- [Narrator] The Earth isn't flat, but the solar system is. About 4.6 billion years ago, the material that makes up our solar system was in the form of a nebula, which is pretty much a big cloud of gas and dust in space. Gravity flattened this material into a disc like this one and then pulled everything towards the center. Eventually, the pressure became so great that the star was formed. The sun, the remaining dust and gas particles collided with each other and eventually formed larger objects like Earth. Together, the sun and all the objects in space that as gravity keeps an orbit, make up our solar system. We call the biggest objects that orbit the sun, planets. Our solar system has eight of them; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. These eight planets are divided by the asteroid belt. Asteroids are rocks that orbit the sun that are much smaller than planets and they're made up of nickel and iron. And unlike the planets, most asteroids aren't round. Beyond Neptune is the Oort cloud, a space at the very edge of the solar system that's filled with comets, which are like space snowballs that are made up of ice and dust. Sometimes comets enter the inner part of our solar system and can even be seen from Earth with long tails of ionized gases. One of the most famous comets is Halley's comet, which only comes near Earth every 75 years or so. Be on the lookout for it in 2061. The four planets on the side of the asteroid belt, closer to the sun, are called the inner or terrestrial planets. The word terrestrial comes from the Latin word for Earth. Terrestrial planets have solid, rocky surfaces, thin atmospheres, few or no moons and no rings. The closest planet to the sun is called Mercury. It's the smallest planet in the solar system and it doesn't have any moons. Next, there's Venus. It's the closest planet to Earth and like Mercury, it doesn't have a moon. Venus shines bright in the sky, like a star. It's about the same size as Earth, but you wouldn't wanna live on it. It's the hottest planet in the solar system and its atmosphere contains an unbreathable mixture of carbon dioxide and sulfuric acid. Next is our home planet, Earth. Earth is the only planet known to have life on it. The last terrestrial planet in our solar system is Mars. Mars has a very thin atmosphere of carbon dioxide and it has two moons. We've sent robots called rovers to Mars in order to explore the planet surface. The planets on the other side of the asteroid belt are called the outer planets. And unlike the inner planets, they're not made out of rock, instead, they're made out of gases and liquids and they have many moons and rings and they're much, much colder and much, much bigger than the terrestrial planets. For instance, the planet Jupiter is so big that it could fit about 1,300 Earths inside of it. We call Jupiter a gas giant, because, you guessed it, it's giant and made up of gas. Its atmosphere is mostly made up of hydrogen and helium. It has more than 50 moons. One of Jupiter's moons, Ganymede, is bigger by volume than the planet Mercury is. Jupiter spins really fast on its axis. This means that a day on Jupiter is a little less than 10 hours long. If you travel another 650 million kilometers or so from Jupiter, you'd see Saturn, another gas giant. In 1610, Galileo Galilei became the first person to see Saturn's rings. Because his telescope wasn't strong enough, he couldn't tell what they were. He thought it looked like the planet had ears. Even though Saturn's rings look smooth from a distance, they're actually made out of rocks and pieces of ice that orbit the planet because of Saturn's gravitational pull. The next planet is called Uranus. It's the sideways planet. Uranus has smaller rings than Saturn does and it's so far away from the Earth that we can only see it with a telescope. Even farther away from Earth, is the big and blue planet, Neptune. Because Neptune is so far away from the sun, it's really cold here. The average temperature of Neptune is about negative 214 degree Celsius. It's also the windiest planet in our solar system with winds going at more than 2000 kilometers per hour. So if you ever visit, bundle up. The planets don't all take the same amount of time to orbit the sun. For example, it takes Jupiter 12 years to orbit the sun just once. And the same amount of time, Mercury has already orbited the sun nearly 50 times and Neptune hasn't even made it around once. Although humans have visited the moon, we haven't built a rocket ship that can allow us to visit our neighboring planets yet. We've been able to learn more about our solar system by using other kinds of technology. Humans have built incredible telescopes that allow us to see into our solar system and beyond, and we've sent probes to other planets and captured stunning images. Even with all this research, scientists are still learning new things about our solar system and its place in the rest of the universe. So maybe one day you could help solve some of the solar system's great mysteries too.