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 exists（存在）somewhere in the universe. They also think we will soon contact these beings（人；生物）.

Why do Shostak and Barnett think intelligent life exists 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 more than 100 billion galaxies,” says Shostak. “And our galaxy, the Milky Way, has more than 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 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.

Have beings from space already visited 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 don’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 might help each other.”

Is there life on Mars (火星) ? We don’t know. We are sure that people on the earth can not live on Mars. Earth people can’t live without oxygen(氧气). There is little oxygen on Mars.

Living on Mars would be like living on a very high mountain. There would be little air. It may be 50℃ at noon and -100℃ at night.

Many people think there may be plants on Mars. We know that Mars has carbon (二氧化碳). This is kind of material (物质) found in all plants and animals on the earth. Mars looks red. In some places, it seems gray. Sometimes the gray color turns gray-green then brown, and then gray again. The gray-green color may be a kind of plant life changing color with seasons.

Will man go to Mars one day? If they do, they will have to take their own oxygen, food , clothing and shelter (住所) with them .

Scientists think that there has been life on Earth for millions of years. However, we haven’t found life on other planets yet. What not?

The Earth is a planet and it goes around the Sun. Seven other planets also go round the Sun. None of them has an environment with air, so people and things cannot grow. The Sun and its planets are called the solar system. The solar system（太阳系）is a small part of our galaxy.

The stars we see at night are the suns in other solar systems. There are more than 200 billion stars in our galaxy, called the Milky Way and our Sun is only one of them.

But scientists have discovered many other galaxies in universe. They are a long way away and their light has travelled for many years to reach us. It is hard to understand how large the universe is.

Scientists have sent lots of spacecraft to look at other planets in our solar system, and some spacecraft have gone beyond the solar system. However, no one has discovered any life in space yet.

But why has no one from other planets sent us a message? Have they tried to send information to us? With so many stars in the universe, are we alone, or is there life out there in space? We don’t know…yet.