
      - RR Lyrae variables
      - Mira variables
      - Semiregular variables
      - Irregular variables
      - Beta Cephei variables
      - Alpha Cygni variables
      - RV Tauri variables
    - Eruptive variables
      - Flare stars
      - T Tauri variables
      - FU Orionis variables
      - R Coronae Borealis variables
      - Luminous blue variables
    - Cataclysmic variables
      - Symbiotic variables
      - Dwarf novae
      - Novae
      - Supernovae
      -
- Type I supernovae
      -
- Type II supernovae
      - Gamma ray bursts
      -
- Long GRBs
      -
  - Collapsars
      -
  - Hypernovae
      -
- Short GRBs
      -
- Dark GRBs
  - Extrinsic variables
    - Rotating variables
      - Alpha2 CVn stars
      - Rotating ellipsoidal variables
    - Eclipsing binaries
      - Algol stars
      - Beta Lyrae stars
      - W Ursae Majoris stars
- Compact stars
  - White dwarfs
    - Black dwarfs
  - Neutron stars
    - Magnetars
    - Pulsars
  - Strange stars
  - Black holes
  - Supermassive black holes
- Planetary systems
- Star systems
  - Single star systems
    - Solar system
  - Multiple star systems
    - Binary stars
      - Optical binaries
      - Visual binaries
      - Astrometric binaries
      - Spectroscopic binaries
      - Eclipsing binaries
      - Close binaries
      -
- Detached binaries
      -
- Semidetached binaries
      -
- Contact binaries
      -
- Unresolved binaries
      - X-ray bursters
    - Triple star systems
- Stellar groupings
  - Star clusters
    - Stellar associations
    - Open clusters
    - Globular clusters
  - Constellations
  - Asterisms
- Galaxy components
  - Galactic bulges
    - Galactic bars
  - Galactic rings
  - Spiral arms
  - Thin disks
  - Thick disks
  - Galactic halos
  - Galactic coronae
- Galaxies
  - Galaxies by morphology
    - Spiral galaxies
    - Barred spiral galaxies
    - Lenticular galaxies
    - Elliptical galaxies
    - Ring galaxies
    - Irregular galaxies
  - Galaxies by size
    - Giant diffuse galaxies
    - Giant ellipticals
    - Dwarf galaxies
    - Ultracompact dwarfs
  - Active galaxies
    - Quasars
      - Blazars
    - Radio galaxies
    - Seyfert galaxies
    - Starburst galaxies
  - Dark galaxies
- Galaxy clusters
- Galaxy cluster clouds
- Superclusters
- Filaments / Voids
- Circumstellar matter
  - Dust disks
  - Interplanetary medium
  - Protoplanetary disks
- Interstellar medium
- Nebulae
  - Emission nebulae
    - Planetary nebulae
    - Supernova remnants
    - Plerions
    - H II regions
  - Reflection nebulae
  - Dark nebulae
    - Molecular clouds
    - Bok globules
    - Proplyds
  - H I regions
- Intergalactic medium
- Cosmic microwave background radiation
- Dark matter
  - MACHOs
  - WIMPs

See also


- Lists of astronomical objects ko:천체 ja:天体 th:วัตถุท้องฟ้า

Gravity

Gravity is the force of attraction between massive particles. Weight is determined by the mass of an object and its location in a gravitational field. While a great deal is known about the properties of gravity, the ultimate cause of the gravitational force remains an open question. General relativity is the most successful theory of gravitation to date. It postulates that mass and energy curve space-time, resulting in the phenomenon known as gravity. The effect of the bending of spacetime is often misunderstood as most people seem to prefer to think of a falling object as accelerating when the facts do not support that assumption. Skydivers do not feel any acceleration (other than from wind resistance). Gravity is acceleration. F=\dot=m\dot+\dotv\,\! means (if the mass is unvarying) that there must be a force that causes a mass to accelerate. For a rocket ship, that is the rocket engine. For the earth, it is the compression of the mass between something on the surface of the earth and the earth's center of mass. The acceleration is in relation to spacetime in that the weight one feels is one's resistance to deviating from one's path in spacetime. The same holds true in the rocket ship except that a rocket engine supplies the force to accelerate an occupant from his spacetime path. There is no difference between the weight he feels because of gravity or the rocket.

Newton's law of universal gravitation

Newton's law of universal gravitation states the following: :Every object in the Universe attracts every other object with a force directed along the line of centers of mass for the two objects. This force is proportional to the product of their masses and inversely proportional to the square of the separation between the centers of mass of the two objects. Given that the force is along the line through the two masses, the law can be stated symbolically as follows. :F = - G \frac where: :F is the magnitude of the (repulsive) gravitational force between two objects :G is the gravitational constant, that is approximately : G = 6.67 × 10−11 N m2 kg-2 :m1 is the mass of first object :m2 is the mass of second object :r is the distance between the objects It can be seen that this repulsive force F is always negative, and this means that the net attractive force is positive. The minus sign is used to hold the same value meaning as in the Coulomb's Law, where a positive force as result means repulsion between two charges. Thus gravity is proportional to the mass of each object, but has an inverse square relationship with the distance between the centres of each mass. Strictly speaking, this law applies only to point-like objects. If the objects have spatial extent, the force has to be calculated by integrating the force (in vector form, see below) over the extents of the two bodies. It can be shown that for an object with a spherically-symmetric distribution of mass, the integral gives the same gravitational attraction on masses outside it as if the object were a point mass.1 This law of universal gravitation was originally formulated by Isaac Newton in his work, the Principia Mathematica (1687). Professor William Whewell of Cambridge University, author of History of the Inductive Sciences (1837) stated: ::The law of gravitation is indisputably and incomparably the greatest scientific discovery ever made, whether we look at the advance which it involved, the extent of the truth disclosed, or the fundamental and satisfactory nature of this truth. [In A Treasury of Science ed. Harlow Shapley et al, Harper & Bros. NY: 1946] The history of gravitation as a physical concept is considered in more detail below.

Vector form

below Newton's law of universal gravitation can be written as a vector equation to account for the direction of the gravitational force as well as its magnitude. In this formula, quantities in bold represent vectors. : \mathbf_ = G \, \mathbf_ or \mathbf_ = - G \, \mathbf_ where :F12 is the force on object 1 due to object 2 :G is the gravitational constant :m1 and m2 are the masses of the objects 1 and 2 :r21 = | r2 − r1 | is the distance between objects 2 and 1 : \mathbf_ \equiv \frac is the unit vector from object 2 to 1 It can be seen, that the vector form of the equation is the same as the scalar form, except for the vector value of F and the unit vector. Also, it can be seen that F12 = − F21. Gravitational acceleration is given by the same formula except for one of the factors m: : \mathbf = G \, \mathbf

Gravitational field

The gravitational field is a vector field that describes the gravitational force an object of given mass experiences in any given place in space. It is a generalization of the vector form, which becomes particularly useful if more than 2 objects are involved (such as a rocket between the Earth and the Moon). For 2 objects (e.g. object 1 is a rocket, object 2 the Earth), we simply write \mathbf r instead of \mathbf r_ and m instead of m_1 and define the gravitational field \mathbf g(\mathbf r) as: : \mathbf g(\mathbf r) = G \, \mathbf so that we can write: :\mathbf( \mathbf r) = m \mathbf g(\mathbf r) This formulation is independent of the objects causing the field. The field has units of force divided by mass; in SI, this is N·kg−1.

Problems with Newton's theory

Although Newton's formulation of gravitation is quite accurate for most practical purposes, it has a few problems:

Theoretical concerns


- There is no prospect of identifying the mediator of gravity. Newton himself felt the inexplicable action at a distance to be unsatisfactory (see "Newton's reservations" below).
- Newton's theory requires that gravitational force is transmitted instantaneously. Given classical assumptions of the nature of space and time, this is necessary to preserve the conservation of angular momentum observed by Johannes Kepler. However, it is in direct conflict with Einstein's theory of special relativity which places an upper limit—the speed of light in vacuum—on the velocity at which signals can be transmitted.

Disagreement with observation


- Newton's theory does not fully explain the precession of the perihelion of the orbit of the planet Mercury. There is a 43 arcsecond per century discrepancy between the Newtonian prediction (resulting from the gravitational tugs of the other planets) and the observed precession.
- The predicted deflection of light by gravity is only half as much as observations of this deflection, which were made after General Relativity was developed in 1915.
- The observed fact that gravitational and inertial masses are the same for all bodies is unexplained within Newton's system. General relativity takes this as a postulate. See equivalence principle.

Newton's reservations

It's important to understand that while Newton was able to formulate his law of gravity in his monumental work, he was deeply uncomfortable with the notion of "action at a distance" which his equations implied. He never, in his words, "assigned the cause of this power". In all other cases, he used the phenomenon of motion to explain the origin of various forces acting on bodies, but in the case of gravity, he was unable to experimentally identify the motion that produces the force of gravity. Moreover, he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was contrary to sound science. He lamented the fact that "philosophers have hitherto attempted the search of nature in vain" for the source of the gravitational force, as he was convinced "by many reasons" that there were "causes hitherto unknown" that were fundamental to all the "phenomena of nature". These fundamental phenomena are still under investigation and, though hypotheses abound, the definitive answer is yet to be found. While it is true that Einstein's hypotheses are successful in explaining the effects of gravitational forces more precisely than Newton's in certain cases, he too never assigned the cause of this power, in his theories. It is said that in Einstein's equations, "matter tells space how to curve, and space tells matter how to move", but this new idea, completely foreign to the world of Newton, does not enable Einstein to assign the "cause of this power" to curve space any more than the Law of Universal Gravitation enabled Newton to assign its cause. In Newton's own words: :I wish we could derive the rest of the phenomena of nature by the same kind of reasoning from mechanical principles; for I am induced by many reasons to suspect that they may all depend upon certain forces by which the particles of bodies, by some causes hitherto unknown, are either mutually impelled towards each other, and cohere in regular figures, or are repelled and recede from each other; which forces being unknown, philosophers have hitherto attempted the search of nature in vain. If science is eventually able to discover the cause of the gravitational force, Newton's wish could eventually be fulfilled as well. It should be noted that here, the word "cause" is not being used in the same sense as "cause and effect" or "the defendant caused the victim to die". Rather, when Newton uses the word "cause," he (apparently) is referring to an "explanation". In other words, a phrase like "Newtonian gravity is the cause of planetary motion" means simply that Newtonian gravity explains the motion of the planets. See Causality and Causality (physics).

Einstein's theory of gravitation

Einstein's theory of gravitation answered the problems with Newton's theory noted above. In a revolutionary move, his theory of general relativity (1915) stated that the presence of mass, energy, and momentum causes spacetime to become curved. Because of this curvature, the paths that objects in inertial motion follow can "deviate" or change direction over time. This deviation appears to us as an acceleration towards massive objects, which Newton characterized as being gravity. In general relativity however, this acceleration or free fall is actually inertial motion. So objects in a gravitational field appear to fall at the same rate due to their being in inertial motion while the observer is the one being accelerated. (This identification of free fall and inertia is known as the Equivalence principle.) The relationship between the presence of mass/energy/momentum and the curvature of spacetime is given by the Einstein field equations. The actual shapes of spacetime are described by solutions of the Einstein field equations. In particular, the Schwarzschild solution (1916) describes the gravitational field around a spherically symmetric massive object. The geodesics of the Schwarzschild solution describe the observed behavior of objects being acted on gravitationally, including the anomalous perihelion precession of Mercury and the bending of light as it passes the Sun. Arthur Eddington found observational evidence for the bending of light passing the Sun as predicted by general relativity in 1919. Subsequent observations have confirmed Eddington's results, and observations of a pulsar which is occulted by the Sun every year have permitted this confirmation to be done to a high degree of accuracy. There have also in the years since 1919 been numerous other tests of general relativity, all of which have confirmed Einstein's theory.

Units of measurement and variations in gravity

tests of general relativity. (ESA image)]] Gravitational phenomena are measured in various units, depending on the purpose. The gravitational constant is measured in newtons times metre squared per kilogram squared. Gravitational acceleration, and acceleration in general, is measured in metres per second squared or in non-SI units such as galileos, gees, or feet per second squared. The acceleration due to gravity at the Earth's surface is approximately 9.81 m/s2, more precise values depending on the location. A standard value of the Earth's gravitational acceleration has been adopted, called gn. When the typical range of interesting values is from zero to tens of metres per second squared, as in aircraft, acceleration is often stated in multiples of gn. When used as a measurement unit, the standard acceleration is often called "gee", as g can be mistaken for g, the gram symbol. For other purposes, measurements in millimetres or micrometres per second squared (mm/s² or µm/s²) or in multiples of milligals or milligalileos (1 mGal = 1/1000 Gal), a non-SI unit still common in some fields such as geophysics. A related unit is the eotvos, which is a cgs unit of the gravitational gradient. Mountains and other geological features cause subtle variations in the Earth's gravitational field; the magnitude of the variation per unit distance is measured in inverse seconds squared or in eotvoses. Typical variations with time are 2 µm/s² (0.2 mGal) during a day, due to the tides, i.e. the gravity due to the Moon and the Sun. A larger variation in the effect of gravity occurs when we move from the equator to the poles. The effective force of gravity decreases as the distance from the equator decreases, due to the rotation of the Earth, and the resulting centrifugal force and flattening of the Earth. The centrifugal force causes an effective force 'up' which effectively counteracts gravity, while the flattening of the Earth causes the poles to be closer to the center of mass of the Earth. It is also related to the fact that the Earth's density changes from the surface of the planet to its centre. The sea-level gravitational acceleration is 9.780 m/s² at the equator and 9.832 m/s² at the poles, so an object will exert about 0.5% more force due to gravity at sea level at the poles than at sea level at the equator [http://curious.astro.cornell.edu/question.php?number=310].

Comparison with electromagnetic force

The gravitational interaction of protons is approximately a factor 1036 weaker than the electromagnetic repulsion. This factor is independent of distance, because both interactions are inversely proportional to the square of the distance. Therefore on an atomic scale mutual gravity is negligible. However, the main interaction between common objects and the Earth and between celestial bodies is gravity, because at this scale matter is electrically neutral: even if in both bodies there were a surplus or deficit of only one electron for every 1018 protons and neutrons this would already be enough to cancel gravity (or in the case of a surplus in one and a deficit in the other: double the interaction). However, the main interactions between the charged particles in cosmic plasma (that makes up over 99% of the universe by volume), are electromagnetic forces. In terms of Planck units: the charge of a proton is 0.085, while the mass is only . From that point of view, the gravitational force is not small as such, but because masses are small. The relative weakness of gravity can be demonstrated with a small magnet picking up pieces of iron. The small magnet is able to overwhelm the gravitational interaction of the entire Earth. Similarly, when doing a chin-up, the electromagnetic interaction within your muscle cells is able to overcome the force induced by Earth on your entire body. Gravity is small unless at least one of the two bodies is large or one body is very dense and the other is close by, but the small gravitational interaction exerted by bodies of ordinary size can fairly easily be detected through experiments such as the Cavendish torsion bar experiment. Cavendish torsion bar experiment Further reading
- Jefimenko, Oleg D., "Causality, electromagnetic induction, and gravitation : a different approach to the theory of electromagnetic and gravitational fields". Star City [West Virginia] : Electret Scientific Co., c1992. ISBN 0917406095
- Heaviside, Oliver, "[http://www.as.wvu.edu/coll03/phys/www/Heavisid.htm A gravitational and electromagnetic analogy]". The Electrician, 1893.

Gravity and quantum mechanics

It is strongly believed that three of the four fundamental forces (the strong nuclear force, the weak nuclear force, and the electromagnetic force) are manifestations of a single, more fundamental force. Combining gravity with these forces of quantum mechanics to create a theory of quantum gravity is currently an important topic of research amongst physicists. General relativity is essentially a geometric theory of gravity. Quantum mechanics relies on interactions between particles, but general relativity requires no exchange of particles in its explanation of gravity. Scientists have theorized about the graviton (a messenger particle that transmits the force of gravity) for years, but have been frustrated in their attempts to find a consistent quantum theory for it. Many believe that string theory holds a great deal of promise to unify general relativity and quantum mechanics, but this promise has yet to be realized. It is notable that in general relativity gravitational radiation (which under the rules of quantum mechanics must be composed of gravitons) is only created in situations where the curvature of spacetime is oscillating, such as for co-orbiting objects. The amount of gravitational radiation emitted by the solar system and its planetary systems is far too small to measure. However, gravitational radiation has been indirectly observed as an energy loss over time in binary pulsar systems such as PSR1913+16). It is believed that neutron star mergers and black hole formation may create detectable amounts of gravitational radiation. Gravitational radiation observatories such as LIGO have been created to study the problem. No confirmed detections have been made of this hypothetical radiation, but as the science behind LIGO is refined and as the instruments themselves are endowed with greater sensitivity over the next decade, this may change.

Experimental tests of theories

Today General Relativity is accepted as the standard description of gravitational phenomena. (Alternative theories of gravitation exist but are more complicated than General Relativity.) General Relativity is consistent with all currently available measurements of large-scale phenomena. For weak gravitational fields and bodies moving at slow speeds at small distances, Einstein's General Relativity gives almost exactly the same predictions as Newton's law of gravitation. Crucial experiments that justified the adoption of General Relativity over Newtonian gravity were the classical tests: the gravitational redshift, the deflection of light rays by the Sun, and the precession of the orbit of Mercury. More recent experimental confirmations of General Relativity were the (indirect) deduction of gravitational waves being emitted from orbiting binary stars, the existence of neutron stars and black holes, gravitational lensing, and the convergence of measurements in observational cosmology to an approximately flat model of the observable Universe, with a matter density parameter of approximately 30% of the critical density and a cosmological constant of approximately 70% of the critical density. The equivalence principle, the postulate of general relativity that presumes that inertial mass and gravitational mass are the same, is also under test. Past, present, and future tests are discussed in the equivalence principle section. Even to this day, scientists try to challenge General Relativity with more and more precise direct experiments. The goal of these tests is to shed light on the yet unknown relationship between Gravity and Quantum Mechanics. Space probes are used to either make very sensitive measurements over large distances, or to bring the instruments into an environment that is much more controlled than it could be on Earth. For example, in 2004 a dedicated satellite for gravity experiments, called Gravity Probe B, was launched to test general relativity's predicted frame-dragging effect, among others. Also, land-based experiments like LIGO and a host of "bar detectors" are trying to detect gravitational waves directly. A space-based hunt for gravitational waves, LISA, is in its early stages. It should be sensitive to low frequency gravitational waves from many sources, perhaps including the Big Bang. Speed of gravity: Einstein's theory of relativity predicts that the speed of gravity (defined as the speed at which changes in location of a mass are propagated to other masses) should be consistent with the speed of light. In 2002, the Fomalont-Kopeikin experiment produced measurements of the speed of gravity which matched this prediction. However, this experiment has not yet been widely peer-reviewed, and is facing criticism from those who claim that Fomalont-Kopeikin did nothing more than measure the speed of light in a convoluted manner. The Pioneer anomaly is an empirical observation that the positions of the Pioneer 10 and Pioneer 11 space probes differ very slightly from what would be expected according to known effects (gravitational or otherwise). The possibility of new physics has not been ruled out, despite very thorough investigation in search of a more prosaic explanation.

Recent Alternative theories


- Brans-Dicke theory of gravity
- Rosen bi-metric theory of gravity
- In the modified Newtonian dynamics (MOND), Mordehai Milgrom proposes a modification of Newton's Second Law of motion for small accelerations.

Historical Alternative theories


- Nikola Tesla challenged Albert Einstein's theory of relativity, announcing he was working on a Dynamic theory of gravity (which began between 1892 and 1894) and argued that a "field of force" was a better concept and focused on media with electromagnetic energy that fill all of space.
- In 1967 Andrei Sakharov proposed something similar, if not essentially identical. His theory has been adopted and promoted by Messrs. Haisch, Rueda and Puthoff who, among other things, explain that gravitational and inertial mass are identical and that high speed rotation can reduce (relative) mass. Combining these notions with those of T. T. Brown, it is relatively easy to conceive how field propulsion vehicles such as "flying saucers" could be engineered given a suitable source of power.
- Georges-Louis LeSage proposed a gravity mechanism, now commonly called LeSage gravity, based on a fluid-based explanation where a light gas fills the entire universe.

Self-gravitating system

A self-gravitating system is a system of masses kept together by mutual gravity. An example is a binary star.

Special applications of gravity

A height difference can provide a useful pressure in a liquid, as in the case of an intravenous drip or a water tower, and can even supply enough power for hydroelectricity. A weight hanging from a cable over a pulley provides a constant tension in the cable, also in the part on the other side of the pulley. pulley Dubuque, Iowa]] Molten lead, when poured into the top of a shot tower, will coalesce into a rain of spherical lead shot, first separating into droplets, forming molten spheres, and finally freezing solid, undergoing many of the same effects as meteoritic tektites, which will cool into spherical, or near-spherical shapes in free-fall. A fractionation tower can be used to manufacture some materials by separating out the material components based on their specific gravity.

Comparative gravities of different planets and Earth's moon

The standard acceleration due to gravity at the Earth's surface is, by convention, equal to 9.80665 metres per second squared. (The local acceleration of gravity varies slightly over the surface of the Earth; see gee for details.) This quantity is known variously as gn, ge (sometimes this is the normal equatorial value on Earth, 9.78033 m/s²), g0, gee, or simply g (which is also used for the variable local value). The following is a list of the gravitational accelerations (in multiples of g) at the Sun, the surfaces of each of the planets in the solar system, and the Earth's moon :
Note: The "surface" is taken to mean the cloud tops of the gas giants (Jupiter, Saturn, Uranus and Neptune) in the above table. It is usually specified as the location where the pressure is equal to a certain value (normally 75 kPa?). For the Sun, the "surface" is taken to mean the photosphere. Within the Earth, the gravitational field peaks at the core-mantle boundary, where it has a value of 10.7 m/s². For spherical bodies surface gravity in m/s2 is 2.8 × 10−10 times the radius in m times the average density in kg/m3. When flying from Earth to Mars, climbing against the field of the Earth at the start is 100 000 times heavier than climbing against the force of the sun for the rest of the flight.

Mathematical equations for a falling body

These equations describe the motion of a falling body under acceleration g near the surface of the Earth. mantle Here, the acceleration of gravity is a constant, g, because in the vector equation above, _ would be a constant vector, pointing straight down. In this case, Newton's law of gravitation simplifies to the law :F = mg The following equations ignore air resistance and the rotation of the Earth, but are usually accurate enough for heights not exceeding the tallest man-made structures. They fail to describe the Coriolis effect, for example. They are extremely accurate on the surface of the Moon, where the atmosphere is almost nil. Astronaut David Scott demonstrated this with a hammer and a feather. Galileo was the first to demonstrate and then formulate these equations. He used a ramp to study rolling balls, effectively slowing down the acceleration enough so that he could measure the time as the ball rolled down a known distance down the ramp. He used a water clock to measure the time; by using an "extremely accurate balance" to measure the amount of water, he could measure the time elapsed. 2 :For Earth \ g=9.8\, \mbox/\mbox^2 \quad For other planets, multiply \ g by the ratio of the gravitational accelerations shown above. Note: "Average" means average in time. Note: Distance traveled, d, and time taken, t, must be in the same system of units as acceleration g. See dimensional analysis. To convert metres per second to kilometres per hour (km/h) multiply by 3.6, and to convert feet per second to miles per hour (mph) multiply by 0.68 (or, precisely, 15/22).

Gravitational potential

For any mass distribution there is a scalar field, the gravitational potential (a scalar potential), which is the gravitational potential energy per unit mass of a point mass, as function of position. It is - G \int dm where the integral is taken over all mass. Minus its gradient is the gravity field itself, and minus its Laplacian is the divergence of the gravity field, which is everywhere equal to -4πG times the local density. Thus when outside masses the potential satisfies Laplace's equation (i.e., the potential is a harmonic function), and when inside masses the potential satisfies Poisson's equation with, as right-hand side, 4πG times the local density.

Acceleration relative to the rotating Earth

The acceleration measured on the rotating surface of the Earth is not quite the same as the acceleration that is measured for a free-falling body because of the centrifugal force. In other words, the apparent acceleration in the rotating frame of reference is the total gravity vector minus a small vector toward the north-south axis of the Earth, corresponding to staying stationary in that frame reference.

History of gravitational theory

The first mathematical formulation of gravity was published in 1687 by Sir Isaac Newton. His law of universal gravitation was the standard theory of gravity until work by Albert Einstein and others on general relativity. Since calculations in general relativity are complicated, and Newtonian gravity is sufficiently accurate for calculations involving weak gravitational fields (e.g., launching rockets, projectiles, pendulums, etc.), Newton's formulae are generally preferred. Although the law of universal gravitation was first clearly and rigorously formulated by Isaac Newton, the phenomenon was observed and recorded by others. Even Ptolemy had a vague conception of a force tending toward the center of the Earth which not only kept bodies upon its surface, but in some way upheld the order of the universe. Johannes Kepler inferred that the planets move in their orbits under some influence or force exerted by the Sun; but the laws of motion were not then sufficiently developed, nor were Kepler's ideas of force sufficiently clear, to make a precise statement of the nature of the force. Christiaan Huygens and Robert Hooke, contemporaries of Newton, saw that Kepler's third law implied a force which varied inversely as the square of the distance. Newton's conceptual advance was to understand that the same force that causes a thrown rock to fall back to the Earth keeps the planets in orbit around the Sun, and the Moon in orbit around the Earth. Newton was not alone in making significant contributions to the understanding of gravity. Before Newton, Galileo Galilei corrected a common misconception, started by Aristotle, that objects with different mass fall at different rates. To Aristotle, it simply made sense that objects of different mass would fall at different rates, and that was enough for him. Galileo, however, actually tried dropping objects of different mass at the same time. Aside from differences due to friction from the air, Galileo observed that all masses accelerate the same. Using Newton's equation, F = m a, it is plain to us why: :F = - = m_1a_1 The above equation says that mass m_1 will accelerate at acceleration a_1 under the force of gravity, but divide both sides of the equation by m_1 and: :a_1 = Nowhere in the above equation does the mass of the falling body appear. When dealing with objects near the surface of a planet, the change in r divided by the initial r is so small that the acceleration due to gravity appears to be perfectly constant. The acceleration due to gravity on Earth is usually called g, and its value is about 9.82 m/s2. Galileo didn't have Newton's equations, though, so his insight into gravity's proportionality to mass was invaluable, and possibly even affected Newton's formulation on how gravity works. However, across a large body, variations in r can create a significant tidal force.

Notes


- Note 1: Proposition 75, Theorem 35: p.956 - I.Bernard Cohen and Anne Whitman, translators: Isaac Newton, The Principia: Mathematical Principles of Natural Philosophy. Preceded by A Guide to Newton's Principia, by I.Bernard Cohen. University of California Press 1999 ISBN 0-520-08816-6 ISBN 0-520-08817-4
- Note 2: See the works of Stillman Drake, for a comprehensive study of Galileo and his times, the Scientific Revolution.
- Max Born (1924), Einstein's Theory of Relativity (The 1962 Dover edition, page 348 lists a table documenting the observed and calculated values for the precession of the perihelion of Mercury, Venus, and Earth.)

See also


- Gravity wave
- Gravitational binding energy
- Gravity Research Foundation
- Standard gravitational parameter
- Weight
- Weightlessness
- n-body problem
- Pioneer anomaly
- Table of velocities required for a spacecraft to escape a planet's gravitational field
- Application to gravity of the divergence theorem
- Gravity field
- Portal:Gravitation
- Scalar Gravity

References


-
-
-

External links


- [http://einstein.stanford.edu/ Gravity Probe B Experiment]
- [http://www.hkshum.net/whatisgravity/ What Is Gravity? - Aimed for Kids 8+ ]
- [http://www.intelligent-forces.com Intelligent Forces Theory] Satirical "Anti-Gravitationalism" website Category:Introductory physics Category:Celestial mechanics ko:중력 ja:重力 ms:Graviti

Natural satellite

The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. There are at least 140 moons within Earth's solar system, and presumably many others orbiting the planets of other stars. The large gas giants have extensive systems of moons, including half a dozen comparable in size to Earth's moon. Mercury and Venus have no moons at all, Earth has one large moon ("The Moon"), Mars has two tiny moons, and Pluto has three, including a large companion called Charon (Pluto and Charon are sometimes considered a double planet).

Origin

Most moons are assumed to have been formed out of the same collapsing region of protoplanetary disk that gave rise to its primary. However, there are many exceptions and variations to this standard model of moon formation that are known or theorized. Several moons are thought to be captured asteroids; others may be fragments of larger moons shattered by impacts, or (in the case of Earth's Moon) a portion of the planet itself blasted into orbit by a large impact. As most moons are known only through a few observations via probes or telescopes, most theories about their origins are still uncertain.

Orbital characteristics

Most moons in the solar system are tidally locked to their primaries, meaning that one side of the moon is always turned toward the planet. Exceptions are Saturn's moon Hyperion, which rotates chaotically due to a variety of external influences, and the outermost moons of the gas giants, which are too far away to become 'locked' (an example is Saturn's moon Phoebe). It is not possible for a moon to have moons of its own: the tidal effects of their primaries would make such a system unstable. However, several moons have small companions in the Lagrangian points of their orbits (e.g., Saturn's moons Tethys and Dione). The recent discovery of 243 Ida's moon Dactyl confirms that some