There are eight planets in orbit around our Sun. These planets form our solar system. The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. There are also rocks, moons, comets, and other objects going around our Sun.
The planet that we live on is perfectly suited to all types of life. Earth has air to breathe and water to sustain us. People are constantly studying the seven other planets in our solar system. So far, there is no evidence that life exists on any of these planets.
All the planets orbit around the same Sun. Planets look a bit like stars in the night sky, but there are differences that we can see even without a telescope. Ancient astronomers knew they were different because they move across the night sky while stars stay in the same place.
The four planets near the Sun are known as the rocky planets because they are primarily made of rock or metals. They are Mercury, Venus, Earth, and Mars (shown above in the right colors and to scale). They have a layer of gas around their rocky cores. The layer of gas changes the weather on each planet. The gas layer on Earth is the air that we breathe. Other planets do not have air that we can breathe. Venus and Mercury are too hot for us to live on. They are too close to the Sun. Mars is too cold for humans to live on. It is too far from the Sun. Earth is just right for us. It has the right kind of air and the right amount of heat.
The years on the four small, rocky planets are all different. A year on Mercury is only about 90 days long. Venus's year is 224 days. A year on Mars is 686 days. These are fairly close to the size of a year on Earth. It takes 365 days for Earth to go around the sun, and that’s our year!
The gas giants are Jupiter, Saturn, Uranus, and Neptune. They are mainly made of gas and dust. They are farther away from the Sun. These four planets are all different sizes. Jupiter is big enough to hold all of the other planets inside of it. All four of these planets have many moons and rings around them. Some astronomers think the rings around Saturn are made of small bits of rock that were once parts of other moons.
The years on the gas giants are very long because they are so far away from the Sun. They make bigger circles as they go around, so it takes much longer. A Jupiter year is almost 12 Earth years. On Saturn, the trip around the Sun takes nearly 30 years. A year on Uranus is 84 years long. Neptune takes almost 165 years to orbit the Sun!
- Some planets do not have moons.
- Planets that are close to the Sun are hot.
- Planets that are far from the Sun are cold.
- Planets look like big, bright stars when we see them at night.
- Planets go around the Sun in paths called orbits.
- Planets spin as they travel around the Sun.
- A planet's year is how long it takes to go around the Sun on its path.
- A planet's day is how long it takes to spin around one time.
Planets and the Gods
The planets have interested people for a long time. When people watched the night sky, they saw the planets move. Stars did not seem to move, so they knew the planets were not the same as stars. They gave the planets names and made up tales about them.
The planets are still named after the gods and goddesses from ancient mythology.
- Mercury (Hermes) is the god of commerce, travel, and thievery in Roman mythology. The planet probably received this name because it moves so quickly across the sky.
- Venus (Aphrodite) is the Roman goddess of love and beauty. The planet is named this since it makes a beautiful sight in the sky. Only the Sun and the Moon are brighter.
- Earth is the only planet whose name does not come from Greek/Roman mythology. The name comes from Old English and Germanic.
- Jupiter (Zeus) was the king of the gods in Roman mythology, making the name a good choice for what is by far the largest planet in our solar system.
- Mars (Ares) is the Roman god of war. The planet probably got its name because of its red color.
- Saturn (Cronus) is the Roman god of agriculture.
- Uranus is the ancient Roman deity of the heavens, the earliest supreme god.
- Neptune (Poseidon) was the Roman god of the sea. Given the beautiful, blue color of this planet, the name is an excellent choice!