The Oort Cloud
The Oort Cloud – a hypothetical spherical Solar System that is the source of long-period comets. Instrumental existence of the Oort cloud is not confirmed, but many indirect evidence points to its existence.
Expected distance to the outer limits of the Oort cloud from the sun is between 50 000 and 100 000 a. is, – a light year. This is about a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and scattered disc, the other two well-known area of trans-Neptunian objects, a thousand times smaller than the Oort cloud. The outer boundary of the Oort cloud defines the gravitational boundary of the solar system – Hill area defined for the solar system of 2.0 mph. year.
The Oort Cloud is thought to include two separate areas: a spherical outer Oort cloud and the inner Oort cloud in the shape of a disc. Objects in the Oort cloud is largely composed of water, ammonia and methane ices. Astronomers believe the objects that make up the Oort cloud formed around the sun, and were scattered far into space gravitational effects of the giant planets early in the solar system.
Although confirmed direct observations of the Oort cloud was not, astronomers believe that it is the source of long-period comets, and comets galleevskogo like arriving in the solar system, as well as many of the Centaurs and Jupiter-family comets. The outer part of the Oort cloud is only estimated boundary of the solar system, and thus, it can easily be exposed to the gravitational forces as passing stars and the Galaxy itself. These forces are sometimes forced comet to a central part of the Solar System. Based on their orbits, the short-period comets may come from the scattered disk, and a few are from the Oort cloud . Although the Kuiper belt and scattered over a remote disk to observe and measure, Oort cloud objects at this point can be expected to consider only four known object: Sedna, 2000 CR105, 2006 SQ372 and 2008 KV42 .
The idea of the existence of such a cloud was nominated Estonian astronomer Ernst Opik in 1932. In the 1950s, the idea was independently proposed by the Dutch astrophysicist Jan Oort as a means to solve the paradox of: in the history of the solar system orbits of comets endure in the long run, the dynamics dictate that a comet must either collide with the Sun or a planet, or should be discarded planetary perturbations of the solar system. In addition, their content of volatile substances means that as they repeatedly approach the Sun, radiation gradually vaporizes them while comets do not disintegrate or not developing insulating crust that prevents further evaporation. Thus, reasoned Oort, a comet may not have formed in their current orbits, and must have spent almost all of its existence in the external cloud.
There are two classes of comets: short-and long-period comets comet. Short-period comets have relatively close orbit with a period of 200 years and a small inclination to the ecliptic plane. Long-period comets have highly eccentric orbits, and the order of thousands. is, and there are all inclinations. Oort noted that there is a peak distribution of aphelion (farthest point from the Sun’s orbit) in the long-period comets – about 20 000 a. is to be expected in this range with a spherical cloud of comets, an isotropic distribution. The relatively rare comets with orbits of less than 10 000 a. is probably passed one or more times through the solar system, and therefore have the orbits of the planets compressed attraction.
The structure and composition
It is believed that the Oort cloud is a vast area from 2000-5000 as well. is, , up to 50 000 a. is,  from the Sun. Some estimates place the outer edge of between 100 000 and 200 000 a. is, This area can be divided into a spherical outer Oort cloud (20 000-50 000 a. AU) and the inner Oort cloud in the shape of a torus (2000-20 and 000. AU). External cloud weakly associated with the Sun and is the source of long-period comets, and possibly family comets Neptune. Inner Oort cloud is also known as cloud Hills, named after the Jack Hills, who proposed its existence in 1981.  Models predict that the inner cloud of tens or hundreds of times more cometary nuclei than in the outer, it is considered a possible source of new comets to supplement relatively meager external clouds, as it gradually depleted. Hills cloud explains the continued existence of the Oort cloud over billions of years.
The outer Oort cloud is believed to contain trillions of comet nuclei larger than approximately 1.3 km (about 500 billion with absolute magnitude brighter than 10.9), the average distance between comets several million kilometers. Its total weight is not known, but, assuming that Halley’s Comet – a suitable prototype for all comets within the outer Oort cloud, the estimated combined mass is 3.1025 kg, or about five times the mass of the Earth. Previously it was thought that the cloud more massive (up to 380 Earth masses), but the latest knowledge in the size distribution of long-period comets have led to much lower ratings. Internal mass of the Oort cloud is currently unknown.
Based on the studies of comets, we can assume that the vast majority of the Oort cloud objects consist of various ices formed by substances such as water, methane, ethane, carbon monoxide and hydrogen cyanide.However, the discovery of the object 1996 PW, an asteroid with an orbit more typical of a long-period comet, suggests that the Oort cloud may be rocky objects.  Analysis of the isotope ratios of carbon and nitrogen in comets as the Oort cloud and Jupiter-family shows only slight differences, despite their rather isolated area of origin. It follows that the objects of these areas come from the original protosolar cloud. This conclusion is also confirmed by studies of particle sizes in the Oort cloud comets and a recent study by the collision of a space probe Deep Impact with comet Tempel 1, belongs to the family of Jupiter.
It is believed that the Oort cloud is a remnant of the original protoplanetary disk that formed around the Sun about 4.6 billion years ago. According to the widely accepted hypothesis of the Oort cloud objects initially formed much closer to the Sun in the same process, which formed the planets, and asteroids, but the gravitational interaction with the young giant planets, such as Jupiter, threw objects at a very elongated elliptical or parabolic orbit. Simulation of the Oort cloud from the origins of the solar system to the current period indicates that the mass of the cloud reached a maximum after about 800 million years after the formation, as the rate of accretion and collision slowed and depletion rate of clouds began to overtake the rate of replenishment.
Julio Angel Fernandez model assumes that the scattered disc, which is the main source of short-period comets in the solar system, could also be a major source of the Oort cloud objects. According to the model, about half of the scattered disk objects moved out in the Oort cloud, while a quarter of the bits in the orbits of Jupiter and Moon ejected on hyperbolic orbits. Scattered disc may be still supplying the Oort cloud material. As a result, one-third of the current objects scattered disk will probably fall into the Oort cloud after 2.5 billion years.
Computer models show that collisions of cometary material during the formation period played a much larger role than previously thought. According to these models, the number of collisions in the early history of the solar system was so great that most comets were destroyed before they reached the Oort cloud. Therefore, the current total mass of the Oort cloud is much smaller than once believed.  The estimated mass of the cloud is only a small part of the ejected material in the 50-100 Earth masses.
Gravitational interactions with neighboring stars and galactic tides changed cometary orbits – have made them more circular. This explains the almost spherical shape of the outer Oort cloud.And the cloud of the Hills, which is more associated with the sun, the result is yet to acquire a spherical shape. Recent studies have shown that the formation of the Oort cloud is definitely consistent with the hypothesis that the solar system was formed as part of a star cluster in the 200-400 stars. These early next star, likely played a role in the formation of clouds, as within the cluster number of close passes stars were much higher than today, resulting in a much more frequent disturbances.
It is believed that comets have two separate areas of origin of the solar system. Short-period comets (with periods up to 200 years) in the conventional theory come from the Kuiper belt or scattered disc, two linked flat discs of icy material, starting in the orbit of Pluto and about 38. is jointly and extending up to 100 a. AU from the Sun. In turn, believe that the long-period comets, such as Comet Hale – Bopp, with periods of thousands of years to come from the Oort cloud. The orbits within the Kuiper belt are relatively stable, and therefore assume that there are only a few of the comet. Scattered disc is dynamically active, and is far more likely the origin of comets. Comets pass from the scattered disc into the sphere of the outer planets, becoming the object, known as centaurs. Then move on to the centaurs internal orbits and become short-period comets.
There are two main families of short-period comets: A family of Jupiter (the large half less than 5 a. AU) and the family of Neptune, or galleevskoe family (a name given because of the similarity of their orbits with the orbit of Halley’s Comet). Family comets Neptune unusual because, even though they are short-period, their primary area of origin – the Oort cloud, and not scattered disc. Assume, based on their orbits, they were long-period comets, and then were captured by the attraction of the giant planets and redirected into the interior of the solar system. This process may also have a significant impact on the orbit of Jupiter-family comets, although most of these comets are thought to have originated in the scattered disc.
Oort noted that the number of returning comets are much smaller than predicted by the model and the problem is still not solved. No known dynamic process can not explain the smaller number of observed comets. Hypotheses for this discrepancy are: destruction of comets because of tidal forces, collisions or heating, loss of volatiles, causing neobnaruzhivaemost some comets or the formation of an insulating crust on the surface. Longitudinal studies of the Oort cloud comets have shown that their prevalence in the outer planets are several times higher than that of the inner planets. This discrepancy could be due to the attraction of Jupiter, which acts as a kind of barrier, a spectacular comet entering the trap and forcing them to face with him, as the comet Shoemaker – Levy 9 in 1994.
Believe that the current position of the majority of comets seen close to the Sun, are explained by the gravitational distortion of the Oort cloud by the tidal forces caused by the Milky Way galaxy. Just as the tidal forces of the Moon bend and distort the Earth’s oceans, causing tides, in the same way galactic tidal forces bend and distort the orbits of bodies in the outer Solar System, pulling them to the center of the galaxy. In the inner solar system, these effects are minor compared to the gravity of the Sun. However, in the outer solar system, the attraction of the sun is weaker and the gradient of the gravitational field Milky Way plays a much more important role. Due to this gradient, galactic tidal forces can distort the spherical Oort cloud, stretching the cloud in the direction of the galactic center and compressing it along the other two axes. These weak galactic perturbations may be sufficient to shift the object of the Oort cloud from their orbits in the direction of the sun. The distance at which the attractive force of the sun gives way to the influence of the galactic tide, the tidal radius is called truncation. He is in the range of 100 000-200 000 a. is, and marks the outer boundary of the Oort cloud.
Some scientists have theorized – perhaps galactic tidal forces contributed to the formation of the Oort cloud, increasing the perihelion of planetesimals with large aphelion. The effects of the galactic tide are quite complex and highly dependent on the behavior of individual objects to the planetary system. However, the cumulative effect can be substantial: the origin of 90% of comets from the Oort cloud could be caused by the galactic tide. Statistical models of the orbits of the observed long-period comets show that the galactic tide – the main source of perturbations of orbits, shift them to the inner solar system.
Oort cloud objects
In addition to the long-period comets, only four known objects have orbits that involve belonging to the Oort Cloud: Sedna, 2000 CR105, 2006 SQ372 and 2008 KV42. The first two, in contrast to the scattered disk objects, perihelion located outside the gravitational reach of Neptune, and, thus, their orbits can not be explained by perturbations of the giant planets. If they were generated in the current area of residence, their orbits were to be initially circular. In other circumstances, accretion (association of small bodies in the large) would not be possible because the large relative velocities between planetesimals would be too disruptive.  Their modern elliptical orbit can be explained by the following hypotheses:
Perhaps the size of the orbit and the perihelion of these objects “raised” the passage of a neighboring star, at a time when the sun was still in the original star cluster.
Their orbits, may not have been as yet unknown planetary body the size of the Oort cloud.
They may have been scattered by Neptune during a period of particularly high eccentricity.
They were dispersed by the attraction of possible massive trans-Neptunian disk at an early age.
Perhaps they were captured by the Sun during the passage of smaller stars.
Capture hypothesis and the “lifting”, it seems most consistent with the observations .
August 18, 2008 at the “Sloan Digital Sky Survey: Asteroids to Cosmology” Washington University astronomers led evidence of origin TNOs 2006 SQ372 from the inner Oort cloud.
Sedna (90377 Sedna catalog CIP) – Trans-Neptunian object, probably a dwarf planet. Was named in honor of the Inuit goddess Sedna marine animals. Was opened November 14, 2003 by American observers Brown, Trujillo and Rabinowitz. Perihelion Sedna is three times farther from the Sun than the orbit of Neptune, and a large part of the orbit is further away (aphelion is approximately 960 a. Ie, exceeding the distance Sun-Neptune is 37 times.) This makes Sedna one of the most distant known objects in the solar system except for the long-period comets.
Sedna was one of the contenders for the status of dwarf planet, and while the MAC is not assigned to it that status, some scholars consider it as such. Although the size of Sedna is about two-thirds the size of Pluto, because of its shape, the distance from the sun, it can be assumed that it is in a complex hydrostatic equilibrium. Spectroscopic analysis showed that Sedna’s surface composition is similar to the similar composition of some other trans-Neptunian objects, and is a mixture of water, methane, nitrogen ice Tolin. Sedna’s surface – one of the Reds in the solar system.
Sedna will take about 11,400 years to complete a full circle in its highly eccentric orbit, which is the nearest point from the Sun is at a distance of 76 a. is, and in the future – and the 900. The Minor Planet Center is currently adheres to the version that TNOs Sedna placed in the scattered disk formed from the Kuiper belt, the “scattered” by gravitational interactions with the outer planets, mostly Neptune. However, this classification is disputed, as Sedna never comes close enough to Neptune to be scattered them, why some astronomers (including at its discoverer), there is an opinion that Sedna is likely attributed to the fact the first known member of the inner part Oort cloud. In addition, there is a suggestion that the orbit of Sedna was changed under the influence of gravity passing near the solar system, stars of open cluster of stars, or even that she was captured by another star system. Also there is the assumption that the orbit of Sedna is a proof that the orbit of Neptune has a large planet. Astronomer Michael Brown, one of the discoverers of Sedna and the dwarf planet Eris, Haumea and Makemake, believes that Sedna is the most important from a scientific point of view, one found to date trans-Neptunian objects, due to its unusual orbit, which is likely to lead to valuable information on the origin and early evolution of the solar system.
Sedna was discovered by Michael Brown (Caltech), Chadwick Trujillo (Gemini Observatory) and David Rabinowitch (Yale University), November 14, 2003. The discovery is part of a telescopic survey started in 2001 by Samuel Oschinom at Palomar Observatory. On this day, observed the object, which moved to 4.6 arc seconds for 3.1 hours relative to the stars, estimates indicated that its distance was about 100 AU. Follow-up observations in November and December 2003 with the SMARTS telescope at Cerro Tololo Inter-American Observatory in Chile, and the telescope Tenagra IV Keck Observatory in Hawaii showed that the object was moving at a distant orbit with high eccentricity. Later, the object was identified in old pictures. These data allowed a more accurate calculation of its orbit.
“Our newly discovered object is cold most distant place known in the solar system – said Michael Brown on his website. – That’s why we feel that it is appropriate to name it in honor of Sedna, the goddess of the seas Eskimos, which is believed to live at the bottom of the cold Arctic Ocean “. Brown also proposed (IAU) and the International Astronomical Union Minor Planet Center, to name any objects found in the future in the orbit of Sedna in honor of the gods of the mythologies of the Arctic peoples.  Following the statement, the name “Sedna” was published before the object was officially numbered. Brian Marsden, director of the Minor Planet, said that the publication is a violation of the protocol and some members of the MAC may vote against it. However, against the published names of objection had been received and was not offered any other name for this object. Committee of the IAU name of small solar system bodies officially awarded Sedna name in September 2004, and suggested that in the case of interest, the names of space objects can be assigned to an official of ordered.
Orbit and rotation
Inclination of the orbit is 11,932 °. In Sedna longest orbital period of the known large objects in the solar system, which is about 11,487 years old(also called evaluation 10,836 years and 11,664 years). Semi-major axis of the orbit of Sedna is a = 509,1 a. is, and she is very elongated orbit, with eccentricity equal to e = 0,8506. Perihelion of the orbit of the most distant ever observed for any objects in the solar system, and are, respectively, 76.1 and. ie, Sedna will it in 2076, and the aphelion of 942 a. e . When opening Sedna its distance is 89.6 a. AU from the Sun, ie, it is twice as far away than Pluto. Eris was discovered later in the same manner at a distance of 97 a. E. Although some long-period comets orbit extends further than Sedna, they are too dim to be detected except approaching perihelion in the Solar System. When approaching your Sedna perihelion in mid-2076 , the Sun appears in the sky, its just like a very bright star, only 100 times brighter than a full moon we observed on the Earth, and too remote to be able to distinguish between its disk the naked eye.
Upon detection of Sedna initially assumed that her unusually long rotation period (20 to 50 days), and that the rotation of Sedna may be slowed by the gravitational pull of a large satellite, similar to Pluto Charon. Conducted by the Hubble Space Telescope search for such a satellite in March 2004, none were found, and subsequent measurement of MMT telescope allowed scientists to make a picture of a shorter period of rotation (about 10 hours), which is much more typical for the object.
Absolute magnitude of Sedna equals 1.56 units , and the albedo is within 0,26-0,36.
At the time of opening in 2003, Sedna was the largest trans-Neptunian object after Pluto. Today it is probably only the fifth, giving Plutoid – Eris, Pluto, Makemake and Haumea.
Prior to 2007, the upper limit of the diameter of Sedna is estimated at 1,800 km, but after observing with a telescope Spitzer this value was reduced to 1,600 km. In 2012, research by the Herschel, allow to estimate the diameter of Sedna in 995 ± 80 km, which is slightly more than 40% the size of Pluto and, therefore, is the object Sedna, smaller than Pluto Charon.
On the artistic illustration of Sedna, NASA provided the journalists portrayed hypothetical satellite of Sedna. However, in April 2004, it was determined that Sedna has no satellites. Determining the mass of the object as possible without sending a space probe.
Observations with the 1.3-meter telescope at the observatory SMARTS Cerro Tololo show that Sedna is one of the red objects in the solar system, almost as red as Mars. Chadwick Trujillo and his colleagues suggest that Sedna is red due to the fact that its surface is covered by a hydrocarbon residue or Tolin formed from simpler organic compounds due to prolonged exposure to ultraviolet radiation. Sedna’s surface has a uniform color and spectrum, which is probably due to the fact that it is less susceptible to the influence of other celestial bodies compared to objects that are closer to the Sun, which will be able to leave the bright spots on the ice surface (such as a centaur (8405) Asbol ). Sedna and two other remote object ((87269) 2000 OO67 and (308933) 2006 SQ372) share color with external classical Kuiper Belt objects and Centaurs (5145) Foul, alluding to a similar region of origin.
Trujillo and his colleagues suggest that Sedna’s surface is made up of approximately 60% of frozen methane and 70% water ice. The presence of methane also supports the theory of the existence of Tolin on the surface of Sedna, as it is formed during irradiation of methane. Maria Baruchchi [en] and her colleagues in the comparison of the spectra of Sedna and Triton found absorption bands belonging ices of methane and nitrogen. Because of this, they suggested Sedna’s surface composition different from the composition proposed by Trujillo and his colleagues: 24% Tolin, similar in type to Tolin, discovered on Triton, 7% amorphous carbon, 10% nitrogen, 26% methanol and 33% methane. The presence of methane and water ice was confirmed in 2006 infrared photometry with the Spitzer Space Telescope. The presence of nitrogen on the surface of Sedna suggests that it is at least for a short time could have an atmosphere. During a 200-year-old closer to perihelion period of maximum temperature on Sedna should exceed 35,6 K (-237,6 ° C). Upon reaching the surface temperature data transfer should occur between the alpha phase and beta-phase of solid nitrogen observed on Triton. Reaching temperatures of 38 K, the vapor pressure of nitrogen is 14 microbars (0.000014 atm). However, the deep red spectral slope indicates a high concentration of organic substances on the surface of Sedna, and weak absorption bands of methane indicate that methane is not formed recently and has a more ancient origin. This means that the surface of Sedna is too cold for methane evaporated, and then returned in the form of snow, as it does on Triton, and probably on Pluto.
Using a model of internal heating by radioactive decay is given the assumption that there have Sedna ability to maintain an underground ocean of liquid water.
Sedna’s discoverers claim that it is the first object of the observed Oort cloud, as its aphelion significantly more than in the known Kuiper belt objects. Other researchers it is referred to the Kuiper Belt.
Sedna’s discoverer, Michael Brown gives three versions of how Sedna could be in its orbit: the gravitational influence of an undiscovered trans-Neptunian planet, a single passing star at a distance of about 500 a. AU from the Sun and the formation of the solar system in the star cluster. The latest version of the scientist considers most likely. However, until the opening of other objects with similar orbits, none of the hypotheses can not be verified.
Discovery of Sedna stimulate discussion about what objects in the solar system to be considered planets.
Sedna will reach perihelion about 2075-2076 years. Closest approach to the Sun will give scientists the opportunity to learn more about (the next approximation to wait about 12 000 years). Although Sedna and included in the list of studies in the solar system by NASA, in the near term is not planning any mission.