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Though we have eight planets in our solar system, they are all quite unique. There seems to be, however, a definite distinction between the inner and outer planets.

Our inner planets, also referred to as terrestrial planets”, are Mercury, Venus, Earth and Mars. All of the inner planets are mostly made up of rock, as opposed to the outer planets, which are mainly gaseous. Our inner planets formed from small grains of dust that collided and formed larger bodies of rock. There they formed along with the Sun during the first 100 million years of our system’s development.

The inner planets are denser and smaller and some have moons, but no rings. Mercury is the closest planet to the Sun and the densest, whereas Venus is the hottest. Earth is, of course, the planet that is abundant in life. Then there is Mars, a planet that has fascinated humans since it was first viewed through a telescope. Recent evidence has leant support to the belief that water once flowed on this now dusty and seemingly lifeless planet. Many hope to find some proof that life once existed on Mars.

Between the inner and outer planets lies the asteroid belt. There, rocks that failed to form a planet, circle the sun. Asteroids are small, rocky fragments left over from the formation of our solar system and can range in size from 952 km to less than 1 km across. The total mass of all the asteroids is less than our Moon. Though there are more than 500,000 of these bodies orbiting, it’s not very crowded: the separation between them is more than 1,000,000 km. Why did these orbiting bodies fail to form another planet? The formation of Jupiter brought an end to any other planet forming in this gap. What makes these especially interesting to astronomers is that these rocks have remained relatively unchanged since the formation of our solar system: they can tell scientists a lot about our early beginnings.

Sometimes these asteroids are dislodged from their orbits and can head toward or away from the sun. Though rare, some have slammed into Earth, playing a major role in the geological formation of our planet.

Beyond the asteroid belt lie the outer planets, also called Jovian planets, Jupiter, Saturn, Uranus and Neptune. What joins these four planets is their gaseous nature. Each one is massive compared to Earth. They have thick atmospheres with hydrogen and helium. Jupiter is the first of the gas giants, and is truly a giant. It has a raging storm, known as the Great Red Spot, which has been raging for at least 400 years. Saturn is considered the jewel of the Jovian planets, its vast ring system a crown around the pale planet. Uranus is a peculiar planet, with its rotation axis at nearly right angles to its orbit around the Sun. Neptune’s supersonic winds rage across the planet at nearly 30 times as far from the Sun as Earth, Neptune's orbit takes 165 Earth years.

In the last two decades we have discovered planets around other stars. In recent years, NASA’s Kepler mission has found 246 confirmed planets, and hundreds of suspected ones outside of our solar system.

More about the Kepler misson.