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The Solar System 2003

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The Solar System - a tiny part of the Universe, a vast, almost empty space.

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It is an alien environment for a human - without atmosphere, without solid land underfoot.

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The Sun, planets and their moons, asteroids and comets are a kind of small islands,

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where mass is concentrated.

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Smaller bodies - meteoroids, interplanetary dust and gas or particles of radiation - photons,

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also do belong to the Solar System.

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A mutual attraction, a gravitational force acts among all of them and governs their motion.

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The Sun is by far the most massive, therefore other bodies orbit around it.

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The closer to the Sun a body is, the higher its orbital speed is.

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The farthest planet is Neptune and then other follow: Uranus, Saturn, Jupiter, Mars, Earth,

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Venus and Mercury.

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The mean distance between the Sun and the Earth, so called Astronomical Unit, is 150 million km.

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The light travels this distance in 8 minutes 20 seconds.

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The diameter of the Earth is only 13,000 km.

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Planets and moons do not shine with their own visible light,

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they only reflect a part of the sunlight.

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The Sun is, on the other hand,

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a star and as such it radiates by the entire surface to all directions.

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Sometimes one can see radiation reflected from the smallest particles of interplanetary dust

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- this phenomenon is called the zodiacal light.

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Meteoroids, stony fragments from fractions of milimetre upto a few metres in diameter,

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can be easily observed at the moment, when they hit the air surrounding our planet.

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Particles move with a relative velocity of about 50 km/s.

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They begin to heat themselves by a friction of air 150 km above the Earths surface.

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A warm and shining column of ionized air, which we see on the sky, is called a meteor,

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folkly speaking a "shooting star".

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Several times per year, for example around the 12th of August or the 18th of November,

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the Earth travels through the streams of cometary meteoroids,

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what causes a meteor stream on the sky.

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Larger and harder meteoroids can "survive" the flight through the atmosphere and they fall

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on the ground as meteorites.

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Bigger bodies are asteroids.

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They typically have an irregular shape and they consist of stony material.

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There orbit about one million asteroids larger than 1 km in the main belt between Mars and Jupiter.

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Small moons of planets, like Phobos or Amalthea, are very similar to asteroids.

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Big moons are comparable to small planets: Jupiters moon Ganymede is even larger than Mercury.

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Bodies with diameters above 1,000 km already have more regular, spherical or elliptical shape.

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The four planets closest to the Sun - Mercury, Venus,

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Earth and Mars - are called Earth-type or terrestrial planets.

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They are relatively similar to each other: they comprise mainly of stony material,

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they have a solid surface and, except Mercury,

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they are covered by a thin layer of gases - an atmosphere.

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The most common chemical elements in the Earths body are iron, oxygen, silicon and magnesium.

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The outer planets Uranus and Neptune are ice giants.

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Their substantial parts are ices of water, methane, and ammonia or a mixture of hydrogen,

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helium and rock. Jupiter and Saturn are gaseous giants. We call them Jupiter-type planets also.

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Their relatively small cores probably consist of rock and ice,

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the extended mantles are made of metallic and molecular hydrogen and helium.

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All giant planets have rings (even thought only rings of Saturn are easily observable)

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and numerous families of regular and irregular moons.

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The Sun. An incandescent gaseous sphere, with a surface temperature of approximately 6,000 deg C.

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There is a hidden natural thermonuclear reactor inside,

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in which nuclei of hydrogen atoms are transformed to nuclei of helium atoms and photons

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and neutrinos are emitted at the same time.

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This source of energy has operated already for four and half billion years.

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Sunspots and eruptions are prominent demonstrations of changes on the Sun.

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Both of them are induced by a strong magnetic field,

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that affects motion of solar material - electrically conductive ionized gases, it means plasma.

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The most extended objects in the Solar System can be comets.

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Their icy-stony cores are tiny (they are a few kilometres in diameter),

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but when they come near the Sun,

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ice starts to sublimate and escaping gas and dragged dust create a coma and a tail.

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Their light hydrogen envelopes may reach a size of upto 100 million km.

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Cometary tails are always directed approximately away from the Sun.

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Molecules of gas and dust grains interact with the radiation and solar wind

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- a stream of charged particles,

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which spread from the Sun to the interplanetary space with a velocity of about 500 km/s.

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Mercury, the first planet, gets the most solar radiation energy.

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There is a temperature over 300 deg C on the illuminated hemisphere,

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but the reverse hemisphere cools quickly downto -200 deg C.

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The surface is covered by impact craters, which were formed by collisions with asteroids and comets.

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A big part of the surface has never been observed by a space probe.

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Venus is covered by a dense atmosphere

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(the pressure on the surface is 90 times higher than on the Earth).

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Regardless it orbits the Sun twice farther than Mercury and it reflects two thirds of solar

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radiation, there is an unprecedented temperature 460 deg C.

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The reason is a strong greenhouse effect

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- carbon dioxide in the atmosphere transmits the visible solar radiation towards the surface

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but it absorbs the infrared radiation radiated from the surface and that way impedes its cooling.

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Radars were able to look below the opaque atmosphere

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- they detected a surface characterized by fractures and volcanos.

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It is possible to deduce, from the number of impact craters,

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that the volcanic activity reformed the most of the surface of Venus 700 to 500 million years ago.

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The third planet from the Sun is the Earth.

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It has a suitable size and distance from the Sun,

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so there may exist not only ice or water vapour but also liquid water on the surface

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- one of the assumptions for creation of life.

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The primordial air was completely converted by the activity of live organisms and it contains mostly

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nitrogen and oxygen today.

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The Earth differs from the rest of planets also by the plate tectonics

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- its crust is cracked to individual plates, that can move on the top of the mantle.

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The ocean floors originate by volcanic eruptions in oceanic ridges and they dissolve

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by the subduction below other plates. New mountains are folded during collisions of the plates.

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This may be important for the life on the dry land,

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because the erosion otherwise would clear all mountains and the entire surface of the Earth would

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be covered by ocean.

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The Earth is followed by the Moon.

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It also has importance for the terrestrial life: it causes, together with the Sun,

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changes of the low and high tide by tidal forces.

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The Moon also stabilizes the rotation axis of the Earth on the long term and thus prevents sudden

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and extreme changes of climate.

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We can observe only one hemisphere of the Moon from the Earth,

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because the Moon rotates around its axis with exactly the same period,

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as it orbits around the Earth. Such phenomenon is called a synchronous rotation.

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The Moon formed approximately 4.45 billion years ago,

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when a Mars-sized body collided with the Proto-Earth.

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A huge amount of fragments was generated by the collision, most of which falled back on the Earth,

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but a part of them formed a disk around the Earth.

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The disk acreted very quickly, probably in a few weeks, into one satellite - our Moon.

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Small bodies, like asteroids, can also appear in the neighborhood of the Earth.

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On average, one asteroid larger than 100 m collides with the Earth every 1,000 years.

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A collision with a 10-km asteroid, which happens once per a few 10 million years,

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can cooperate in the course of large extinction of animal and plant species.

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The best known example is the extinction of dinosaurs 65 million years ago.

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Mars. The planet distinctive by its red colour, which is caused by iron oxides.

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The elevation map and distribution of impact craters on the surface suggest,

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that the north hemisphere could be covered by ocean 3 billion years ago.

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The relics of running water are erosion shapes in numerous valleys.

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The largest of all is Vallis Marineris, 4,000 km long, 7 km deep.

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The valley drained water from the Tharsis region.

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The Tharsis is of volcanic origin and one can find here the biggest volcanos in the Solar System

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- for example Olympus Mons, 27 km high and with the base 600 km in diameter.

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Mars is rather a calm planet from the geological point of view for the last two billion years.

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Weak wind in the light atmosphere plays with small sand grains,

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seasonal changes lead to the regular growing of polar caps in winter and to their shrinking

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in summer.

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We can find the asteroid number (243) Ida, for example, on an orbit between Mars and Jupiter.

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This 50-km asteroid belongs to the Koronis family,

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what is possible to recognize by similar orbits and similar colors of its members.

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These asteroids made up a single body, which was however shattered by a huge impact.

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The current moon of Ida - one-kilometre Dactyl - was formed that way.

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Jupiter has the mass higher than all other planets and smaller bodies of the Solar System together.

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It radiates twice as much energy, as it receives from the Sun.

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The source is evidently a slight shrinkage of the planet and a transformation of the rotational

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energy to heat.

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There is an enormous storm, called the Great Red Spot,

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observed for a few hundreds of years in the atmosphere of Jupiter.

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Probably the most interesting Jupiters moons, out of tens, are Io and Europa.

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Tides of Jupiter are so strong on Io,

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that they deform the whole moon and heat its interior to a temperature of a few thousand degrees

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Celsius. It produces never-ending volcanic activity on the surface.

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The volcanos erupt sulfur to a height of a few hundred kilometres and reshape the surface

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with unbelievable rate.

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Europa is, on contrary, a very smooth moon, covered by water ice.

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But the structure of ruptures and magnetometric measurements prove the existence of sub-surface

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liquid ocean.

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Saturn became famous by beauty of its bright rings.

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Regardless they have radius over 100,000 km, they are at most a few hundred metres thick.

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They look as a series of thousands of differently bright and differently transparent ringlets,

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but in reality they are composed of individual icy-stony fragments with a typical size of 10 cm.

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"Spaces" and other structures in the rings are caused by gravitational perturbations of small moons

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orbiting directly inside the rings or out of them.

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The moon Mimas is responsible for the most prominent Cassini division.

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Uranus is almost not visible on the sky with the naked eye.

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It was discovered by chance using a telescope, by William Herschel in 1781.

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The rotation axis of Uranus is interesting

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- it lies almost in the plane of orbit and Uranus exposes in turn the north and the south pole

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towards the Sun.

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The discovery of Neptune was one of the greatest triumphs of the celestial mechanics

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in the 19th century.

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Adams and Le Verrier calculated the position of the unknown planet from the observed perturbations

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of Uranuses orbit, and Galle then indeed found it on the sky.

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We observe wind with a velocity of hundreds kilometres per second in the upper layers

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of the atmosphere, likewise on other giant planets.

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Spots and storms, which are probably the demonstration of the internal heat source,

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appear on Neptune, similarly as on Jupiter and Saturn (but not on Uranus).

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No space probe has yet visited Pluto and Charon.

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The best ground-based telescopes resolve only a few bright and dark areas on them.

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However, hundreds of other bodies were discovered behind Neptune,

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moving on similar orbits as Pluto or even farther. The whole population is called the Kuiper belt.

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They are icy bodies, often very dark, reflecting only about 4 % of solar radiation.

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The equilibrium temperature on their surfaces is only a few tens of degrees above the absolute zero

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(it is -273 deg C).

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There is a spherical Oort cloud also around the inner Solar System.

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It is not observable directly, but we infer its existence by new long-periodic comets,

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which reach the inner part of the Solar System evenly from all directions.

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There is an area farther away, where the gravity of foreign stars starts to dominate...

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Apart from the eight planets in the Solar System,

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there is also known a few hundreds of extrasolar planets, which orbit foreign stars.

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Contemporary astronomical instruments do not allow to observe these distant planets directly,

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but their properties are calculated from photometric and astrometric measurements of the mother

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stars. The most of extrasolar planets, discovered up to now,

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are bigger than Jupiter and they orbit in the distance less than the Earth orbits around the Sun.

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There exist hundreds of billions of stars in our Galaxy.

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There are tens of billions of galaxies in the entire observable universe...

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What is the past and the future of the Solar System?

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We know, from the analysis of the decay of radioactive elements in primitive meteorites,

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that these meteorites solified 4.56 billion years ago.

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The Sun formed at the same time and the complete planetary system did also.

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Stars, even whole star clusters,

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arise from interstellar gas-dust clouds (with a main component molecular hydrogen).

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There sets in a gravitational collapse, a strong increase of the density, in their coolest parts,

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when the temperature is only a few degrees above the absolute zero.

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Then the pressure and temperature become high and thermonuclear reactions are ignited.

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That is the time of a star birth. A flat disk from the remaining matter forms around it.

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Collisions, which lead to a consequent coalescens of small bodies to bigger ones,

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happen frequently in the disk.

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Finally, only a few large bodies remain, in which the most of the mass is concentrated.

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We call this process the accretion.

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Embryos of planets, planetesimals, are then heated by radioactive decay of unstable elements.

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The mentioned collisions support the heating also.

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Larger bodies are partly or completely melted, what sets up their spherical shape.

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A core arises from heavier rocks by differentiation,

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lighter elements remain in a mantel and a crust.

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There may arise cores of giant planets in greater distances from the Sun,

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because there are enough icy planetesimals, which cannot exist in the proximity of the Sun.

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As soon as the mass of the core exceeds the particular critical value,

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it starts to acrete surrounding gas and the mass of the planet increases many times.

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Finally, strong ultraviolet radiation and star wind cause,

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that the mother nebula heats and blows up to the surroundings space.

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The Solar System thus gains almost the present look.

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The complete process of the creation spans approximately 100 million years.

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As we know from observations of other stars and from models of star evolution,

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the Sun will calmly shine for about 6 billion years.

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Then the reserves of hydrogen in the core will be exhausted,

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the whole interior will be "rebuild" and the Sun will change to a red giant.

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The inner planets may be completely destroyed.

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The red giant will explode as a nova in later phases, it will expand its envelope,

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what will be observable as a planetary nebula for a short time.

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Only a naked cooling core - a white dwarf - will remain from the Sun.

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There is still a lot of open questions in the exploration of planetary systems.

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What can we await in the forthcoming years? Cassini probe will explore Saturn and its moon system

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in detail, the exploration of Mars will continue for example by Mars Express and Mars Exploration

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Rover, Mercury will be mapped by Messenger and BepiColombo probes,

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New Horizons probe will be sent to Pluto.

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Stardust probe should bring samples of dust from the comet Wild 2,

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Hayabusha should obtain a piece from the near-Earth asteroid Itokawa,

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Dawn interplanetary probe will become an orbiter of the asteroids Vesta and Ceres.

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We probably will discover hundreds of other extrasolar planets, some of them as small as our Earth.

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Future big space telescopes will allow to obtain spectra of their atmospheres.

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If we find spectral lines appertaining to nitrogen and oxygen molecules, we can hope,

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that there may exist biosphere on distant planets.

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However, thousands of another, surprising discoveries cannot be predicted...