Twinkle, Twinkle, Little Star
How far up does the sky go? It seems to go on forever. No one knows where space ends. No one even knows if it has an end. So let us explore a small piece of it, our system of planets.
People have always gazed at the night sky. Long ago, they saw the moon and the stars just as we do today. But some people noticed something more. They saw that some stars moved slowly across the night sky. Now we know that the moving objects are not stars at all. They are planets circling the sun.
About five billion years ago, the sun and planets were formed. They were made from clouds of gas and dust. Gravity pulled the chunks and pieces together to make lumps of matter. More and more dust was added to them. This made the lumps get bigger. The largest lump became the sun. It is a giant star. It shines brightly because it is an enormous ball of fire. Eight smaller lumps circle the sun. These are the planets. They reflect light from the sun.
The four planets farthest from the sun are Neptune, Uranus, Saturn, and Jupiter. No astronaut will ever land on any of these four. That is because they are made of gas. Each of these planets has rings around it. Each planet also has several moons. Jupiter is the largest planet. Its diameter is eleven times that of the Earth.
Four smaller planets are closer to the sun. They are Mars, Earth, Venus, and Mercury. All of them are made of solid rock. Mercury is the closest to the sun. It is very hot. The temperature there can rise to 840 degrees. That’s almost twice as high as the top setting on an oven!
About four hundred years ago, the telescope was invented. People were able to look closely at the planets for the first time. The early astronomers were surprised. Saturn’s rings were beautiful. Mars had huge craters. Jupiter had several moons. One of the moons was even bigger than Mercury.
Our planet Earth travels around the sun. It is on a path between those of Mars and Venus. We think that Earth is one of the few planets in the universe that has liquid water. Our distance from the sun gives us just the right amount of heat. If we were much closer, the oceans would boil away. If we were much farther away, the oceans would turn to ice. But the temperature is just right. So life in all of its forms is possible on Earth.
Scientists now know that there are other suns besides our own. These suns also have planets circling them. They are very, very far away. Is there life on any of them? We do not know. They might have creatures that are smarter than we can imagine. Or Earth may be the only planet anywhere that has life. As more of space is explored, we may someday find out.