The Kuiper belt is a region in the Solar system which begins for the Neptune. But scientists at the moment don’t know where it comes to an end. We don’t know what occurs on the outer edge of the Kuiper belt and where it is, but we know that it it is very far: some open objects of the Kuiper belt have unusual orbits which in 2000 times more, than distance between Earth and the Sun.
Opening of the Kuiper belt
Nobody predicted detection of the Kuiper belt. Nobody wrote work in which it would be told: “Look for objects of such brightness, such size and in such quantity here”. But there were assumptions. The most known of them is an assumption of Gerrard Koyper, American astronomer of the Dutch origin. In 1951 he wrote work in which said that it is strange that the Solar system comes to an end on Pluto, and, perhaps, it proceeds also after it. It sounds normally for modern readers. But, besides, Koyper told: “If on border of the Solar system there were small objects, gravitation of Pluto (whom we consider by the same massive celestial body as Earth, or it is more) very long time ago would destabilize orbits of these objects, and this region would be empty”. Koyper was wrong about Pluto: it not so массивен, contains only 0,2% of mass of Earth and doesn’t render such effect on surrounding celestial bodies. The irony consists that Koyper didn’t assume existence of what began to be called subsequently the Kuiper belt. He assumed that it isn’t there. It is an example of the law of Stigler: “No discovery was called in honor of the pioneer”. Stigler’s law was opened by Robert Merton that it proves this statement.
To Koyper scientists also made different assumptions. One of them was made in 1943 during World War II by the Irish by the name of Kenneth Edzhvort. He wrote one or two sentences in the article and told: “Perhaps, there are some celestial bodies on the edge of Solar system which too dim that we saw them (he called them clusters), and, perhaps, they treat comets”. But it not the scientific assumption, it isn’t based on anything, and with it nothing can be made. It reminds records of Nostradamus who in the 16th century accidental predicted World War II and murder of the president Kennedy. If you write something fuzzy, you leave for future generations a scope for thoughts. Someone can decide that you knew what you told about though actually it was not so.
When we began to look for the Kuiper belt in 1986, computers were such feeble that nobody could calculate dynamics of Solar system. It was necessary to work with approximate digits which developed analytically, and it is very difficult. At that time there was a great interest to that from where short-period comets because their estimated source — an Oort cloud — wasn’t found yet come. The Uruguayan astronomer Julio Fernandez wrote article in 1980, having assumed that for the Neptune there can be an area from where short-period comets come. This article was already similar to the scientific assumption. Unlike Koyper and Edzhvort’s operations, it seems convincing in a retrospective. But it didn’t motivate scientists on searches, including us. Sounds badly, but it was just one more article.
First objects of the Kuiper belt
The scientific method is often described as assumptions which are proved by observations. But the science often works not so. In astronomy almost nothing opens by means of assumptions and almost all important opens accidental. Theories are often created to describe new things which give in to observations. Seldom happens so that the made assumption is confirmed by observations. We are just insufficiently good for this purpose. Nevertheless without suitable model in 1985 we wouldn’t know that the fact that on boundaries of Solar system it is empty, seems to the strange. Beyond Saturn were Uranium, the Neptune and Pluto — three objects. At the same time an internal part of Solar system is complete of different objects: asteroids, comets, other planets. And it was very strange: why the Solar system shall be empty with the edge and complete of objects inside? That is why we decided to conduct a research. It is empty because all objects are remote, or it empty because far objects too dim that we noted them. We didn’t think of the Kuiper belt, didn’t think of what is for the Neptune, we were happy that we know at least what is beyond Saturn, and any more there is nothing was to speak. As a result we began a research which called “a research of slow objects”. It was aimed at finding something beyond Saturn.
It has turned out that it is very difficult to count distance to an object if you don’t use special geometry to direct the telescope directly towards the Sun. When you do it, the speed of the movement of an object on the sky inversely proportional to distance because of parallax. It as two planes: the one that flies above at a speed of 50 miles/hour crosses the sky longer, and what flies low at the same speed, crosses the sky very quickly. We can measure distance proceeding from speed. We used this simple tactics of observation opposite to the Sun, and then used parallax to measure distance. That is why we called it “a research of slow objects”. We looked for slowly moving objects because, most likely, these objects are located very far.
We for years couldn’t find anything interesting. We have found many objects like asteroids in Solar system, but haven’t found anything beyond Saturn, and looked for it. We have spent about 5 years for this research and didn’t find anything valuable up to 1992. And then have found an object. He was not just behind an orbit of Saturn — he was far outside the known region of Solar system. We called this object 1992 QB1. It was the farthest object which was ever observed in Solar system.
It was is fascinating. The matter is that until you find the first object, you don’t know whether that is useless, what are you doing, know whether in the correct direction you look for. You don’t even know whether is there what to look for. But as soon as you find one object, all doubts disappear. It so influences all work, views that you pass for all psychological barriers. The fact that it seemed impossible becomes a commonplace when it is already made. I worked together with Jane Lu who was post-dock at that time. After we have found 1992 QB1, we have begun to find also other objects. We have found about 40 or 50 objects within the next several years. Other scientists have joined this game, and to the middle of 2016 total number of the known objects made nearly 2 000. It is a lot of.
Objects of the Kuiper belt and migration of planets
Soon we have made many surprising discoveries concerning the Kuiper belt. For example, we have found out that there are different types of objects of the Kuiper belt. We have given them different names: classical, resonant, scattered and isolated. They dynamically differ from each other — generally for the reasons connected with gravitational control of the Neptune which is quite massive planet (by 16 times massivny Earth) and is not so far from some objects of the Kuiper belt. The Neptune imposes dynamic structure on the Kuiper belt because of the gravitational influence. We have proved that we Pluto — it simply one of big objects of the Kuiper belt, have defined distribution of the sizes and masses in the Kuiper belt and have understood that it is only an iceberg top: from objects which we saw we have taken 100 000 objects of the Kuiper belt more than one hundred kilometers and one billion objects more than one kilometer. It is amazing that earlier they were completely unknown.
In spite of the fact that there is a lot of objects of the Kuiper belt, we have found out that their weight is quite small and equal only 10% of the mass of Earth. It was a riddle: how these bodies if they have such small weight are formed? Not enough material is widespread on the large volume of the Kuiper belt. These bodies grow very slowly. Models of small mass of the Kuiper belt became a hot topic. They have been based on the idea that the Kuiper belt was much more massive when it has begun to be formed — in 20 or 40 times massivny Earth. But the most part of weight has been lost.
The key to understanding of loss of weight consists in other observation made by us. It consists that objects of the Kuiper belt “are attached” by an orbital resonance of the Neptune. It means that their siderichesky cycle time divided into the siderichesky period of the Neptune is the relation of small integers. For example, in a resonance from 3 to 2 Neptune bypass the Sun three times for the same time for which objects of the Kuiper belt manage to round the Sun only two times. It means that force of an attraction of the Neptune affects bodies in that orbit therefore force grows as when we swing a swing and force is increased over time.
This opening has made Rena Malkhotra of Arizona in the 1990s soon after opening of the Kuiper belt. Observation of the first resonant objects has led to emergence of this fine model. But the question is in how to tighten these objects in a resonance. If just to scatter Kuiper belt objects, the few from them will enter such resonance what we observe. Rena has explained also it. She made a start from Fernandez and Uyingga Yip’s works in which it was said that planets migrate. Radiuses of orbits of planets not always were such as now: The Neptune, for example, at first was closer to the Sun, and then moved in the direction from him.
And while he departed further, his resonances were pushed out and collected Kuiper belt objects. It is similar to how snow gathers in a shovel when we in it push it. As the resonance crossed the Kuiper belt, objects to him “stuck”. It explains why in an orbital resonance there are a lot of objects. This only explanation for why in a resonance with the Neptune there are so many bodies. The Kuiper belt shows that planets were created not in those orbits in which they are now. They migrate.
Influence on Solar system
The Kuiper belt has strongly affected understanding of an origin and dynamics of Solar system. Before the Solar system was similar for hours: a set of the planets rotating around the Sun easy, steadily it is predictable and it is even boring. After detection of the Kuiper belt, and especially resonant objects because of which planets migrate unusual opportunities have appeared. If planets were carried away there where they are now, they, perhaps, have passed through each other resonances. If this is so, then they have shaken Solar system, and there were different chaotic processes. In some models loss of 99,9% of objects of the Kuiper belt could result from severe concussion of Solar system which happened as a result of interactions between Jupiter and Saturn which has resulted from migration of planets.
The understanding that the structure of the Kuiper belt depends on migration of planets has changed the direction of researches of Solar system. Features which haven’t been expected and which nobody predicted were surprisingly important for understanding of our place in this system. Influence of the Kuiper belt on studying of Solar system and evolution of her formation was huge. Our understanding of an origin of architecture of Solar system strongly differs from the fact that we thought earlier. And now we understand that the Solar system works not like clock-work.
Comets usually not really big (about a kilometer in the diameter), and they lose weight (she goes to a tail). We can consider how long the comet can lose weight to our measures. And it occurs not really long — about 10 000 years. The comet nucleus can’t be the same age, as Solar system which already 4,5 billion years. Most likely, they have appeared in Solar system recently. In other words, they only appear in Solar system somewhere near Earth and as soon as they appear, begin to evaporate. The question is in that from where they undertake.
There are two answers to this question. The first has been formulated in the 1950th years by the Dutch astronomer Jan Oort. He has found out that long-period comets (those whose orbits are more senior than 200 years) have an elliptic orbit of very big size which extends randomly. Approximately equal quantity comes from the different parties: from the northern hemisphere, from southern, from a spherical and isotropic source. The spherical source is called an Oort cloud. It looks as the big bee swarm surrounding Solar system. He is huge, in 50 000 or 70 000 times more of distance between the Sun and Earth. It is a source of long-period comets. We don’t watch objects in an Oort cloud because they too dim for our telescopes. Everything that we know about an Oort cloud, including data on his existence, has been received from comets which were beaten out from an Oort cloud by gravitation of the stars which are flying by by.
On the other hand, korotkoperiodichesky comets (with the period less than 200 years) have rather small and round orbit. They are distributed not randomly, and, on the contrary, are combined with the plane of orbits of the Solar system. Same question: from where they undertake? Oort said that they come from an Oort cloud, but Jupiter could catch them and break their orbits so that they created a disk. This idea was accepted from the 1950th to the 1980th years. But it turned out that to Jupiter enough long-period comets from an Oort cloud are difficult to grab and to do them korotkoperiodichesky.
The Kuiper belt which we know delivers to the Solar system korotkoperiodichesky systems. And as the belt is much closer (50 astronomical units instead of 50 000 astronomical units of an Oort cloud), we can watch it, but not just objects which flew in near-earth space. It is one more reason for which the Kuiper belt so made a noise among astronomers.
Kuiper belt and other star systems
Residual disks are analogs of the Kuiper belt which are around other stars. Many stars of the same type, as the Sun, have disks from dust in which parts of dust in a disk can’t long live. We can consider how long there is a dust, and this term is small. The fact that the star still has dust (or residual dust) a disk, means that dust appears from some source. Model of the Kuiper belt are the best dust source known to us. One difference consists that the majority of residual disks more massivna, than the Kuiper belt. It meets with that thought that the Kuiper belt was much more массивен, than it is now. If to look at massive residual rings, it is possible to understand how the young Solar system looked.
Future directions of researches
Detection of the Kuiper belt has given us the best understanding of how the Solar system is arranged, but all of us can’t see her far parts yet. We can’t watch an Oort cloud because it is too far also objects insufficiently bright. Even it isn’t so simple to find external parts of the Kuiper belt. We assume that the Kuiper belt mixes up with an Oort cloud, and would like to know where and as it occurs. We would like to measure orbital structure of a belt in more detail. Then we would have stronger guesses about an origin and evolution of Solar system. For example, resonant capture works differently if planets migrate slowly and smoothly and if they migrate quickly and in the jumping mode. Measurements of orbits of objects of the Kuiper belt can potentially tell us as the Neptune, and, perhaps, even migrated as well as as long he did it. We have constructed models which adapt to new observations of Solar system, but some features remain unclear. The outer edge of the classical Kuiper belt is not the natural sequence of the offered models. Future observations can help to solve this problem, but it is more important to construct new models to improve the general understanding of the device of Solar system. Eventually we would like to explore the Kuiper belt by means of the space vessel. Unfortunately, the existing rocket technologies aren’t ready to this task. In the next decades progress will come from observations by means of land and space telescopes.