|Schematic representation of the solar system up to the Kuiper belt, with the sun, the eight planets, the dwarf planets and the most important asteroids and moons|
|Inner planets||1. Mercury|
|Earth orbit cruiser|
|4. Mars||Phobos , Deimos|
|Asteroid belt||Vesta , Juno , Ceres , Pallas|
|Outer planets||5. Jupiter||Io , Europe , Ganymede , Callisto|
|6. Saturn||Tethys , Dione , Rhea , Titan , Iapetus|
|Centaurs||Chariklo , Chiron|
|7. Uranus||Miranda , Ariel , Umbriel , Titania , Oberon|
|8. Neptune||Triton , Nereid|
|Trans-Neptunian objects||Kuiper belt||Eris , Pluto , Haumea , Makemake , Gonggong , Quaoar , Orcus|
|Sedna , 2012 VP 113|
The solar system is the planetary system that includes the sun , the planets orbiting it (see list of planets in the solar system ) and their natural satellites , dwarf planets and other small bodies such as comets , asteroids and meteoroids as well as the entirety of all gas and dust particles that pass through the Attraction of the sun are bound to these, includes.
Areas of the orbits
(in million km).
The vertical colored bars mark the scope between the smallest and largest orbital distance to the sun.
The sun is the central star of the solar system. Since it has 99.86% of the total mass of the system, it is very close to the barycenter of the solar system . In the order of their distance from the Sun, the terrestrial planets Mercury, Venus, Earth and Mars, which make up the inner part of the planetary system, follow . The outer part is formed by the gas planets Jupiter, Saturn, Uranus and Neptune. Other companions of the sun are dwarf planets, millions of asteroids (also called asteroids or minor planets) and comets, which are mainly found in three small body zones of the solar system: the asteroid belt between the inner and outer planets, the Kuiper belt beyond the outer planets and the Oort cloud on the very outside.
The orbits of the planets are only slightly inclined to the plane of the earth's orbit, by a maximum of 7 °, so they lie in a relatively flat disk. Most of the minor planets known so far (2019), especially those of the Kuiper Belt, have an inclination of less than 30 °. For the Oort cloud, however, a spherical shape is assumed.
Within the areas of space dominated by the individual sun companions - their hill spheres - there are often smaller celestial bodies as circumferential companions of these objects. After the well-known earth moon , they are also referred to as moons, but also as satellites or satellites. With the exception of the Earth's moon and the Pluto moon Charon , at least the planets and dwarf planets are considerably smaller than their main body. Moonless exceptions among the planets are only Mercury and Venus. A definitely lower limit, above which one no longer speaks of a moon, as with the components of the rings of the gas planets, has not yet been officially established.
At about 1.39 million kilometers, the diameter of the sun is far larger than the diameter of any other object in the system. The largest of these objects are the eight planets, the four moons of Jupiter Ganymede , Callisto , Europa and Io (the Galilean moons ), the Saturn moon Titan and the Earth's moon . Jupiter accounts for two thirds of the remaining mass of 0.14%. (See also List of Largest Objects in the Solar System .)
As a result of the formation of the solar system, all planets, dwarf planets and the asteroid belt move in their orbit around the sun in the same sense of rotation , which is called right-handed . They circle the sun counterclockwise when viewed from the north. Most of the larger moons also move in this direction around their main body. Most of the larger bodies of the solar system also rotate in a right-handed direction. Of the planets, only Venus rotates in the opposite direction, and the axis of rotation of Uranus is almost in its orbit.
Zone of the planets
Closest to the sun are the inner , earth-like planets Mercury (distance to the sun 57.9 million km or 0.39 AU ), Venus (108.2 million km or 0.72 AU), earth (149, 6 million km or 1 AU) and Mars (227.9 million km or 1.52 AU). Their diameter is between 4878 km and 12756 km, their density between 3.95 g / cm³ and 5.52 g / cm³. Within the habitable zone around the sun, however, there are only Earth and, depending on the model, Mars.
The so-called asteroid belt , a collection of minor planets, is located between Mars and Jupiter . Most of these asteroids are only a few kilometers in size (see List of Asteroids ) and only a few are 100 km or more in diameter. At around 960 km, Ceres is the largest of these bodies and is considered a dwarf planet. The orbits of the asteroids are partly very elliptical, some even cross Mercury ( Icarus ) or Neptune ( Dioretsa ) orbits . The outer planets include the gas giants Jupiter (778.3 million km or 5.2 AU) and Saturn (1.429 billion km or 9.53 AU) as well as the ice giant Uranus (2.875 billion km or 19.2) AE) and Neptune (4.504 billion km and 30.1 AU) with densities between 0.7 g / cm³ and 1.66 g / cm³.
The mean distances of the planets from the sun can be approximated by mathematical series such as the Titius-Bode series . This certain regularity of the orbital distances is likely to be due to resonance effects during the formation of the solar system. The fact that the mean distance of the asteroid belt can also be classified in this series, but not that of Neptune, gave and still gives rise to speculation about cosmic catastrophes.
Mercury and Venus can come closest with a minimum distance of 0.26 AU. The minimal distance between Venus and Earth is slightly greater. If one takes the mean orbit radii, then Venus and Earth are the planets with the smallest distance to each other (41 million km or just under 0.28 AU).
The planets Mercury, Venus, Mars, Jupiter and Saturn were already known as wandering stars in ancient times and were associated with individual gods. The names chosen after the gods of Roman mythology have prevailed. The planets Uranus and Neptune, discovered in 1781 and 1846, and the dwarf planet Pluto , discovered in 1930 - also classified as a planet until 2006 - were named in a similar way for reasons of tradition.
- Note on the order of the planets
In order to be able to memorize the planets in their order - according to increasing distance from the sun - more easily, various mnemonics were devised, mostly so-called donkey bridges in the form of an acrostic , e.g. B.
- M a V ater e rklärt m ir j Eden S onntag u nseren N night sky. alternatively "... u ur N achbarplaneten."
A motto that also takes into account the small bodies is:
- M a V ater e rklärt m ir a n j edem S onntag u ur n atural k osmische O rdnung.
Since the 1990s, thousands of objects have been found moving beyond the orbit of Neptune. Almost all of these objects are 4.5–7.5 billion km (30–50 AU) from the Sun and form the Kuiper Belt there . It is a reservoir for comets with medium orbital periods. The objects in this zone are probably almost unchanged relics from the formation phase of the solar system; they are therefore also called planetesimals . The Kuiper Belt contains a number of dwarf planets such as Pluto , (136199) Eris , (136472) Makemake , (136108) Haumea, and a number of other objects that are likely dwarf planets by their size.
The solar wind acts unhindered in interplanetary space up to the Kuiper belt and displaces the interstellar matter . Slows down beyond the Kuiper belt and the particle of the sun compressed by the interaction with the interstellar medium and forms the outer shell of the Heliosphäre the Helio shell (heliosheath) . The boundary layer between the heliosphere and the interstellar medium is the heliopause at a distance of about 120 AU (4 times the distance between the Neptune and the Sun).
Outside the heliopause, the Oort cloud is theoretically located up to a distance of about 1.5 light years (about 100,000 AU) from the sun . Due to the influence of the gravitation of passing stars, bodies are presumably released from it and fall as long-period comets into the inner regions of the solar system. Some of these comets then remain on highly elliptical orbits near the sun, others are disturbed and deflected by the planets, especially Jupiter, so that they are catapulted out of the solar system or crash onto planets or into the sun.
There is no generally accepted definition of how far the solar system extends. The extent of the solar system has often been equated with that of the heliosphere. But with the discovery of distant Trans-Neptunian objects it was proven that there are also objects gravitationally bound to the sun beyond the heliopause.
Since astronomical dimensions are difficult to imagine for most people, a scaled-down model of the solar system or a visit to a planetary path is helpful in order to visualize the proportions and distances of the objects.
Local stellar neighborhood
The individual star closest to the Sun is the red dwarf Proxima Centauri . Its distance from the solar system is about 4.22 light years or 268,000 AU . The star is orbited by an exoplanet (→ Proxima Centauri b ), which was discovered in 2016 and is located in the habitable zone . Its minimum mass corresponds roughly to the mass of the earth. Proxima Centauri is very likely gravitationally bound to the binary star system Alpha Centauri , which is 4.34 light years (1.33 parsecs ) away from the Sun. The main components of Alpha Centauri are two sun-like stars , which have also been thought to have a planetary companion around Alpha Centauri B since 2012. The Alpha Centauri system thus contains both the three stars closest to the Sun and the closest extrasolar planet.
The stellar neighborhood of the solar system (see list of nearest stars ) is dominated by low-mass red dwarfs. Of the 64 stars within 5 parsecs around the Sun alone 49 red dwarf stars of are spectral class M. Only two stars (Alpha Centauri A and Tau Ceti ) are like the sun to the spectral type G. The brightest and most massive star in this area is Sirius with 2.12 solar masses, which is also the brightest star in the earthly night sky. In the meantime, planetary systems or individual exoplanets have been detected or indications have been found for several of the nearby stars, namely Alpha Centauri (including Proxima Centauri), Epsilon Eridani , Ross 128 , Tau Ceti, YZ Ceti , Wolf 1061 , Luytens Stern , Gliese 674 , Gliese 687 , Gliese 832 , Gliese 876 , Groombridge 34 and Kapteyns Stern .
The average star density in this area with a radius of five parsecs around the solar system is about 4 stars per 1000 cubic light years (a cube with an edge length of 10 light years), the average distance between the stars in the solar neighborhood is about 6 light years.
The area near the sun is poor in giant stars and star clusters. The closest red giants are Pollux and Arcturus , 34 and 37 light years away, respectively. The nearest blue giant star is Elnath, 130 light-years away . The nearest open star cluster , the Hyades , is 153 light years away. The closest known black hole is part of the A0620−00 system about 3000 light years away.
The galactic region around the solar system is largely free of interstellar dust , as the Sun has crossed a region called the Local Bubble for about five to ten million years . It measures around 200 light years along the galactic plane and around 600 light years perpendicular to it and consists of very hot and extremely dilute gas, mainly hydrogen , which keeps the interstellar dust away. Within this bubble, the solar system is currently moving through a local interstellar cloud known as a local flake . The solar system has been crossing the local flake for approx. 100,000 years and is expected to leave it again in 10,000 to 20,000 years. In the local cloud there are an average of 0.26 atoms per cubic centimeter with varying particle density. The temperature of the cloud is about 6000 Kelvin, a little hotter than on the surface of the sun. The local bubble is the result of supernovae that exploded over the past 10 to 20 million years. Most of the gas in the bubble is in turn shielded in the area of influence of the sun by the solar wind storming towards it.
Even more bubble was 500 light-years in the direction of the constellation Scorpio discovered and Loop I mentioned. It has a diameter of about 1000 light years. At its center is the young Scorpius-Centaurus Association . It is believed that the Milky Way is riddled with hundreds of such hot bubbles.
Milky Way System
Like all stars, the sun and its companions are part of a star cluster or a galaxy . With at least 100 billion (some estimates go up to 400 billion) other stars, it is a member of the Milky Way , a barred spiral galaxy with a diameter of around 100,000 light years. The solar system is located between two of the spiraling star concentrations, between the Perseus arm and the Sagittarius arm , in a local fork, the Orion arm . It lies around 15 light years north of the galactic plane of symmetry, is around 27,000 light years from the galactic center and orbits it at a speed of around 240 km / s for around 210 million years, a galactic year . In addition to this galactic rotation, according to current values from the beginning of the 21st century, the sun is moving at 19.7 km / s in the direction of the sun apex , which with a galactic longitude of 57 ° and a galactic latitude of 22 ° in the direction of the constellation Hercules lies. The sun crosses the disk plane about every 30 million years.
The position of the middle orbit plane of the planets of the solar system does not correspond to the equatorial plane of the galaxy, but is strongly inclined against it. The northern ecliptic pole is located in the constellation Dragon , on the celestial sphere only about 30 degrees from the galactic equator (in the band of the Milky Way that shimmers in the night sky ). The southern pole of the Earth's orbit is in the constellation Swordfish . The north pole of the galaxy is 30 degrees above the ecliptic in Berenike's hair , the galactic south pole in the sculptor . The center of the galaxy is close to the plane of the earth's orbit, in perspective in the constellation Sagittarius . Little of the bright central thickening, the bulge , appears in visible light because it is surrounded by large amounts of interstellar dust in the disk area.
The direction of rotation of the Milky Way system does not match that of the planets around the sun. Seen from the north, the galactic disk rotates clockwise, as if the spiral arms were being dragged from the central area, and thus counter to the direction of rotation of the solar system.
Many astronomers suspect that the spiral structure in the distribution of stars is due to density waves of still unknown origin and that the gas and dust masses of the galactic disk accumulate on them during their rotation and are thereby stimulated to form new stars, see density wave theory . Some paleontologists saw periodic patterns in dated mass extinctions and impact craters and made these density waves, the above oscillation through the disk plane or an undiscovered companion of the sun, see p. Nemesis , responsible for throwing comets off the Oort cloud . However, the existence of such patterns has now been refuted.
The current theory of the formation of the solar system is based on the Kantian nebular hypothesis , according to which large bodies emerged at about the same time from a rotating cloud of gas and dust. The German philosopher Immanuel Kant formulated the idea of a primordial cloud in his work General Natural History and Theory of Heaven in 1755 , but it has only been taken up again by astronomers in the last few decades .
According to current knowledge, about 4.6 billion years ago, instead of the solar system, an extensive molecular cloud moved around a common center within the Milky Way system. The cloud consisted of more than 99% of the gases hydrogen and helium as well as a small proportion of micrometer-sized dust particles made up of heavier elements and compounds such as water , carbon monoxide , carbon dioxide , other carbon compounds , ammonia and silicon compounds. The hydrogen and most of the helium had already been created during the Big Bang . The heavier elements and compounds were created inside stars and released as stardust when they exploded . Parts of the matter cloud contracted as a result of its own gravity and condensed. The impetus for this could have been the explosion of a relatively nearby supernova , the pressure waves of which migrated through the cloud. These densifications led to the formation of probably several hundred or even thousands of stars in a star cluster , which probably disintegrated into free single or double stars after a few hundred million years. In the following, the development of that "fragment" of the cloud of matter from which the solar system was formed - the solar nebula - is considered .
Since the angular momentum must be maintained during the contraction , an already minimal rotation of the collapsing nebula has increased ( pirouette effect ). The resulting centrifugal forces acting outwards led to the cloud forming into a rotating accretion disk .
Almost all of the matter in the solar nebula fell into the center and formed a protostar that continued to collapse. Inside this gas body, pressure and temperature rose until a nuclear fusion process was ignited, in which hydrogen nuclei fuse to form helium nuclei. The energy released in the process generated a radiation pressure that counteracted gravitation and stopped further contraction. A stable star - the sun - was formed.
According to investigations from 2010 (by the scientists Audrey Bouvier and Meenakshi Wadhwa), the age of the solar system was calculated to be around 4.5682 billion years (with a deviation of +200,000 to −400,000 years) using isotope decay.
Formation of the planets
The rotating accretion disk developed into a protoplanetary disk which, according to the previous model, led to the formation of planetesimals, the building blocks of the planets, via the clumping of dust particles (coagulation). These kilometer-sized structures possessed enough mass to unite with other planetesimals to form larger objects due to their gravity . The time of the formation of the mountain-sized planetesimals, and thus the beginning of the planet formation, could be determined by investigations on certain meteorites (see age).
According to more recent models, gravitational instabilities could also lead to self-reinforcing mass concentrations and thus to the formation of planetesimals. The growth was not even. The heaviest objects exerted the greatest gravitational forces, attracted matter from a large radius and could thus grow even faster. The protojupiter eventually disturbed other planetesimals with its gravitational field and influenced their growth. It probably also prevented the formation of a larger body between the orbits of Mars and Jupiter, which led to the formation of the asteroid belt.
The distance between the protoplanets and the young sun had a decisive influence on the processes of planet formation . In the vicinity of the sun, non-volatile elements and compounds condensed , while volatile gases were torn away by the strong solar wind . Here the inner planets, Mercury, Venus, Earth and Mars, with solid silicate surfaces were created. In the colder outer regions, the emerging planets were also able to hold onto volatile gases such as hydrogen, helium and methane . The gas planets Jupiter, Saturn, Uranus and Neptune were formed here.
Some of the matter that was not captured by the planets combined to form smaller objects, the comets and asteroids. Since these celestial bodies have remained almost unchanged since the early days of the solar system, exploring them can provide important information about its history. The investigation of meteorites also brought very valuable insights . These are fragments of planetoids that got caught in the gravitational field of the earth.
New insights into the formation of planets in general emerge from observations made with an instrument at the Paranal Observatory that went into operation in 2014 , a camera called SPHERE ( Spectro-Polarimetric-High-Contrast Exoplanet Research ), which for the first time captured protoplanetary disks in which concentric Orbits around the central star are visible, which are free of gas and dust. These orbits provide information about the exoplanets that arose from the gas and dust on these orbits.
Even if the basic principles of planet formation are already widely understood, there are still numerous open and not insignificant questions.
One of the problems is the distribution of the angular momentum over the sun and the planets: the central body contains almost 99.9% of the mass of the entire system, but has only about 0.5% of the angular momentum; the main part of this is in the orbital angular momentum of their companions.
Furthermore, the inclination of the equatorial plane of the sun in relation to the mean plane of the planets of about 7 ° is a mystery. Due to their extremely dominant mass, the sun (unlike the earth, for example) should hardly stagger due to the interaction with them. In its early days it may have had a dwarf star as a companion or it received a “visit” from a neighboring star of the original star cluster, which overturned the protoplanetary disk by about 7 ° due to its attraction , while the sun remained largely unaffected due to its small spatial extent. In addition, the general validity of the statements about the formation of planetary systems must be questioned, since exoplanets have also been discovered whose orbits run counter to the rotation of their central star, which would not be possible according to the model described above.
- List of the largest objects in the solar system
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