The four planets closest to the Sun are called the rocky planets. Their names are Mercury, Venus, Earth, and Mars. They are called rocky because they have hard, solid ground you could stand on, just like the ground outside your house.

Mercury is the smallest planet and the closest to the Sun. It has almost no air to trap heat, so it is boiling hot during the day and freezing cold at night. A year on Mercury is very short because it zips around the Sun so fast.

Venus is about the same size as Earth, but it is covered in thick, cloudy air that traps heat. This makes Venus the hottest planet of all, even hotter than Mercury. Its clouds are full of poison, so nothing can live there.

Earth is our home, and it is the only planet we know of where living things can survive. It has air to breathe, water to drink, and just the right temperature for plants, animals, and people. That is why we call it a special blue planet.

Mars is known as the Red Planet because its dusty ground is full of rusty red dirt. It is smaller than Earth and very cold. Scientists send robots called rovers to drive around Mars and look for signs that water once flowed there.

All four rocky planets are smaller than the giant planets farther out. Because they are made of rock and metal, they are heavy and solid, which makes them very different from the huge, puffy gas planets.