The Solar System
Our sun is surrounded by a large number of bodies of various sizes, from microscopic
micrometeoroids (grains one-thousandth of a millimetre across) to the giant planet Jupiter.
All these bodies together are known as the Solar System, because the Sun holds them
close to itself by its gravitation, forcing them to travel around it in elliptical orbits.
The chief members of the Solar System are the Sun and the nine planets. Some of
these planets (Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto) have orbiting
satellites. We now know of the existence of about 60 of them in the Solar System, for
thanks to interplanetary probes we are discovering new, smaller satellites. In recent times
these natural satellites, which came into being along with the planets themselves, 4600
million years ago, have been joined by artificial ones, launched by man. They contain
scientific instruments to help us learn more about the Sun, the planets and their natural
satellites.
We can also consider as satellites the huge number of small bodies a few metres
across which orbit Jupiter, Saturn and Uranus, forming their 'rings'. Satellites and rings
move in the vicinity of their planets, which means in the region where the gravitational
attraction of the planets is greater than that of the Sun. If a satellite escapes from the
gravitational pull of a planet, it ceases to be that planet's satellite. It becomes a planet in
its own right, and its movement is decided by the Sun.
Since the time of Nicolaus Copernicus it has been known that the planets orbit the
Sun along orbits which lie in approximately the same plane - the plane of the ecliptic. The
astronomer Johannes Kepler of Prague deduced from the observations of Tycho Brahe
that the orbits of the planets are elliptical, with the Sun at one of the foci. The nearer a
planet (or other body) is to the Sun, the more quickly it orbits. We see the planets from
Earth against the background of the constellations of the Zodiac, which are distributed
along the ecliptic. If we observe the sk for a long time (several days of even weeks), we
find that the planets move in relation to the Zodiac, seeming to pass from one
constellation to the other. That is why they are called planets- the Greek word planetein
meaning to wander.
Much smaller bodies than planets are planetoids (called also asteroids or minor
planets). You cannot see them with the naked eye, and some of them can be observed
only with the world's largest telescope. Through a telescope the planetoids look like stars,
and only after some hours' observation does it become clear that they are moving in
relation to the stars. The total number of planetoids is estimated at some 400,000, through
only a few thousand have actually been observed. Their total mass is about
one-thousandth that of the Earth. The diameter of the largest, Ceres, is 1020 km (634
miles), which Adonis at 300 m (984 ft) is one of the smallest. Most of the planetoids move
in the belt between the orbits of Mars and Jupiter. Only a few (such as Adonis) come
nearer the Earth, or go beyond the orbit of Jupiter (such as Hidalgo). The smallest planet,
Pluto, is sometimes considered to be a planetold. With its satellite Charon it forms a
double planetoid. Some planetoids have names given to them by their discoveries. The
planetoids within the orbit of Mars are probably extinguished comets. Whose envelope of
ice and snow has been exhausted. They are in fact the cores of short-period comets
(made of rocks), which can never form a coma and a tail again.
The smallest members of the Solar System are the meteoroids -various sized lumps
of rock. They orbit the Sun in elliptical orbits, like the rest of the large bodies. The smallest
meteoroids are composed of a few hundred molecules only, while the largest are
measured in fractions of a kilometre and approach the size of the planetoids. The
meteoroids are known collectively as the meteoroid complex. Its meteoroids are scattered
in and ellipsoidal space around the Sun and it stretches along the plane of the ecliptic far
beyond the Earth's orbit. You can observe the Sun's rays diffused and reflected by the
meteoroid complex before sunrise in autumn or after sunset in spring. It looks like a
shining cone in the constellation of the Zodiac. For this reason we speak of it as the
Zodiacal light.
The furthest planet of the Solar System is Pluto. It is about 40 times further away from
the Sun than the Earth. To use scientific terms, Pluto is about 40 astronomical units away
from the Sun. An astronomical unit is the average distance between the Earth and the Sun:
150 million km (93, 204,000 miles)-or more precisely 149.6 million km. Though Pluto is
often considered to be the most distant planet this is not always the case. Its orbit is an
elongated ellipse and Pluto can come nearer to the Sun than Neptune. From 1979 till
1999 the most distant planet is Neptune. But neither Pluto nor Neptune are on the edge of
the Solar System. The Sun's attraction reaches out much farther. The solar wind ceases
at a distance of some 100 astronomical units. A comet can occur several hundred times
further out than that, at a third of the distance to the nearest star, Proxima Centauri, which
is 4.25 light years away from the Sun, or around 269,000 astronomical units. The comets,
between 30,000 and 100,000 astronomical units away, form the outer envelope of the
Solar System.
Our own star, the Sun, is not the only one accompanied by a planetary system. It is
likely that there are thousands of millions more in the Milky Way with such systems. But as
yet not even the most powerful telescopes are able to observe their planets directly, since
the light these reflect is too weak.