As a yellow dwarf star, the Earth's sun blankets the planet in light, energy and heat. The moon, Earth's only natural satellite, lights up the night sky when it is full. It affects the height and strength of the tides in all of its phases, and is the fifth largest moon in our solar system. The moon, like the sun is not a planet. Scientists posit that it was formed when after an astronomical body as large as Mars collided with the Earth.
TL;DR (Too Long; Didn't Read)
In a solar system, the planets circle around the sun, and moons circle around the planets. You'll also find asteroids, comets and meteoroids traveling a path around the sun. Sometimes comets or asteroids visit from other solar systems outside the Earth's. Scientists speculate that there are tens of billions of solar systems in the Milky Way Galaxy alone, of which are solar system is just a small part.
Until 2006, astronomers had no formal definition of the word planet. In 1991, an object larger than Pluto was discovered far out in the Kuiper Belt, starting an intense debate about the meaning of the word. The International Astronomical Union settled on a description in 2006. The first criterion is that a planet must orbit its sun. In addition, the object must be large enough for the force of gravity to make it spherical. Finally, a planet has cleared its orbit of any other objects, such as asteroids, by attracting them to the planet’s surface or hurling them into space.
The moon is often visible from Earth, but many planets have these satellites. For instance, Jupiter has 63 moons, while 47 orbit Saturn while Mercury and Venus have none. A moon is a natural satellite that revolves around a planet, minor planet or dwarf planet. Pluto, which is classified as a dwarf planet, has three moons: Charon, Nix and Hydra. Moons vary a great deal in size and shape, but most are made from the dust and gas that were going around planets during the formation of the solar system.
A star is a sphere of hydrogen and helium held together by gravity. The pull of gravity would make the star collapse into itself if not for the pressure of nuclear fusion in its core. Heat and light energy are released by this process. That brightness is why you can see stars from such great distances. Astronomers cannot count the actual number of stars in Earth’s Milky Way galaxy. They do estimate, based on the amount of visible light and mass in the galaxy, about 100 billion stars shine there.
Which Is Which
The sun and moon are not planets when you consider the objects in space they orbit. For the sun to be a planet, it would have to orbit another sun. Although the sun is in a orbit, it moves around the center of mass of the Milky Way galaxy, not another star. The sun fits the definition of a star, because it is a giant ball of gases consisting of hydrogen and helium, with nuclear reactions going on inside. The Earth’s moon is also not a planet because it orbits one. For the moon to be a planet, it would be in orbit directly around the sun.
About the Author
Living in upstate New York, Susan Sherwood is a researcher who has been writing within educational settings for more than 10 years. She has co-authored papers for Horizons Research, Inc. and the Capital Region Science Education Partnership. Sherwood has a Ph.D. in curriculum and instruction from the University at Albany.