The solar system is made up of the objects that move around our sun in a regular path. Things were not always this way. The sun and the objects that encircle it started out in a very different form.
It is believed that our solar system began as a large cloud of gas and dust. The cloud had a round shape and spun slowly. The spinning caused the gas and dust to flatten into a large disc. The mass of gas and dust at the center became the sun. The gas and dust were pulled together by gravity so that nuclear reactions started. The rest of the dust and gas outside of this moved so slowly that it started to cling together in places. Eventually, enough material stuck together to form planets.
There are many different objects in our solar system. Of course, there are planets. Each one is unique. Some planets are made of rock or metal elements. Mercury, Venus, Earth, and Mars are like this. Other planets are more like gas and ice. Jupiter, Saturn, and the others are like this. The planets have moons — sometimes more than one.
Our solar system has a major asteroid belt. An asteroid is basically a rock. The largest is about 10km across. Huge numbers of these rocks circle the sun between Mars and Jupiter. Sometimes the asteroids crash into each other and move towards Earth. When they pass near our planet, they heat up and burn. We see them as shooting stars.
Lastly, there are comets which are made of ice and gas. They also go around the sun but not with a normal path. When they are near the sun, they start to melt and form a tail. Far from the sun, where space is cold, the comets remain frozen solid.
1. All of the following are mentioned in paragraph 2 EXCEPT（ ）.
2. The word this in paragraph 3 refers to（ ）.
3. According to paragraph 4, what happens when an asteroid burns near our planet?
