“Twinkle Twinkle Little Star” is a popular song for children. But have you ever wondered why the stars twinkle? I’m afraid many adults don’t understand this question, let’s unlock the mystery of the stars!

When we look up to the sky on a clear, dark night, our eyes can see about 6,000 stars in the sky. Some stars look like they are always twinkling and winkling. They are very beautiful. In fact, most of the stars are shining with a steady(稳定的) light. Not all stars twinkle. The stars that twinkle are generally stars. In addition to the Sun, the nearest star to the Earth, the neighboring star(比邻星), is also more than 4 light years away from Earth. Because the distance is too far, the stars we see are only a small point. Also, from the distant star travels a long way through the atmosphere(大气) down to us on Earth. At the same time, the air is moving in the atmosphere of Earth. The movement of air causes the starlight to get slightly bent. To our eyes, this makes the star twinkle. You will notice that stars closer to the horizon appear to twinkle more than other stars. This is because there is more atmosphere between you and a star near the horizon. However, there is no atmosphere in space to make the stars seem to twinkle. That’s why the Hubble telescope(哈勃望远镜), which is in space, can take such clear pictures of other galaxies(星系).

For thousands of years, humans have explored (探索) the Earth. Nowadays, we are exploring space. Astronomers (天文学家) are the modern day explorers. Now, many astronomers are looking for new planets and new places for humans to live in the future. But where do astronomers start looking?

First of all, astronomers look for a star. That's because our own Earth moves around a star (the Sun).More importantly, it is the correct distance (距离) from the Sun for heat and light. So when astronomers have found the star, they look at the planets around it. In recent years, astronomers have found nearly 400 new planets with stars. However, many of these planets are either too near the star or too far away.

However, if the planet is in a good position, astronomers look for three key things: water, air and rock. Water is important because all life needs water. Humans can drink it and also grow plants with water. And plants produce air for humans to breathe and food to eat. Rock on a planet is also important. That’s because there is often water under the rock.

After many years of scrutinizing, astronomers have found a planet that is similar to the Earth. It’s Gliese 581g and it’s near a star. The astronomers think it has water and rock and the average (平均) temperature is between－31°C and－12°C. That’s cold, but not colder than Antarctica or the Arctic Circle, for example. Gliese 581g is bigger than the Earth. A year on Gliese 581g is only 37 Earth days instead of 365.But astronomers do not think these are big differences and some of them think Gliese 581g will be a new Earth. However, Gliese 581g is twenty light years from the Earth.

The idea of life in outer space has been talked about for a long time. Some scientists say that life development on Earth was far too unlikely for it to have happened anywhere else. Things had to be perfect for us to make it on this planet.

Other scientists say that space is too big. Stars and other planets are far too numerous for there to be no other life in the universe.

For many years, there have been reports of visitors from other planets. People all around the world have claimed to see alien spaceships or even aliens themselves.

There have been glimpses of these so-called UFOs (unidentified flying objects) flying through the air and they have even ben captured on video. Some Americans believe that the U.S. Army found an alien spaceship crashed in the desert and then lied to the press about it. Although these sightings may be true, scientists have not found significant evidence that aliens exist.

If you go out into the countryside on a clear night and look up, you can see thousands of stars. Those stars make up a tiny part of our universe. There are more stars, planets, and galaxies than we can count. Even the smartest scientists can’t even come close to defining how big space is. The number of possible stars and planets out there is bigger than our ability to count. If we are really on the only planet that can sustain life, then we are very special in a universe full of amazing things.