In this topic you will learn about how Earth provides things needed to support life.

The solar system is the Sun and the objects that are traveling around it. The objects around the Sun include nine planets. Planets are any of the nine objects that travel around the Sun and shine by reflecting its light. Planets travel in an orbit. Planets do not give off light as stars do. They reflect the light from their star, the Sun. Earth and the other planets reflect sunlight. Earth is the third planet from the Sun.

Gravity is a force of attraction, or pull, between any object and any objects around it. Gravity depends on two measurements—mass and distance. The more mass in an object, the greater the pull in the object's direction. The closer two objects are, the stronger the pull of gravity between them.

Earth's gravity is very strong because Earth is so massive. The Sun has more mass than Earth or any other planet. Since it is much more massive than Earth, its gravity is much stronger too. The Sun's gravity holds all the objects in the solar system together.

However, gravity alone is not the only reason why the planets stay in their orbits. Gravity alone would pull the Earth into the Sun, because the Sun is so massive. That doesn't happen because the planets are all moving. The planets have a property called inertia. Inertia is the tendency of a moving object to keep moving in a straight line. It is gravity and inertia that keep the planets in their orbits.

Earth has a solid surface layer, mostly covered by a layer of water, all surrounded by layers of gases. The lithosphere is the hard, outer layer of Earth, about 100 kilometers thick. The rocky surface that makes up the top of the lithosphere is the crust. The crust includes the continents and the ocean floors.

The crust has the soil and many other resources. Earth's resources are materials that help support life on Earth. The hydrosphere is Earth's water. Earth's water is a valuable resource. There is so much water that it covers most of the lithosphere. Most of this water is called the ocean. It is salty because of minerals that have been washed into it over the ages.

The Sun is continually interacting with the lithosphere and the hydrosphere. Plants and other producers trap the Sun's energy and produce food. They also produce the gas oxygen.