 We explain the Solar System. This is the Sun, the star of our Solar System. Every object in its vicinity is attracted towards it. It is the biggest energy source and the gravitational center in our Solar System. To understand gravity, let's go back some billions of years ago. During this time, the space was not filled with stars, but with a gigantic gas cloud. The elements in this cloud were drawn to each other. The more massive objects become, the stronger they attract other objects. This effect is called gravitation. Most of the matter accumulated in one place. A very big, hot ball, which makes up for an impressive 98% of all the matter in the Solar System, the Sun. From the center of our Solar System, it exerts gravitational force at all objects around it. Besides gravity, other forces can influence an object in space. Together, these forces define the movement of all objects. If a celestial body revolves around another, we call this movement an orbit, like our Moon, which orbits the Earth. Eight planets orbit the Sun. The inner ones are Mercury, Venus, Earth, and Mars. And the outer planets are Jupiter, Saturn, Uranus, and Neptune. The asteroid belt and the so-called trans-Neptune objects, like the dwarf planet Pluto, are also revolving around the Sun. But the Sun is not only the gravitational center of our Solar System, it is also a source of energy. Fueled by nuclear fission, the Sun emits light and a steady stream of particles, the solar wind. Sometimes you can see it from Earth. This spectacle is called Aurora Borealis, or the Northern Lights, mostly seen in the vicinity of the poles of our planet. The distance between the Earth and the Sun is called an astronomical unit. This entity is used to measure distances in space. The last planet of the Solar System, Neptune, is 30 AU away from the Sun. The solar wind travels up to 150 AU. Its reach is called the Heliosphere. But the Solar System is much bigger. Its boundaries reach as far as the influence of the Sun's gravity. It extends to a place called the Oort Cloud, where most of the comets are believed to come from, up to 100,000 astronomical units away. The places at the edge of our Solar System are largely unexplored, and we are still working to uncover the secrets hidden in the depths of space. But our Sun would not be a real star without being surrounded by an air of mystery, would it?