Oort Cloud is surrounded bysolar systemSphere ofcloud cluster, full of many inactivecomet, about 50000~100000 astronomical units away from the sun, and the maximum radius is about 1 light year, that is, the sun andAdjacent starA quarter of the distance.Astronomers generally believe that the Oort Cloud is the remnant of the nebula that formed the sun and its planets 5 billion years ago and surrounds the solar system.
It was proposed by the Oort Cloud. The scientific community has different theories about their formation. But today, astronomers believe that Oort Cloud is the remnant of the nebula that formed the sun and its planets 5 billion years ago, and is surrounded bysolar system。
The most widely accepted assumption is that the Oort cloud object is actuallyKuiper belt The area closer to the sun is similar to other planets and asteroids, but because they are often affected by the gravity of large planets, and later driven out of the solar system by the strong gravity of still young large gas planets, such as Jupiter and other celestial bodies, they have extremely elliptical or parabolic orbits and are scattered in the outermost layer of the solar system.At the same time, this process also deviates their orbitsEclipticAnd form the Oort cloud in a spherical shape.The orbits of some distant celestial bodies are perturbed by nearby stars, making them round, and can stay far away from the sun for a long time.And awayplanetsThe Kuiper Belt is formed by the objects of the Kuiper Belt, which are not affected by the big planets and are scattered in the disk area near the ecliptic.This theory explains why the Oort Cloud is not as close to the ecliptic as the orbits of the Kuiper Belt and the eight planets, but is in a unique spherical shape.
Phenomenon description
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In 1932,EstoniaAstronomer Ernst Julius Opik proposed that comets are clouds from the outer edge of the solar system.But in 1950, Dutch astronomersJane Hendrick Ault(Jan Hendrick Oort) pointed out that there is a contradiction in this inference, that is, a comet will be destroyed by many factors if it keeps going back and forth inside and outside the solar systemlife cycleNever as old as the solar system.What the cloud cluster receivessolar radiationWeak, very stable, with more than millions of comet nuclei, which can constantly generate newcomet, to replace the destroyed.In addition, it is believed that the total mass of all Oort Cloud comets will be 5 to 100 times that of the Earth.
Among the discovered asteroids, only Sedna/90377 asteroid is considered to beOort cloud With orbits between 76 and 850Astronomical unitBetween, the orbit is closer to the sun than expected, possibly from the inner layer of the Oort Nebula.If the conjecture is correct, then the distance of the Oort nebula must be closer to the sun than estimated, and the density will be higher.
IfcometJust fast flying ice, where did they come from and how did they get here?In 1950,NetherlandsAstronomer Jane Oort concluded that there are a large number of comets in the outer edge of the solar system, later known as Oort Nebula.
In the four centuries since the invention of the telescope, only a small number of comets in the Oort Nebula have entered the solar system.Comets, protected by the cold upper space, are considered to be the remnants of early nebulae when the solar system was formed.
Olt's theory is based on years of observation of comets.The time interval between comets means that most comets have long circular motion tracks.Olt thinks the comet originated fromExoplanetA cloud like region billions of miles away.The region is so far away that the sun cannot integrate it into the solar system.
In the early 1980s, researchers began to revise Ault's theory.According to their understanding, the Oort Nebula floats on the edge of the solar system and is vulnerable to the gravitational effect of nearby stars.
According to their calculations, sometimes these forces will drag comets from the Oort Nebula toInterstellar space。In this way, they are closer to the sun.At this time, Jupiter's gravity will either push them into smaller orbits or drive them out of the solar system.Only 5% of the comets have ever returned to their homes, where the number of comets will decrease gradually.
But this theory seems to be inconsistent with the series of comets that are seen steadily across the Earth's sky every year.In order to solve this contradiction, scientists added another point of view to Ault's theory in 1991.According to this view, there is a larger celestial body outside the inner layer of the Oort Nebula, and the inner ring is like a reservoir, constantly providing new comets for the outer ring.
Although the Oort Nebula has yet to be discovered, most astronomers believe that it does exist.They also believe that it is composed of debris left over from the formation of the solar system.staySpace probeUntil then, the existence of the Oort Nebula will be a mystery.
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Cometary nebula
Where comets come from is a fascinating question as well as a puzzling one.When astronomers study the source of comets, they often have to make statistical analysis on the orbits of comets to see how they are affected byMegaplanetsWhat is the orbit before gravitational perturbation? Find the law from it.In 1950, Holt, a Dutch astronomer, pointed out that 41Periodic cometThe original orbit ofPlutoThere is a huge "ice house", or a huge "cloud", outside the orbit.This cloud extends to about 2.2 billion kilometers away from the sun.All comets in the solar system come from this cloud cluster, so people call it Comet Cloud or Oort Cloud.
Today, the distance of the Oort cloud is generally set at about 150000 astronomical units, roughlyPluto4000 times the distance.It will take more than two years for the fastest light to come to our solar system from there, so it will take a long time for comets here to circle the sun. They can only be seen when they are hundreds of millions of kilometers away from the sun.Most of their time in orbit is spent away from the sun.withIkeya Guan CometFor example, its speed near the perihelion is 500 kilometers per second, and it only takes two hours to run half of the distance to the sun, but it will take more than 1000 years to run the distance from the sun.The period of Comet Ikeya Guan is not long, someLong-period cometIt takes millions of years to travel a week.So although astronomers estimateComet Oort CloudThere may be 100 billion comets in thecometOnly five or six on average.
Since the comet cloud is very far away from the sun, there is no large and round sun in the position of the comet cloud. The sun has really become a "common star" with brightness higher than that seen on the earthSiriusIt's still darker.But the comet cloud is even farther away from other stars than can be imagined. The comet cloud cannot get the light and heat of any star, so it looks like an "iceberg".
Comets come from this icebergGuests on the icebergIt is also a series of icebergs, large and small, with a diameter of more than 10 kilometers, larger than the highest peak on the earthMount QomolangmaIt is also spectacular, and the small one is only a few tens of meters.These icebergs are made of a large amount of ice and dust.In addition to water ice, there are carbon monoxide icecarbon dioxideIce (dry ice), ammonia ice, methane ice, etc.Because there are a lot of dust materials mixed in the ice material, the iceberg looks gray and black, not as crystal and lovely as the Antarctic iceberg we see on TV.American astronomerWhippleGive them a very vivid name, called "dirty snowball".
Due to the disturbance of the stars passing through the sun's adjacent area to the original cometcometLeave the comet cloud, turn your head, or run outside the solar system, or run toward the inner solar system.Most comets follow hyperbola or parabola orbits when they move towards the sun. After thousands of years of travel, they are captured with telescopes as they get closer to the sun.Some cometsMegaplanetsWhen they meet, the orbit is perturbed and becomes an elliptical orbitAperiodic cometIt became a new periodic comet and began to settle down in the solar system.
In 1958, some American astronomers believed that there was another comet storehouse in the solar system, namely the so-called "Kuiper Comet Belt".This ring-shaped comet is carried awayNeptuneThe orbit is not far, and it is estimated that there are at least thousands of comets in the belt.Short period cometAll from this comet bank.Compared with the Oort cloud, this comet belt is much closer to the Earth.Kuiper beltAfter the proposal, some astronomersLarge telescopeSegmental observations were made in this area, but nothing was found.There are three possibilities. The first possibility is that there are not as many comets as expected in the Kuiper Belt, the second possibility is that the Kuiper Belt may be located farther away, and the third possibility is that there is no Kuiper Belt at all.
In the late 1980s, after five years of hard searching, American astronomers David Jowett and Jane Rue finally found the first celestial body outside the orbit of Pluto (referred to as Extraterrestrial Object for short) on September 14, 1992, named 1992QB1, which is 41 astronomical units away from the sun.In March and December 1993, they successively discovered three extrasolar objects, whose distances from the sun were 46, 32 and 35 AU respectively.In the same year, British astronomers also found two celestial bodies 33 and 34 astronomical units away from the sun.By the end of 1997, astronomers had discovered 56 extrasolar objects (including Pluto).
Astronomers call them celestial bodies because they are not sure that they are planetsasteroid, or comet, but in any case, this is also an important and remarkable discovery in astronomy.
Astronomers have not yet fully understood the movement and internal structure of comets, so whether it is the Oort Cloud or the Kuiper Belt, it is a hypothesis of the origin of comets and has not been finally confirmed.Astronomers agree that the formation period of comets from the original solar nebula is basically the same as that of the sun and planets, and comets are a natural by-product of the creation of the solar system.
As for the origin of comets, it can be said that there are different opinions. Up to now, there is no consensus.One view is that,celestial bodies Volcanic eruptions on the Earth threw a lot of materials into space, and comets were formed from these materials.Such views can be called "eruption theory".The other view, called "collision theory", believes that in a very remote era, two celestial bodies in the solar system collided with each other, resulting in a large number of pieces of material, forming comets in the solar system.These hypotheses have some problems that are difficult to explain and are difficult to be recognized by most astronomers.
Primordial cloud hypothesis
Among the hypotheses about the origin of comets, the so-called "primordial cloud hypothesis" has been widely introduced and appreciated by a considerable number of scientists.Based on the statistical study of the orbits of a large number of comets, the primordial cloud hypothesis believes that: long-period cometsElliptical orbitMany of the aphelion points are between 30000 and 100000 astronomical units, which leads to the conclusion that there is a material group called "protocloud" at the edge of the solar system, which is about 150000 astronomical units away from the sun. Like a huge envelope, comets are formed from the material in it.Primordial clouds are often called "comet clouds" because this hypothesis was first proposed by Dutch astronomers in the 1950sAultIt is also called "Oort Cloud".The Oort Cloud is like the main "homeland" of comets.
According to Alter's estimation, there may be as many as 100 billion comets in the envelope of the comet cloud.This is really a huge comet "warehouse"!Each of these comets takes millions of years to circle the sun.They are mainly attracted by nearby stars, some comets are affected by Jupiter, etcMegaplanetsThe influence of gravity becomes periodic comet.Other comets may be thrown out of the solar system.
Structure and composition
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The space occupied by the Oort cloud is extremely large. Its nearest distance from the sun is 2000~5000 astronomical units (0.03~0.08 light years), and its farthest distance is 50000 astronomical units (0.79 light years).The farthest distance is estimated at 100000~200000 astronomical units (1.58~3.16 light years) in some literatures.The Oort cloud can be divided into a spherical outer cloud cluster with a radius of 20000~50000 astronomical units (0.32~0.79 light years) and an annular inner cloud cluster with a radius of 2000~20000 astronomical units (0.03~0.32 light years).The outer layer is less constrained by the inner solar system, and it is a long-period comet (possibly also a Halley type comet) that is enteringNeptuneThe former place of origin within the orbit.The inner layer, also known as the Hill's Cloud, proposed its existence in 1981Jack G. Hills(Jack G. Hills).The theoretical model predicts that the number of comet nuclei in the inner layer cloud cluster is dozens or even hundreds of times more than that in the outer layer.The thin outer layer will gradually disappear over time. Some scholars believe that the inner layer continues to add new comets to the outer layer, which is the reason why the Oort Cloud still exists billions of years after its formation.
Among the outer celestial bodies, there may be trillions of objects larger than 1 km, andAbsolute magnitudeThere are billions of more than 11 (that is, more than 20 kilometers in diameter), which are tens of millions of kilometers away from each other.The total mass of the Oort cloud is still uncertain, but if we assume that the comet nuclei in the outer layer areHalley's CometThe mass is similar, and the total mass is estimated to be 3 × 10twenty-fiveThe kilogram is about five times the mass of the earth.In the early estimation, the quality of the Oort cloud was higher (up to 380 Earth masses), but after more accurate understanding of the size distribution of long-period comets, the estimation was correspondingly reduced.There is no similar quality estimate for the inner cloud cluster.
According to the observation of the nature of comets, most of theOort Cloud ObjectAre composed of water icemethane、ethane、carbon monoxideandHydrogen cyanideIs composed of "ice".However,1996 PWThe appearance ofD-type asteroidBut the orbits belong to long-period comets.Its discovery has led some theorists to suspect that the Oort cloud may also contain 1% to 2% of asteroids.It is pointed out that the long period comets and Jupiter family cometscarbonnitrogenisotopeThe difference in ratios is not significant, although the origins of the two are quite different.This means that both types of comets originate from the originalSolar nebula。Studies on the particle size of Comet Oort Cloud, and on theTempel 1 Post impact studies support this conclusion.
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No human madeSpace probeArrive at Oort Cloud.Among the probes leaving the solar system, even the ones with the fastest speed and the farthest distanceVoyager 1It will take 300 years to reach the Oort Cloud, and it will take 30000 years to cross it.In addition, theRadioisotope thermal motorBefore and after 2025, it will no longer be able to provide enough power for scientific instruments of the same industry, so it cannot be used for any substantive exploration of the Oort Cloud.The other four are leaving the solar system（Voyager 2、Pioneer 10 、Pioneer 11 andNew Horizon No）You will also be unable to work when you arrive at Oort Cloud.
