The solar system is comprised of our Sun and the retinue of celestial objects gravitationally bound to it. Traditionally, it 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 <name> system" (e.g. "the Alpha Centauri system" or "the 51 Pegasi system").
In broad terms, the charted regions of our solar system consist of the Sun, 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 (♅/[image]), Neptune (♆), and Pluto (♇), though Pluto's status has been thrown into question by the discovery of 2003UB313 . 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 roughly 150 million 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
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.
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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.
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.
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.
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.
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 2003 UB313, 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 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.
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 our solar system, therefore, is completely unknown.
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. 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.
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 solar systems forming.
The nature and structure of the solar system were long misperceived, for at least two reasons:
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:
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.
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.
| (million kilometres) The bottom and top of each colored bar represent a planet's nearest approach to the sun and farthest distance from the sun. | |
| | |
All attributes below are measured relative to the Earth:
| ** Soon after its discovery in 1930, Pluto was classified a planet by the International Astronomical Union. However, based on additional discoveries since that time, some astronomers have suggested reconsideration of that decision. | ||||||
| Planet | Equatorialdiameter | Mass | Orbitalradius (AU) | Orbital period(years) | Day(days) | Moons |
|---|---|---|---|---|---|---|
| Mercury | 0.382 | 0.06 | 0.38 | 0.241 | 58.6 | none |
| Venus | 0.949 | 0.82 | 0.72 | 0.615 | -243 | none |
| Earth* | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1 |
| Mars | 0.53 | 0.11 | 1.52 | 1.88 | 1.03 | 2 |
| Jupiter | 11.2 | 318 | 5.20 | 11.86 | 0.414 | 63 |
| Saturn | 9.41 | 95 | 9.54 | 29.46 | 0.426 | 49 |
| Uranus | 3.98 | 14.6 | 19.22 | 84.01 | 0.718 | 27 |
| Neptune | 3.81 | 17.2 | 30.06 | 164.79 | 0.671 | 13 |
| Pluto** | 0.24 | 0.0017 | 39.5 | 248.5 | 6.5 | 3 |
Of the other objects, Ganymede has the largest mass (0.02).
Note: Although 2003 UB313 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.
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:
| Minor planet | Equatorialdiameter | Mass | Orbital radius(AU) | Orbital period(years) | Day(days) |
|---|---|---|---|---|---|
| 1 Ceres | 0.075 | 0.000 158 | 2.767 | 4.603 | 0.3781 |
| 90482 Orcus | 0.066 - 0.148 | 0.000 10 - 0.001 17 | 39.47 | 248 | ? |
| 28978 Ixion | ~0.083 | 0.000 10 - 0.000 21 | 39.49 | 248 | ? |
| (55636) 2002 TX300 | 0.0745 | ? | 43.102 | 283 | ? |
| 20000 Varuna | 0.066 - 0.097 | 0.000 05 - 0.000 33 | 43.129 | 283 | 0.132 or 0.264 |
| ~.0768 | ~0.000 67 | 43.339 | 285 | ? | |
| 50000 Quaoar | 0.078 - 0.106 | 0.000 17 - 0.000 44 | 43.376 | 285 | ? |
| ? | ? | 45.64 | 308 | ? | |
| ~.3 | ? | 67.709 | 557 | ? | |
| 90377 Sedna | 0.093 - 0.141 | 0.000 14 - 0.001 02 | 502.040 | 11500 | 20 |
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).
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.
| Our Solar System |
| Sun | Mercury | Venus | Earth (Moon) | Mars | Asteroid belt |
| Jupiter | Saturn | Uranus | Neptune | Pluto | Kuiper belt | Scattered disc | Oort cloud |
| See also astronomical objects and the solar system's list of objects, sorted by radius or mass |