Our Sun is not unique in the universe. It is a common middle-sized yellow star which scientists have named Sol. This is why our system of planets is called the Solar System. There are trillions of other stars in the Universe just like it. Many of these stars have their own systems of planets, moons, asteroids, and comets. The Sun was born in a vast cloud of gas and dust around 5 billion years ago. Indeed these vast nebulae are the birth places of all stars. Over a period of many millions of years, this gas and dust began to fall into a common center under the force of its own gravity. At the center, an ever growing body of mass was forming. As matter falls inward, it generates a tremendous amount of heat and pressure. As it grew, the baby Sun became hotter and hotter. Eventually when it reached a temperature of around 1 million degrees, its core ignited, causing it to begin nuclear fusion

With no more gas or dust, the planets, minor planets, moons, comets, and asteroids stopped growing. You may have noticed that the four inner planets are much smaller than the four outer planets. Why is that? Because the inner planets are much closer to the Sun, they are located where the solar winds are stronger. As a result, the dust and gas from the inner Solar System was blown away much more quickly than it was from the outer Solar System. This gave the planets of the inner Solar System less time to grow. Another important difference is that the outer planets are made up largely of gas, and water, while the inner planets are made up almost entirely out of rock and dust. This is due to the same reasons. As the outer planets grew larger, their gravity had time to accumulate massive amounts of gas, water, as well as dust.

In astronomy mythology,Mercury was the Roman version of the god Hermes. He was the messenger for the other gods, and for this reason Mercury is often depicted in pictures with winged sandals. In addition to delivering messages, he was also the protector of travelers, and merchants.

The planet Venus has long been one of the most misunderstood of all the inner planets. Like the Earth, Venus has an atmosphere. However, this atmosphere is far thicker than that of the Earth, making it difficult for modern science to penetrate. Because we cannot see the surface of the planet Venus visually, there is much we still do not know about how this planet looks, and what it is like. However, using special instruments and probes scientists have in recent years unlocked many of the secrets long hidden by this mysterious world.

In astronomy mythology, Her Greek name was Gaea. Earth was the mother of the mountains, valleys, streams and all other land formations. She was married to Uranus. The Earth is the biggest of all the terrestrial planets. A terrestrial planet is a dense planet found in the inner Solar System. Our planet is an oasis of life in an otherwise desolate universe. The Earth's temperature, weather, atmosphere and many other factors are just right to keep us alive. To learn more about Earth, please visit our Earth Sciences website.Earth Sciences website The Earth has one moon. Its name is Luna. To learn about Luna Click Here.Click Here

Mars was the Roman god of war and agriculture. It may not seem like these two things go together, but they do. Mars protected those who fought for their communities, and stayed home to raise crops for food. If you weighed 70 pounds on the Earth, you would weigh about 27 pounds on Mars. Mars excites scientists because its mild temperament is more like the Earth's than any of the other planets. Evidence suggests that Mars once had rivers, streams, lakes, and even an ocean. As Mars' atmosphere slowly depleted into outer space the surface water began to permanently evaporate. Today the only water on Mars is either frozen in the polar caps, or underground. Mars has higher mountains, and deeper canyons than any other planet. The largest canyon on Mars would stretch from New York City to Los Angeles on the Earth. That makes the Grand Canyon look tiny. It Also has the Solar Systems biggest volcano, Olympus Mons.

As you observe the moon from your back yard you will probably notice right away that some areas are darker than others. This is because long ago there were many comets and asteroids, many more than today, which frequently hit the worlds of our Solar System. Every once in a while a very large object would crash hard enough into the Moon to break through its surface rock causing lava to ooze out. When you see a dark spot you are looking at an ancient bed of lava rock from one of these impacts. If a large object struck the Moon today It would not cause a volcano however, this is because the core of the Moon has almost completely cooled down, meaning that there is not any lava in the core to ooze out it is solid rock through and through. Many scientists consider the Earth, and Luna, or Moon a double planet system. This is because they are so close in size. In a photograph of the Moon you can see large scars called craters. Craters are caused by collisions between the Moon, and other objects such as asteroids, or comets.

Jupiter Jupiter known as Zeus in Greece over threw his father Saturn to become king of the gods. He then split the Universe with his brothers Neptune and Pluto. If you traveled to Jupiter on vacation you would be very heavy. If you weigh 70 pounds on the Earth, on Jupiter you would weigh 185 pounds. Jupiter's great red spot visible in the picture to the right is where a giant storm has been raging for at least 300 years. This storm's super hurricane winds blow across an area larger than the Earth. This giant planet has no solid surface. Under its atmosphere is a large liquid ocean of hydrogen and water. What lies in between that ocean and the atmosphere? Actually there is no in between. The atmosphere slowly gets thicker and thicker until it becomes part of the ocean. In other words Jupiter's ocean has no surface on which you could float a boat. The sky slowly becomes the ocean.

Saturn Saturn was the god of agriculture, he was called Cronus by the Greeks. He is the son of Uranus, and father of Jupiter. Saturn over threw his father to become king of the gods, but was then over thrown himself by his son Jupiter. Because Saturn is bigger than the Earth, you would weigh more on Saturn than you do here. If you weigh 70 pounds on Earth you would weigh 82 pounds on Saturn. Saturn is a favorite object for many observers. Its beautiful rings are 169,800 miles wide, but less thick than a football field. In many ways Saturn is similar to Jupiter, but it is much smaller. Under the clouds of methane and helium the sky gradually turns into liquid until it becomes a giant ocean of liquid chemicals. Saturn has several hundred rings. However it is not the only planet with rings. Jupiter, Uranus, and Neptune also have rings.

Uranus In astronomy mythology, Uranus was the lord of the skies and husband of Earth. He was also the king of the gods until his son Saturn overthrew him. It would take you many years to fly a rocket to Uranus. When you arrived you would weigh more because Uranus is bigger than the Earth. If you weigh 70 pounds on the Earth you would weigh 82 pounds on Uranus. Uranus is very odd. Unlike all the other planets and most of the moons in our Solar System Uranus spins on its side. It is believed that long ago a very large object smashed into this planet. The crash was so powerful that it completely changed the direction of Uranus' spin. Uranus may have an ocean of water beneath its clouds. It has a large rocky core, and because of the tremendous pressure could possibly contain trillions of large diamonds. Uranus is almost identical to the planet Neptune. Scientists think Uranus has a huge water ocean underneath its clouds.

Neptune Neptune was originally only the god of water, but was later extended to the ocean when he became associated with the Greek god Poseidon. If you weigh 70 pounds on the Earth you would weigh 84 pounds on Neptune. Neptune has a giant storm much like the storm on Jupiter. This storm is often called The Great Dark Spot. We do not know how long this storm has been active, because it is so far away that we could not get a good view of Neptune until modern times when we began to send robot explorers. For many centuries people did not know that this planet even existed. It was discovered by Johann Galle and Heinrich D'Arrest in Neptune has six rings which circle the planet.

Pluto Pluto was thought to be the god to whom all men must eventually go. Romans believed him to be the god of the underworld. His name in Greece was Hades. Because Pluto is so small you would be very light if you visited. If you weigh 70 pounds on Earth, you would only weigh 4 pounds on Pluto. Pluto is smaller than 7 of the moons in the Solar System. Because it is so small many scientists don't consider it a planet at all. In 1999 a group of scientists attempted to redesignate Pluto as a comet.On August 24th, 2006 Pluto's status was officially changed from planet to dwarf planet. For decades children have been taught that there are nine planets in the Solar System. However, with this change, there are now only eight planets. Pluto has never been visited by space craft. Because we have never sent probes to this planet we know very little about it.

comets Many people think that a comets tail is always following behind it, but actually the coma, or tail can either be behind the comet or in front of it. Which way the tail is pointing depends on where the Sun is. That's right, the Sun's heat and radiation produce a wind called the Solar Wind, as a comet gets close to the Sun it begins to melt. The gas and dust that melt off are blown away from the Sun by the solar winds. So if a comet is traveling towards the Sun then the tail will follow behind, but if the comet is traveling away from the Sun the tail will be in front of the comet. Imagine a place far far away, at the very edge of the Solar System. A place where millions of comets can be seen swishing around in every direction. These Icy comets are orbiting the Sun in two different places, both of which are very distant. One place is called the Oort cloud, and the other is called the Kuiper Belt.

asteroids An asteroid is a large rock in outer space. Some, like Ceres, can be very large, while others are as small as a grain of sand. Due to their smaller size, asteroids do not have enough gravity to pull themselves into the shape of a ball. Astronomers group asteroids into different categories based on the way they reflect sunlight. The asteroid belt is divided into an inner belt and an outer belt. The inner belt which is made up of asteroids that are within 250 million miles of the Sun contains asteroids that are made of metals.