3 . ① Is there intelligent (有智力的) life on other planets? For years, scientists said “no” or “we don’t know.” But today this is changing. Seth Shostak and Alexandra Barnett are astronomers (天文学家). They believe intelligent life is somewhere in the universe. They also think we will soon contact (联系) them.

② Why do Shostak and Barnett think intelligent life is on other planets? The first reason is time. Scientists believe the universe is about 12 billion years old. “This is long enough for other planets to have intelligent life,” say Shostak and Barnett. The second reason is size, the universe is huge. “Tools like the Hubble Telescope (哈勃望远镜) have shown that there are at least 100 billion galaxies,” says Shostak. “And our galaxy, the Milky Way, has at least 100 billion stars. Some planets going around these stars might be similar to the earth.”

③ In the past, it was hard to look for signs of intelligent life in the universe. But now, powerful telescopes (高倍望远镜) allow scientists to discover smaller planets—the size of Mars or Earth—in other solar systems. These planets might have intelligent life.

④Has anyone from space already visited the earth? “Probably not,” says Shostak.“It’s a long way away. However, intelligent beings may contact us in other ways, such as radio signals (信号). In fact, they may be trying to communicate with us now, but we can’t receive their messages. However, this is changing. By 2025, we could make contact with other life forms in our universe and we might help each other.”

4 . ①Which planet is the most beautiful in our solar system? Many people may answer “Saturn” (土星). Saturn is a funny-looking planet. It has beautiful rings. Of all the planets in the solar system, Saturn’s rings are the biggest and brightest. According to scientists, however, these rings will one day disappear completely.

②How did Saturn get its rings in the first place? Science has no sure answer. But there is a popular theory: Some small planets ran into each other with Saturn’s moons. Earth has only one moon. But Saturn has at least 60 moons. And this left behind a large amount of ice and rock pieces. These pieces are different in size. Some are as small as a grain of sand. Others are as large as a house.

③These small pieces began to travel around Saturn. Saturn’s gravity (引力) pulled them toward it. At the same time, the speed of their orbit (轨道) tried to throw them into space. Over time, these pieces of ice and rock formed rings around Saturn.

④A new study, however, suggests that Saturn’s gravity is winning. About ten tons of “ring matter” is falling into Saturn every second. If this continues, the planet’s ring system will disappear in 100 million to 300 million years’ time.

⑤According to scientists, Saturn’s rings are between 10 million and 100 million years old. “We’re lucky to be able to see Saturn’s huge ring system.” One of the scientists said. “Some of the planets in our solar system only have very thin rings today. Perhaps we just missed seeing their huge ring systems!”

⑥Stars and planets die and are born in the universe. Will the change of Saturn’s rings affect our planet?

Is there intelligent life on other planets? For years, scientists said “no” or “we don’t know”. But today this is changing. Seth and Alex are astronomers (天文学家). They believe there must be intelligent life on other planets. They also think we might soon communicate with these beings.

Why do Seth and Alex think intelligent life exists on other planets? The first reason is time. Scientists believe the universe (宇宙) is about 12 billion years old. “This is long enough for other planets to have intelligent life,” say Seth and Alex. The second reason is size—the universe is so vast. “Tools like the Hubble Telescope (哈勃望远镜) have shown that there are at least 100 billion galaxies (星系) in the universe,” says Seth. “And our galaxy, the Milky Way, has at least 100 billion stars. Some planets going around these stars might be similar to Earth.”

In the past, it was hard to look for signs of intelligent life. But now, powerful telescopes allow scientists to discover smaller planets—the size of Mars or Earth—in other solar systems. These planets might have intelligent life.

Have intelligent beings from space already visited Earth? “Probably not,” says Alex. “It’s a long way away. However, intelligent beings may contact us in other way, such as using radio signals. In fact, they might try to communicate with us in the past, but we didn’t have the right tools to receive their messages. However, this is changing. By 2025, we could make contact with other life forms in our universe and we even might help each other.”

6 . Mars is the fourth planet from the sun. It is home to both the highest mountain and the deepest, longest valley (山谷) in the solar system (太阳系). Olympus Mons, the highest mountain on Mars, is about 27 kilometers high, about three times as tall as Qomolangma. And the deepest valley on Mars reaches as deep as 10 kilometers and runs through east to west for about 4,000 kilometers, about one fifth of the distance around Mars and close to the width of Australia.

Mars is much colder than Earth because it’s farther away from the sun. The highest temperature can reach 20℃ while the lowest can be as low as -140℃. Just like Earth, the sun gives Mars seasons. However, the seasons that Mars experiences are more extreme than Earth’s. For example, when Mars is farthest from the sun, the southern part of Mars experiences a very long and cold winter, about six months on Earth.

Mars lies farther from the sun than Earth does, so the red planet has a longer year—687 days compared to 365 for our home world. However, the two planets have similar day lengths, 24 hours and 40 minutes for Mars and 24 hours for Earth.

Mars is far from an easy planet to reach but many countries like the USA, Russia, China and Japan are always trying to send people there. Robots have got the tickets to Mars and it’s believed that humans will get the tickets by the 2030s, although it is still a big challenge now. And scientists say it will take at least six months for people to travel to Mars.

7 . Astronomers (天文学家) can tell just how hot the surface of the moon gets. The side of the moon toward the sun gets two degrees hotter than boiling water. The night side reaches 243 degrees below zero.

In a lunar eclipse (月食) the earth’s shadow falls on the moon. Then the moon’s temperature may drop 300 degrees in a very short time.

A temperature change like this cannot happen on the earth. Why does it happen on the moon? Astronomers know that the surface of the moon is dust. On the earth, the rocks store (储存) heat from the sun. When the sun goes down, the rocks stay warm. But the dust of the moon cannot store heat. So when the moon gets dark, the heat escapes quickly. The moon gets very cold.

Scientists think that there has been life     1     the earth for hundreds of millions of years. However, we have not found life on any other planets yet.

The earth is a planet and it     2     (go) around the sun. Seven other planets also go around the sun. None of them has an environment     3     that of the earth, so scientists do not think they will find life on     4     (they). The sun and its planets are called the solar system, and our solar system is a small part of a much     5     (large) group of stars and planets, called the Galaxy or the Milky Way. There are     6     (billion) of stars in the Galaxy, and our sun is only one of them.

Scientists have also     7     (discover) many other galaxies in the universe. They are very far away and their light has to travel     8     many years to reach us. So how large is the universe? It is     9     (possible) to imagine.

Scientists have sent spaceships to the planet Mars to take     10     (photo). They have even sent spaceships to travel outside the solar system. However, no spaceship has traveled far enough to reach other stars in our Galaxy.

The moon is the earth’s closest neighbor. It is the brightest object in the night sky. Scientists have studied the moon for thousands of years.

The moon is about 4.5 billion years old. It is made of rocks. It is covered with craters (火山口). They were made when objects hit the moon’s surface.

The moon has no weather. That means there is no wind or rain. Footprints (脚印) left by astronauts could last billions of years.

The moon has eight phases (月相). Once a month, we see a full moon. Sometimes we can’t see a full moon at all. The shape we see depends on how the earth, the sun, and the moon line up.

Only 12 people have walked on the moon. U.S. astronaut Neil Armstrong was the first one in 1969. He took a photo of Edwin Aldrin.