There are three types of planets in our solar system: terrestrial planets, gas giants, and ice giants.

The terrestrial planets (including Earth!) are solid, and made of metal and rock. The gas giants, for example Jupiter, are mostly made of hydrogen and helium--the same helium that goes into balloons! The ice giants such as Neptune are made of some hydrogen and helium, but much less than the gas giants, and also include oxygen, carbon, and sulfur. Some of the oxygen and hydrogen is combined in the ice giants to form frozen water, or ice!

Technically, dwarf planets are not really planets at all! The most common definition of a planet is that it must orbit a star, be a sphere due to its own gravity, and it must be big enough that no other objects orbit the star on the same path. Let's talk about each of these criteria:

1. A planet must orbit a star. To orbit means to travel around a large object in space. When an object is large enough, like a star, it has a strong gravitational field. The force of gravity from the star attracts other objects to it. Some of these get sucked into the star, but if an object is moving fast enough and far enough away from the star, it instead continuously travels around the star in a pattern called an ellipse, which is like a stretched circle. All the planets in our solar system orbit the sun. For the earth, orbiting the sun takes approximately 365 days, or one year!

2. A planet must form a sphere due to its own gravity. The force of gravity is attractive--it pulls things together. Every object has a gravitational field and attracts everything around it. The bigger the object, the more gravity, and the stronger the pull. When an object is big enough, it pulls in on itself, and squishes into the smallest possible space, which happens to be a sphere.

3. A planet must have no other objects orbiting the star on the same path. An object that is big enough will attract other smaller objects to it when traveling through space. In the case of a planet, as the planet orbits the sun, it attracts smaller objects, and after enough time, there are no objects left on the same orbit.

Objects that are not big enough to clear smaller objects from their orbit are called dwarf planets. Pluto, for example, is a dwarf planet because although it orbits the sun and is a sphere due to gravity, it is too small to clear all objects from its path. However, not all scientists agree that Pluto should be labeled a dwarf planet and not a "real" planet like Earth, and some argue that the definition of a planet should be changed.

Here are some interesting sites where you can read all about planets: here, and here.

The link says that "Our solar system contains three types of planets: rocky, terrestrial worlds (Mercury, Venus, Earth, and Mars); gas giants (Jupiter and Saturn); and ice giants (Uranus and Neptune). Planets orbiting distant stars come in an even wider variety, including lava worlds and “hot Jupiters.”