The Sun is a huge mass of hot， glowing gas in the Milky Way Galaxy. The strong gravitational pull of the Sun holds Earth and the other planets in the Solar System in orbit.
The Sun is made up mainly of hydrogen and helium. Like other stars， the Sun’s energy is generated by nuclear reactions at its core. The Sun represents over 99.86 percent of the total mass of the Solar System.
Pluto was discovered in 1930， and it was long considered the ninth planet of the Solar System. Now， Pluto has been classi.ed as a plutoid.
Neptune is the eighth planet from the Sun; its atmosphere， composed of hydrogen， helium and methane， gives Neptune its bluish color.
Uranus is the seventh planet from the Sun， also the third largest planet. Uranus is composed mainly of rock， ice and hydrogen.
Saturn is the sixth planet from the Sun， also the second largest planet. The .rst person to see the rings was the Italian scientist Galileo in 1610， using one of the earliest telescopes.
Jupiter is the .fth planet from the Sun， also the largest. Unlike the rocky inner planets of the Solar System， Jupiter is a ball of dense gas and has no solid surface.
Asteroid belt is circular region between Mars and Jupiter containing the greatest number of asteroids.
Mars is the fourth planet from the Sun. Mars''s crust contains iron oxide， giving it its reddish color. Topic No. Solar System
Earth is the third planet from the Sun. Up to now， it is the only planet with evidence of life.
Venus is the second planet from the Sun. Mercury is the planet closest to the Sun. It is marked by extreme variations in temperature.
2 Earth地球
Earth is the only planet that we know of that provides the right conditions for life. Earth is just the right distance from the Sun so that temperatures are neither too hot nor too cold for living things to survive.
There is liquid water， and there is oxygen in the atmosphere， both vital for life. Earth‘s atmosphere and oceans help to control temperatures.
Earth is home to millions of different living species， from tiny bacteria to giant blue whales.
Earth is a miracle and life is a mystery. Today， our life is just a link of a chain of numerable living beings. The engine of life is linkage. Everything is linked. Nothing is self-suf.cient.
Our Earth lies on a balance， in which every being has a role to play， and exists only through the existence of other beings. It is a subtle， fragile harmony， which is easily shattered.
Earth is made up of a series of layers that formed early in the planet’s history， as heavier material gravitated toward the center and lighter material .oated to the surface.
The dense， solid， inner core of iron is surrounded by a liquid， iron， outer core. The lower mantle consists of molten rock， which is surrounded by partially molten rock in the asthenosphere and solid rock in the upper mantle and crust.
Between some of the layers， there are chemical or structural changes that form discontinuities. Lighter elements， such as silicon， aluminum， calcium， potassium， sodium， and oxygen， compose the outer crust.
Earth’s atmosphere is the layer of gases that surrounds the planet. We live in the inner layer of Earth’s atmosphere， the troposphere. The stratosphere extends from the top of the troposphere to about 50 km above Earth’s surface.
Above the stratosphere， a drop in temperature marks the beginning of the next layer， the mesosphere.
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