Over the past five years, astronomers have counted in our solar system several dozen satellites (at the time of preparation of this article was 86), moving around their planets in the opposite direction of rotation! They are called non-regular (as opposed to the regular traded in the forward direction with respect to their planets).
It seems that the irregular satellites have in the old days, when the gravitational influences of the planets newborn plucked smaller body size from their original orbits and transferred them to the back of the orbit. Scientists realized that the study of these processes may shed light on the earliest stages of formation of the solar system.
"Travel" giant planets.
Ironically, the first irregular satellite, named Triton, was detected in 1846 in the penultimate of the known planets in the solar system — Neptune. This satellite, we can say lucky because most irregular satellites because of their small size and low luminosity is very difficult to detect. Besides, unlike the regular satellites are scattered over a large
space. For example, the most remote of the regular satellites of Jupiter is at a distance of two million kilometers of the planet and the known irregular satellites of Jupiter rotate away from him at 30 million miles away!
Recall that the giant planets in our solar system are Jupiter, Saturn, Uranus and Neptune. Until recently, for Jupiter "number" 8 regular and 55 irregular satellites, beyond Saturn — 21 regular and 26 irregular, with Uranus — 18 regular and irregular 9, beyond Neptune — 6 and 7 of regular and irregular past Pluto — 1 regular satellite. Now the picture has changed dramatically. It turned out that the four giant planets have many irregular satellites. According to the extrapolation of the available astronomical data, each of the giant planets should be about a ton and (!) Irregular satellites of more than a kilometer in diameter! In the system of Jupiter's moons, these dimensions vary from 180 miles away (in the Himalayas) to two kilometers in the smallest "fellow travelers". These satellites are moving on the most complex in the solar system — loop-like orbits. Since irregular "fellow travelers" are at a very great distance from their planets they almost equally influence of the gravitational forces of the planet and the Sun. The result is a rapid precession of the orbit (rotation of the major axis of the ellipse it).
Rhythms of the cosmos.
If the specified corresponds to the precession frequency of a planet around the Sun, the satellite is in resonance. In this situation gradually accumulated effects of solar gravity, distorting the satellite's orbit: it is an ellipse is drawn to such an extent that a satellite or planet is faced with his or breaks into the gravitational "arms" of the Sun! Analysis of the various influences on the motion of the satellite in the system resulted in the discovery of Jupiter to 17 fragments crashed in a collision of a larger "companion."
One of the few well-studied irregular satellites of Saturn's moon Phoebe is. It has been researched interplanetary space probe "Cassini" in June 2004. With it, managed to get a clear picture of numerous craters and spectra of reflected sunlight, indicating the presence of ice on Phoebe from water and carbon dioxide. The satellites of Neptune and Triton Nereid investigated probe "Voyager 2". too, were covered with ice. Consequently, they are formed away from the sun.
The remains of "building blocks".
There is reason to believe that the irregular satellites are the remains of "building blocks" of planet formation. Like the nuclei of comets and asteroids, they first turned around the sun, and then were captured by the planets. The famous comet Shoemaker-Levy 9, for example was captured by complex gravitational interactions and has been on a temporary orbit around Jupiter, and then in 1994 was torn apart by the gravitational forces of the planet, and fell on her. If this had not happened, hundreds of years later the comet would appear again on the heliocentric orbit! Astronomers have known for several cases of the return of objects in space around the Sun after a temporary "exile" of Jupiter.
in the 70-ies of the last century, theorists have proposed three options for the script capture satellites. In the first option "candidate" in the satellites, if it is small in size, burn up in the atmosphere of the planet, by which flies If the "candidate" is large in size, it will pass through it and continue to move forward in its orbit around the Sun. Only when the average size of "straying" heavenly body will be captured by the planet.
The second locking mechanism works in the growth stage of the planet. Asteroids and other "candidates" in the satellites fall into a kind of gravitational trap because of the sharply increased mass of the planet. As in the previous scenario, this option does not explain the satellites of Neptune and Uranus, which did not experience a sharp increase in its mass.
In 1971 he was offered a third version of the problem that interests us, which is based on the collision of two bodies in space. But it later emerged that the capture can occur without a collision. In May 2006, astronomers have suggested a different version of the triple capture: When it system "satellite — planet" is broken by gravitational forces, one of the components is ejected from the system, and the second — comes into its orbit.
Where did the "fellow travelers"?
According to the latest astronomical discoveries, all four giant planets in the solar system are surrounded by irregular satellites. In search of their origin, scientists have focused on the study of scenarios triple capture "candidates". That may explain the triple capture nearly the same number of irregular satellites of the giant planets.
However, the question remains: where did these satellites? Scientists propose two possible answers to this question. Under the first option, "candidates" could be the nucleus of comets and asteroids caught in the same area of the solar system, the planets and their captors. The second answer is based on the assumption that approximately 700 million years after the formation of the planets solar system was abundantly littered planetesimals ("germ" of the future of celestial bodies). At one point during the "lead-out" of large planets in their current orbits, and as a result of strong gravitational interactions of Saturn and Jupiter myriad of asteroids and comets have been scattered in space. Some of these wandering bodies could be captured by the large planets. The main part of the scattered "candidates" in the satellites became more beyond Neptune in the so-called Kuiper belt.
Now actively continue the study of systems of irregular satellites. Has failed to show that captures "candidates" in the satellites took place in the early stages of the evolution of the solar system, as in more recent times, the conditions for this are not there. In addition, the similarity of systems of irregular satellites of the giant planets indicates the appearance of these "fellow travelers" as a result of the triple capture — the only mechanism suitable for both Neptune and Jupiter for, thus, irregular satellites are centered around the giant planets keep the memory about events that occurred millions of years ago.