The Solar System
Where do planets come from?The answer lies in the stars,or at least in the birthplaces of the stars,which are vast clouds of gas and dust,scattered through space.Our Sun was born in such a place,more than four and a half billion years ago.This is how it happened. Attracted by its gravity,enough gas and dust pulls itself together into a disc.Inside the disc,solid pieces come together-each one growing as it takes in more material and eats up smaller neighbors.The winners will become the planets.Meanwhile,building pressure heats up the center of the disc to one million degrees-hot enough for nuclear reactions to begin.In a flash,the young Sun turns itself on,blowing the disc away,starting an infant solar system.
Our solar system can be divided into three distinct parts made of rock,gas and ice.As this astronomer explains:
"The scale model of our solar system that I've set up showing the different distances of the planets illustrates these parts very clearly.For example,here orbiting close to the Sun are four small rocky planets:Mercury,Venus,Earth,Mars.Too small at birth to hold on to the lightest elements,these planets represent rare concentrations of minerals in a universe that is 98% hydrogen and helium gas."
Between the last rocky planet,Mars and the first of the gas planets,Jupiter,lies a rocky wasteland called "the Asteroid Belt".And beyond this,"A new kind of solar system,this one made of gas,takes over.Planets here are composed mainly of hydrogen and helium. They're enormous.They're widely spaced.They command vast systems of rings and moons, as though each one were almost an imitation of the Sun itself.Jupiter is the largest and nearest,then comes Saturn,Uranus and Neptune."
And just like the Asteroid Belt marks the boundary between the rocky and gas planets, astronomers now believe a similar region called "the Kuiper Belt" exists just beyond the orbit of Neptune,the last of the gas planets.Because it's so far from the Sun's warmth, anything in this Belt is rich in ice and frozen gas.
One thousand times farther away than the Kuiper Belt,hundreds of billions of comets drift at the edge of the solar system.Occasionally,the Sun's gravity pulls some comets closer, allowing us to see them and study them,letting scientists learn more about the beginnings of our solar system.
Translation to be continued~