Have you
ever tried to count the stars in the night sky? Nobody knows how many there
are. A group of stars is called a galaxy, and there are millions of galaxies in
the universe. Our sun is one of the smallest stars on the edge of our galaxy,
the Milky Way.
Let’s
imagine the tour of our solar system. We begin our journey from the sun, but of
course we could never live there- the temperature on the surface is 6000°C! The
first planet we see is Mercury. It is strange because the same side always
faces the sun, so one half of the planet is very hot and the other side is
extremely cold. It orbits the sun once every 88 days. The next planet we see is
cloudy Venus, which has an atmosphere of poisonous gases and temperatures of up
to 500°C! Our Earth is the next planet we pass on our journey. After Earth is
Mars, which is the nearest planet to the Earth. Mars is not very warm.
Temperatures can fall to -100°C. Spacecraft have landed on Mars a number of
times, first in 1976 and more recently in 1997. We now know more about Martian
rocks and the Martian atmosphere. Some scientists now believe there used to be
primitive life on Mars because there is frozen water there.
The next planets on our tour are
giants. The enormous Jupiter has a moon called Europa where scientists think
there may also be water. Next is Saturn, famous for its colorful rings of rock
and ice which go round it. We know very little about the other two giant
planets, Uranus and Neptune. Pluto was also included as a planet until 2006.
Then, the scientist agreed that Pluto was not a planet because it is too small
and does not have an orbit like the other planets.
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