Solar system
 |
Major features of the solar system (not to scale) |
The
solar system comprises the
Sun and the retinue of
celestial objects gravitationally bound to it: nine
planets and their 165 known
moons,
[ ] as well as
asteroids,
meteoroids,
planetoids,
comets, and
interplanetary dust.
The principal component of the solar system is the
Sun (
astronomical symbol ☉); 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. (The currently hypothetical
Oort cloud, should its existence be confirmed, would also hold a substantial percentage).
[ ] 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. One planet,
Pluto, is also a member of the Kuiper belt. In order of their distances from the Sun, the major planets are
Mercury (),
Venus (♀),
Earth (),
Mars (♂),
Jupiter (),
Saturn (),
Uranus (/
),
Neptune (), and
Pluto (). All planets but two are in turn orbited by
natural satellites (usually termed "moons" after Earth's Moon) and the largest are 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.
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 uncovered systems very different compared to Earth's solar system. Although the term "solar system" is frequently applied to these other
star systems, literally, it should strictly refer to Earth's only: 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".
|
The orbits of the bodies in the solar system to scale (clockwise from top left) |
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. Other units in common use include the
gigametre (abbreviated as "Gm," one billion (10
9) kilometres) and the
terametre ("Tm", one trillion (10
12) kilometres).
Pluto is roughly 38 AU (5.7
Gm) from the Sun, while
Jupiter lies at roughly 5.2 AU (0.778 Gm).
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 major planets, with the exception of Pluto, lie very close to the ecliptic, while
comets and
kuiper belt objects often lie at significant angles to it. All of the major planets, and most other objects, also orbit with with the Sun's rotation.
All objects tied to the Sun have
elliptical orbits. An ellipse is a circle stretched in one dimension, therefore 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 most major planets follow nearly circular orbits, with perihelions roughly equal to their aphelions, Pluto and the objects of the Kuiper belt follow highly elliptical orbits, with large difference between perihelion and aphelion.
There is a direct relationship between how far away a planet is from the Sun, and how quickly it orbits. Mercury, which is closest to the Sun, not only has the smallest orbital circumference but also travels the fastest, while Pluto, since it is much farther from the Sun, travels more slowly.
By and large, the planets within our solar system are arranged so that each is roughly double the distance from the Sun as the one before it.
Venus is roughly twice as far from the Sun as
Mercury,
Earth is roughly double the distance as Venus,
Mars double that of Earth, and so on. The exception is Neptune, which lies roughly halfway between Uranus and Pluto.
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 (possibly a nearby
supernova) 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 planitsimals which formed there were relatively small (comprising only 0.6% the mass of the disc) 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, 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.
|
The Sun as seen from Earth. |
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 though 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 ("
metals" in astronomical parlance) 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 kilomtetres per hour, creating a very tenuous "atmosphere" (the
heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the
interplanetary medium. Small quantities of
cosmic dust (some of it arguably
interstellar in origin) 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.
|
The inner planets, their sizes to scale |
The four
inner or
terrestrial planets are characterised by their dense,
rocky composition and few or no moons or 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 (i.e. Mercury and Venus).
The four inner planets are:
Mercury
Mercury (0.4 AU), the closest planet to the Sun, is also the least massive of the inner planets, at only 0.055 Earth masses. It has no atmosphere, 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 (0.7 AU), the first truly terrestrial planet, is of comparable mass to the
Earth (0.815 Earth masses), 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 with temperatures reaching 800+ degrees
Celsius.
Earth and Moon
The largest and densest of the inner planets,
Earth (1 AU) is also the only one to demonstrate unequivocal evidence of current geological activity. 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 (1.5 AU), 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 (
Deimos and
Phobos) thought to be captured
asteroids.
Asteroids are objects smaller than planets that are composed in significant part of rocky, non-volatile minerals.
|
Image of the main asteroid belt and the Trojan asteroids. |
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. It contains tens of thousands (possibly millions) of asteroids 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 of that of the
Earth.
[ ] In contrast to its various depictions in
science fiction, the main belt is very sparsely populated; several probes have passed through it without incident. Asteroids with a diameter of less than 50 m are called
meteoroids. The largest asteroid,
Ceres, has a diameter of almost 1000 km; large enough to become spherical under its own gravity, which would make it a planet by some definitions of the word.
Asteroids in the main belt 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. 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 is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.
|
The four outer planets (not to scale) |
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.
Jupiter and
Saturn are true giants, at 318 and 95 Earth masses, respectively, and composed largely of hydrogen and helium.
Uranus and
Neptune 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." The term
outer planet should not be confused with
superior planet, which designates those planets which lie outside
Earth's orbit (thus consisting of the outer planets plus
Mars).
Jupiter
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. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the
Great Red Spot. Three of its 63
satellites,
Ganymede,
Io and
Europa, share elements in common with the terrestrial planets, such as volcanism and internal heating. Ganymede has a larger diameter than either Mercury or Pluto.
Saturn
Saturn (9.5 AU), famous for its extensive
ring system, has 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, like Ganymede, is larger than both Pluto and Mercury; it is also the only satellite in the solar system with a substantial atmosphere.
Uranus
Uranus (19.6 AU) at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its
axial tilt lies at over ninety degrees to the
ecliptic. Its core is remarkably cold (compared to the other gas giants; it is still several thousand degrees Celsius) and radiates very little heat into space.
Neptune
Neptune (30 AU), though slightly smaller than Uranus, is slightly more massive, at 17 Earth masses, and radiates more internal heat than Uranus, but not as much as Jupiter or Saturn. Its peculiar ring system is composed of a number of dense "arcs" of material separated by gaps. Neptune has 13 moons. Its largest moon,
Triton, is geologically active, with geysers of
liquid nitrogen.
Comets are small bodies (usually only a few kilometres across) composed largely of volatile ices, which possess highly eccentric orbits, generally having a
perihelion within the orbit of the inner planets and an
aphelion far beyond Pluto. When a comet approaches the Sun, its icy surface begins to sublimate, or boil away, creating a
coma; a long tail of gas and dust which is often visible with the naked eye.
There are two basic types of comet: short-period comets, with orbits less than 200 years, and long-period comets, with orbits lasting thousands of years. Short-period comets are believed to originate in the
Kuiper belt, while long period comets, such as
Hale-Bopp (pictured), are believed to originate in the
Oort Cloud. Some comets with
hyperbolic orbits may originate outside the solar system. Old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids.
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 coma just as comets do when they approach the sun.
[ ] |
Artist's rendering of the Kuiper Belt and hypothetical Oort cloud. |
The area beyond Neptune, often referred to as the
outer solar system or simply the "
trans-Neptunian region", is still largely unexplored.
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 and 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 100,000
Kuiper belt objects with a diameter greater than 50 km, the total mass of the Kuiper belt is relatively low, perhaps barely equalling the mass of the Earth.
[ ] Many Kuiper belt objects have multiple satellites and most have orbits that take them outside the plane of the ecliptic.
Pluto and Charon
|
Pluto and its three known moons |
Pluto (38 AU average) is officially the solar system's smallest planet, though astronomers also consider it part of the Kuiper belt population. Like other objects in the Kuiper belt, Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at
perihelion (within the orbit of Neptune) to 49.5 AU at
aphelion.Pluto has a large moon (the largest in the solar system relative to its own size), called
Charon, as well as two much smaller moons called Nix and Hydra. Like the Earth/Moon, Pluto and Charon are often considered a
double planet, since both orbit a common
barycenter between the two bodies.
Kuiper belt objects which, like Pluto, possess a 3:2 orbital resonance with Neptune (ie, they orbit twice for every three Neptunian orbits) are called
Plutinos. Other Kuiper belt objects have different
resonant orbits (2:1, 4:7, 3:5 etc) and are grouped accordingly. The remaining Kuiper belt objects, in more "classical" orbits, are classified as
Cubewanos.
|
black: scattered disc; blue: classical Kuiper belt; green: resonant KBOs inc. Pluto. |
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 by the gravitational influence of Neptune's migration (
see Formation of the solar system). Most scattered disc objects have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. Their orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers, such as
David Jewitt, consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."
[ ]2003 UB313 ("Xena")
One particular scattered disc object has renewed the old debate about
what constitutes a planet since it is at least 5% larger than Pluto with an estimated diameter of 2400 km (1500 mi). It currently has no name, but has been given the
provisional designation , though its discoverers have nicknamed it "
Xena", after the television character.
[ ]The object 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, at 44 degrees, more so than any known object in the solar system except the newly-discovered object , also known as "Buffy".
[ ] Like Pluto, it is believed to consist largely of rock and ice, and has a moon
[ ]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. The solar wind extends to a point roughly 130 AU from the Sun, whereupon it surrenders to the surrounding envionment of the
interstellar medium. It is generally accepted, however, that the Sun's gravity holds sway to the
Oort cloud. This great mass of up to a trillion icy objects, currently hypothetical, is believed to be the source for all long-period
comets and to surround the solar system like a shell from 50,000 to 100,000 AU beyond the
Sun, or almost a quarter the distance to the next star system. The vast majority of the solar system, therefore, is completely unknown; however, recent observations of both our solar system and others have led to an increased understanding of what is or may be lying at its outer edge.
[ ] |
an artist's conception of Sedna |
Sedna
Sedna is a large, reddish Pluto-like object with a gigantic, highly elliptical 10,500-year orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion.
Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the
scattered disc or the
Kuiper Belt as it has too distant a
perihelion to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, one which also may include the object , which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years.
[ ]Heliopause
|
The Voyagers entering the heliosheath |
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.
[ ] |
artist's conception of the Local Bubble |
|
presumed location of the solar system within our galaxy |
The solar system is located in the
Milky Way galaxy, a
barred spiral galaxy with a diameter estimated at about
100,000 light years containing approximately 200 billion stars. The galaxy is a spiral, and our Sun resides in one of the outer spiral arms, known as the
Orion Arm or
Local Spur.
The immediate galactic neighborhood of the solar system is known as the
Local Fluff, an area of dense cloud in an otherwise sparse region known as the
Local Bubble, an hourglass-shaped region roughly 300
light-years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.
Estimates place the solar system at between 25,000 and 28,000 light years from the
galactic center. Its speed is about 220
kilometres per second, and it completes one revolution every
226 million years. The apex of solar motionis near the current location of the bright star
Vega.
[ ] 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 remarkable 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. The solar system also lies well outside the star-crowded environs of the galactic centre. The opposing gravitational tugs from so many close stars within the galactic centre would have prevented planets from forming.
[ ]Extrasolar systems neighboring Earth's have been found to be different. For instance, many extrasolar planetary systems contain a "
hot Jupiter"
; a planet of comparable size to Jupiter that nonetheless orbits very close to its star, at, for instance, 0.05 AU. It has been hypothesised that while the giant planets in these systems formed in the same place as the gas giants in Earth's solar 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.
For many thousands of years, people, with a few notable exceptions, did not believe the solar system existed. The Earth was believed not only to be stationary at the centre of the
universe, but to be categorically different from the divine or ethereal objects that moved through the sky. The conceptual advances of the 17th century, led by
Nicolaus Copernicus,
Galileo Galilei,
Johannes Kepler, and
Isaac Newton, led gradually to the acceptance of the idea not only that Earth moved round the Sun, but that the planets were governed by the same laws that governed the Earth, and therefore could be similar to it.
Telescopic observations
The first exploration of the solar system was conducted by telescope, with astronomers learning that the Moon and other planets possessed such Earthlike features as craters, ice caps, and seasons.
Galileo Galilei was the first to notice the physical nature of our solar system. He discovered that the
Moon was cratered, that the Sun was pocked with sunspots, and that Jupiter had four sattellites in orbit around it.
Giovanni Domenico Cassini and
Christian Huygens followed on from Galileo's discoveries by discovering the rings of Saturn, and Saturn's moon
Titan, respectively.
In 1682,
Edmund Halley realised that repeated sightings of a
comet were in fact recording the same object, returning regularly once every 75-6 years. This proved once and for all that comets were not atmospheric phenomena, as had been previously thought, and was the first evidence that anything other than the planets orbited the Sun.
In 1781,
William Herschel was looking for binary stars in the constellation of
Taurus when he observed what he thought was a new comet. In fact, its orbit revealed that it was a new planet,
Uranus, the first ever discovered.
In 1802,
Giuseppe Piazzi discoverd
Ceres, a small world between Mars and Jupiter that was initially considered a new planet. However, subsequent discoveries of thousands of other small worlds in the same region led to their eventual seperate reclassifacation:
asteroids.
In 1846, discrepancies in the orbit of Uranus led many to suspect a large planet must be tugging at it from farther out.
Urbain Le Verrier's calculations eventually led to the discovery of
Neptune.
Further discrepancies in the orbits of the planets led
Percival Lowell to conclude yet another planet, "
Planet X" must still be out there. After his death, his Lowell Observatory conducted a search, which ultimately led to
Clyde Tombaugh's discovery of
Pluto in 1930. Pluto was, however, found to be too small to have disrupted the orbits of the outer planets, and now, thanks to the discovery of many other similarly sized objects in its vacinity, considered, much like Ceres, to be a member of a population.
Observations by spacecraft
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 planned Phoenix Mars lander |
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. That should change with the successful launch of the
New Horizons spacecraft on
19 January 2006. This
unmanned mission is scheduled to fly by Pluto in July 2015. Should it prove feasible, the mission will then be extended to observe a number of
Kuiper Belt objects.
[ ]Through these unmanned missions, humans 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.
*
List of solar system objects**
by mass**
by radius*
Attributes of the largest solar system bodies*
Hypothetical planets*
Astronomical symbols*
Geological features of the solar system*
Numerical model of solar system*
Solar nebula*
Table of planetary attributes*
Timeline of discovery of Solar System planets and their natural satellites*
Timeline of solar system astronomy*
Solar system model
nds-nl:Zunnesysteem*
NASA's Solar System Exploration site*
NASA's Solar System Simulator*
NASA/JPL Solar System main page*
The Nine Planets - Comprehensive solar system site by Bill Arnett *
Planetary data*
SolStation: Stars and Habitable Planets*
Celestia - Free 3D realtime space-simulation (OpenGL)*
SPACE.com: All About the Solar System
