What Is The Solar System?
The Solar System, showing the Sun, inner planets, asteroid belt, outer planets and a comet. (Not to scale!)
At the center of the Solar System is the Sun. It is a star, like the billions of other stars in the sky. The other stars are very very far away, so they look tiny. The Sun is important to us because it gives us heat and energy that allows life. None of the life on Earth could exist without the Sun.
The rest of the things in the Solar System orbit (travel around) the Sun. The planets are the largest of these. Each planet is a little like the Earth. But the planets are also very different from each other.
Many of the planets have moons. A moon is like a small planet. Mercury has no moons. Earth has one. Jupiter has 63!
The planets closest to the Sun are called the inner planets. These are Mercury, Venus, Earth and Mars. Then comes a big ring of asteroids, chunks of rock much smaller than planets. This ring is called theasteroid belt. Then come the outer planets: Jupiter, Saturn, Uranus, Neptune and Pluto.
Beyond the orbit of Neptune is another big ring of things like the asteroids, called the Kuiper belt. Kuiper (said "KYE-per") was the last name of the person who first wrote about it. Most of the things in the Kupier belt are hard to see through telescopes.
After the Kuiper belt comes the Oort cloud. Scientists think this is where comets come from. It is very far away, many more times further away than Pluto is from the Sun (over a thousand times). It is near the edge of the Solar System. (Yes, "Oort" was the last name of the person who first wrote about it.)
Amazing Solar System
Who discovered the Solar System?
Anyone who looks up at the sky enough on clear nights can see seven bright objects. These are the Sun, our Moon, Mercury, Venus, Mars, Jupiter, and Saturn. People have known about them for a very long time. Ancient people thought they were related to gods. In Babylon, they named the days of the week after them. Almost everyone was sure that all these things were orbiting the Earth. They did not know we lived in a Solar System.
In 1543, Nicholaus Copernicus figured out that the planets orbit the Sun. Only the Moon orbits the Earth. But he was afraid to say so for most of his life. Then Galileo Galilei pointed a telescope at the sky. He found moons orbiting Jupiter. He was certain Copernicus was right, and he got in trouble for saying so. It took seventy years to convince scientists that the planets orbit the Sun.. Now, almost everyone on Earth understands that we live in a Solar System.
People made better telescopes and found more things in the sky such as moons, new planets, and asteroids. More things are being found today. Recently, astronomers found some things a lot like Pluto. One of them, called 2003 UB313, is bigger than Pluto. Maybe it should be called a planet. Maybe Pluto should not be called a planet any more.
How have we explored the Solar System?
Before the telescope, people explored the sky with their eyes. They saw how the planets seemed to "wander" through the sky. They learned to predict where the Sun, the moon, and planets would be in the sky. They built some observatories -- places for watching the sky. They watched the Sun and stars to tell the time of year. In China, they even knew when the moon would block the Sun. Most people thought that celestial bodies could cause war or peace on Earth.
After telescopes were first made, people kept making them better. Astronomers saw that planets are not like stars. They are worlds, like the Earth. They could see that some planets have moons. They began to think about what these worlds were like. At first, some thought that the other planets and moons had people or animals living on them. They thought about how it would be to live on these other worlds. Then they made telescopes better and saw that there are no plants or animals on the Moon or on Mars.
Now, we can explore by going to some of the other worlds. Twelve Astronauts walked on the Moon about 30 years ago. They brought rocks and dirt back to Earth. Spacecraft flew by Venus, Mars, and the outer planets. The pictures they took showed us a lot of what we know about these worlds. Robots landed on Mars in 1971, 1976, and 1997. They took thousands of pictures of the planets. Two robots, "Spirit" and "Opportunity", are working on Mars right now. They send photos and movies back to Earth. They also check rocks to find out what the rocks are made of.
So far, we have not found any life except on Earth. Maybe tiny one-celled life once lived on Mars. Maybe there is life under the ice on Jupiter's moon Europa. New spacecraft are being planned to look for life on these worlds.
What are made up the Solar System?
Our Solar System is part of the Milky Way galaxy. Galaxies are big mixes of dust, gas, stars, and other things. Inside our Milky Way galaxy are clouds of dust and gas where stars are born. Our Solar System was created in this kind of cloud. A part of the cloud began to get smaller and less spread out. It formed a big, spinning disk of gas and tiny pieces of dust. This disk was thickest at the middle. The middle slowly collapsed until it became the Sun. We are still trying to learn how the planets were formed. Most scientists think that they were formed from the left over gas and dust.
This is how it could have happened. The rest of the disk continued to spin around the Sun. The tiny pieces of dust hit each other and some of them stuck together, next the bits of dust slowly collected to form grains, these in turn joined to form lumps the size of gravel, then pebbles, and then rocks. The rocks crashed together into mountains. The mountains crashed together to make bigger things. These big things swept up most of the rest of the disk to form the planets, moons, and asteroids.
The Sun got hotter as it collapsed. It began to glow. The temperature at the center reached a million degrees. The Sun started to make a lot of light and heat. This light and heat swept away most of the leftover dust and gas between the inner planets. This light and heat are the sunlight we see and feel every day on Earth.