Encyclopedia
The
Solar System or
solar system is the
stellar system comprising the
Sun and the retinue of celestial objects
gravitationally bound to it: the eight
planets, their 162 known
moons, three currently identified
dwarf planets and their four known moons, and thousands of small bodies. This last category includes
asteroids, meteoroids,
comets, and interplanetary dust.
The principal component of the Solar System is the
Sun ; a
main sequence G2 star that contains 99.86% of the system's known
mass and dominates it gravitationally. Because of its large mass, the Sun has an interior density high enough to sustain
nuclear fusion, releasing enormous amounts of
energy, most of which is radiated into space in the form of
electromagnetic radiation, including visible
light. The Sun's two largest orbiting bodies,
Jupiter and
Saturn, account for more than 90% of the system's remaining mass. .
In broad terms, the charted regions of the Solar System consist of the Sun, four rocky bodies close to it called the
terrestrial planets, an inner belt of rocky
asteroids, four
gas giant planets, and an outer belt of small, icy bodies known as the
Kuiper belt. In order of their distances from the Sun, the planets are Mercury ,
Venus ,
Earth ,
Mars ,
Jupiter ,
Saturn ,
Uranus , and
Neptune . All planets but two are in turn orbited by
natural satellites , and every planet past the
asteroid belt is encircled by
planetary rings of dust and other particles. The planets, with the exception of Earth, are named after gods and goddesses from
Greco-Roman mythology.
From 1930 to 2006,
Pluto , one of the largest known
Kuiper belt objects, was considered the Solar System's ninth planet. However, in 2006 the
International Astronomical Union created an official
definition of the term "planet". Under this definition, Pluto is reclassified as a
dwarf planet, and there are eight planets in the Solar System. In addition to Pluto, the IAU currently recognizes two other
dwarf planets:
Ceres , the largest asteroid, and
Eris, which lies beyond the Kuiper belt in a region called the scattered disc. Of the known dwarf planets, only Ceres has no moons.
For many years, the Solar System was the only known example of planets in orbit around a star. The discovery in recent years of many
extrasolar planets has led to the term "solar system" being applied generically to all
stellar systems. Technically, however, it should strictly refer to Earth's system only, as the word "solar" is derived from the Sun's
Latin name,
Sol. Other stellar systems or
planetary systems are usually referred to by the names of their parent star; "the
Alpha Centauri system" or "the
51 Pegasi system".
Layout
Most objects in orbit round the Sun lie within the same shallow plane, called the
ecliptic, which is roughly parallel to the Sun's equator. The planets lie very close to the ecliptic, while
comets and
kuiper belt objects often lie at significant angles to it. All of the planets, and most other objects, also orbit with the Sun's rotation in a counter-clockwise direction as viewed from a point above the Sun's north pole. There is a direct relationship between how far away a planet is from the Sun, and how quickly it orbits. Mercury, with the smallest orbital circumference, travels the fastest, while Neptune, being much farther from the Sun, travels more slowly.
A planet's distance from the Sun varies in the course of its year. Its closest approach to the Sun is known as its
perihelion, while its farthest point from the Sun is called its
aphelion. Though planets follow nearly circular orbits, with perihelions roughly equal to their aphelions, many comets, asteroids and objects of the Kuiper belt follow highly elliptical orbits, with large differences between perihelion and aphelion.
Astronomers most often measure distances within the solar system in astronomical units, or AU. One AU is the average distance between the Earth and the Sun, or roughly 149 598 000 km .
Pluto is roughly 38 AU from the Sun, while
Jupiter lies at roughly 5.2 AU.
Informally, the Solar System is sometimes divided into separate "zones"; the first zone, known as the
inner Solar System, comprises the
inner planets and the main asteroid belt. The
outer solar system is sometimes defined as everything beyond the asteroids; however, it is also the name often given to the region beyond Neptune, with the gas giants as a separate "middle zone."
One common misconception with regards to the Solar System is that the orbits of the major objects are equidistant. Due to the vast distances involved, many representations of the Solar System tend to simplify these orbits, with equal spacing between each object. However, with certain exceptions, it can generally be stated that the farther a planet or belt is from the Sun, the greater the distance between it and the previous orbit. For example,
Venus is approximately 0.33 AU farther out than Mercury, whereas
Jupiter lies 1.9 AU from the farthest extent of the
asteroid belt, and Neptune's orbit is roughly 20 AU farther out than that of Uranus. Attempts have been made to determine a correlation between these distances but to date there is no accepted theory that explains the respective orbital distances.
Planets, dwarf planets, and small solar system bodies
A
planet, according to the recent definition passed by the
International Astronomical Union General Assembly on August 24, 2006, is any body in orbit around the Sun that a) has enough mass to form itself into a spherical shape and b) has cleared its immediate neighborhood of all smaller objects. Eight objects in the Solar System currently meet this definition; they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Dwarf planet is a newly defined classification for stellar objects. The key difference between planets and dwarf planets is that while both are required to orbit the Sun and be of large enough mass that their own gravity pulls them into a nearly round shape, dwarf planets are not required to
clear their neighborhood of other celestial bodies. Three objects in the solar system are currently included in this category; they are
Pluto , the asteroid
Ceres, and the
scattered disc object
Eris. The IAU will begin evaluating other known objects to see if they fit within the definition of dwarf planets. The most likely candidates are some of the larger asteroids and several Trans-Neptunian Objects such as
Sedna,
Orcus, and
Quaoar.
The remainder of the objects in the Solar System were classified as
small solar system bodies. A small solar system body is a term
defined in 2006 by the
International Astronomical Union to describe Solar System objects which are neither
planets nor
dwarf planets.
- All other objects ... orbiting the Sun shall be referred to collectively as "Small Solar System Bodies" .... These currently include most of the Solar System asteroids, most Trans-Neptunian Objects , comets, and other small bodies.
As of 2006, the IAU considers the following bodies to be SSSB's:
- all asteroids except Ceres
- all centaurs
- all trans-Neptunian objects, including Kuiper belt & Scattered disc objects, with the exception of Pluto and Eris
- all comets
Formation
The current hypothesis of Solar System formation is the
nebular hypothesis, first proposed in 1755 by
Immanuel Kant and independently formulated by
Pierre-Simon Laplace. The nebular theory holds that the Solar System was formed from the gravitational collapse of 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, a disturbance squeezed the nebula, pushing matter inward until gravitational forces overcame the internal gas pressure and it began to collapse. As the nebula collapsed, conservation of
angular momentum meant that it spun faster, and became warmer. 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
protoplanetary disk with a gradually contracting protostar at the center.
From this cloud and its gas and dust, the various planets formed. The inner solar system was too warm for volatile molecules like water and methane to condense, and so the planetesimals which formed there were relatively small and composed largely of compounds with high melting points, such as silicates and
metals. These rocky bodies eventually became the
terrestrial planets. Farther out, the gravitational effects of
Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the
asteroid belt. Farther out still, beyond the frost line,
Jupiter and
Saturn developed as large gas giants, while
Uranus and
Neptune captured much less gas and are known as ice giants because their cores are believed to be made mostly of ice, that is, hydrogen compounds.
The gas giants were massive enough to retain a "primary atmosphere" of hydrogen and helium captured from the surrounding solar nebula. The terrestrial planets eventually lost their retained hydrogen and helium, and subsequently generated their own "secondary atmospheres" via volcanism, comet impacts, and, also in Earth's case, the evolution of life.
After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin
thermonuclear fusion, which increased until
hydrostatic equilibrium was achieved.
The young Sun's
solar wind then cleared away all the gas and dust in the
protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets.
Sun
The
Sun is the Solar System's parent star, and far and away its chief component. It is classed as a moderately large
yellow dwarf. However, this name is misleading, as on the scale of stars in our galaxy, the Sun is rather large and bright. Stars are classified based on their position on the
Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperature. Generally speaking, the hotter a star is, the brighter it is. Stars which follow this pattern are said to be on the
main sequence, and the Sun lies right in the middle of it. This has led many astronomy textbooks to label the Sun as "average;" however, stars brighter and hotter than it are rare, whereas stars dimmer and cooler than it are common. The vast majority of stars are dim red dwarfs, though they are under-represented in star catalogues as we can observe only those few that are very near the Sun in space.
The Sun's position on the
main sequence means, according to current theories of stellar evolution, that it is in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for
nuclear fusion, and been forced, as older
red giants must, to fuse more inefficient elements such as
helium and
carbon. The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today.Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through its life cycle, and will eventually begin moving off the main sequence, becoming larger, brighter and redder, until, about five billion years from now, it too will become a red giant.
The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe's evolution. As such, it contains far more elements heavier than hydrogen and helium than older
population II stars such as those found in
globular clusters. Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could be enriched with them. For this reason, the very oldest stars contain very little "metal", while stars born later have more. This high "metallicity" is thought to have been crucial in the Sun's developing a
planetary system, because planets form from accretion of metals.
The Sun radiates a continuous stream of charged particles, a
plasma known as
solar wind, ejecting it outwards at speeds greater than 2 million kilometres per hour, creating a very tenuous "atmosphere" , that permeates the solar system for at least 100 AU. This environment is known as the
interplanetary medium. Small quantities of
cosmic dust are also present in the
interplanetary medium and are responsible for the phenomenon of
zodiacal light. 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.
Earth's
magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.
Inner planets
The four
inner or
terrestrial planets are characterised by their dense, rocky composition, few or no moons, and lack of ring systems. They are composed largely of minerals with high melting points such as silicates to form the planets' solid crusts and semi-liquid
mantles, and metallic dust grains such as
iron, which forms their
cores. Three of the four inner planets have atmospheres. All have
impact craters, and all but one possess tectonic surface features, such as
rift valleys and
volcanoes. The term
inner planet should not be confused with
inferior planet, which designates those planets which are closer to the Sun than the Earth is .
The four inner planets are:
Mercury
Mercury , the closest planet to the Sun, is also the least massive of the planets, at only 0.055 Earth masses. Mercury has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replenished. Mercury is surrounded by an extremely small amount of helium, hydrogen, oxygen, and sodium. This envelope of gases is so thin that the greatest possible atmospheric pressure on Mercury would be about 0.000000000002 kg/cm² . The atmospheric pressure on the Earth is about 1.03 kg/cm² . It has no
natural satellite, and, to date, no observed geological activity save that produced by
impacts. Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the Sun's gravity. The MESSENGER probe should aid in resolving this issue when it arrives in Mercury's orbit in 2011.
Venus
Venus , the first truly terrestrial planet, is of comparable mass to the
Earth , and, like 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. However, it is much drier than Earth and its atmosphere is 90 times as dense and is composed overwhelmingly of
carbon dioxide and
sulfuric acid. Unlike Earth, evidence suggests that Venus's crust is not divided into
tectonic plates but instead comprises a single very thick rind. Venus has no natural satellite. It is the hottest planet, despite being farther from the sun than Mercury, with temperatures reaching more than 400 degrees
Celsius. This is most likely due to the amount of
greenhouse gases in the atmosphere.
Earth
The largest and densest of the inner planets,
Earth is also the only one to demonstrate unequivocal evidence of current geological activity. Earth is the only planet known to have life. Its liquid
hydrosphere, unique among the terrestrials, is probably the reason Earth is also the only planet where multi-
plate tectonics has been observed, because 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, because its orbit around the Sun never actually loops back on itself when observed from above. The Moon possesses many features in common with other terrestrial planets, though it lacks an iron core.
Mars
Mars , at only 0.107 Earth masses, is less massive than either Earth or Venus. It 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 recent evidence suggests this may have been true until very recently. Mars possesses two tiny
moons thought to be captured
asteroids.
Asteroid belt
Asteroids are mostly small solar system bodies that are composed in significant part of rocky, non-volatile minerals.
The main
asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be the remnants of a small terrestrial planet that failed to coalesce due to the gravitational interference of Jupiter. Asteroids range in size from hundreds of kilometers to as small as dust. All asteroids save the largest,
Ceres, are classified as small solar system bodies; however, a number of other asteroids, such as
Vesta and
Hygeia, could potentially be reclassed as
dwarf planets if it can be conclusively shown that they are spherical. The asteroid belt contains tens of thousands - and potentially millions - of objects over one kilometre in diameter. However, despite their large numbers, the total mass of the main belt is unlikely to be more than a thousandth of that of the
Earth. In contrast to its various depictions in
science fiction, the main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with a diameter of less than 50 m are called meteoroids.
Ceres
Ceres is the largest astronomical body in the asteroid belt and the only known
dwarf planet in this region. It has a diameter of slightly under 1000 km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the nineteenth century, but was reclassified as an asteroid as further observation revealed additional asteroids.
Asteroid groups
Asteroids in the main belt are subdivided into
asteroid groups and 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. The asteroid belt also contains main-belt comets which may have been the source of Earth's water.
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
