When we look at all of the celestial bodies in our solar system, there are many distinctions and categories to decipher. The gas giants (or jovian planets) – Jupiter, Saturn, Uranus, and Neptune – are the largest, independently orbiting planets in our solar system, largely composed of gaseous atmospheres. The terrestrial planets – Mercury, Venus, Earth, and Mars – are characterized by solid surfaces, and they are much smaller. There are additional dwarf planets distributed throughout our solar system; Pluto, Ceres, and Makemake are all well documented dwarf planets. There are additional comets, asteroids, meteors, and other free debris that constantly travel throughout our solar system.
When we look up, we might sometimes catch a glimpse, with a naked eye, of Venus or Jupiter shining brightly amongst a backdrop of stars, or a dusky sky, but the two spheres that shout out for the most attention, shine brightly in day and night: the sun and the moon. As a yellow dwarf star, our solar system’s sun blankets the planet in light, energy and heat, and the moon, Earth's only natural satellite, lights up the night sky when it reflects this very same energy. They are neither planets nor dwarf planets, instead falling into different categories.
TL;DR (Too Long; Didn't Read)
In a solar system, the planets orbit around the sun, and moons (also called natural satellites) orbit around planets. You'll also find asteroids, comets and meteoroids traveling a path around the sun. Comets or asteroids can visit from other solar systems outside the Earth's, and scientists speculate that there are tens of billions of solar systems in the Milky Way Galaxy alone, of which the solar system is just a small part.
Until 2006, astronomers had no formal definition of the word planet. In 1991, the first of the Kuiper Belt objects were discovered, starting an intense debate about the meaning of the word. The International Astronomical Union (IAU) settled on a definition of a planet in 2006, following the discovery of Eris, a dwarf planet slightly more massive than Pluto nestled deep in the Kuiper Belt.
The first criterion is that a planet must orbit its sun. In addition, the object must be large enough for the force of its gravitational pull 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.
In more recent scientific pursuits, planets in other solar systems – called exoplanets – have become a more frequent topic of interest. Many of these exoplanets are thousands of light years away from Earth, contained in their own solar and planetary systems.
The moon is often visible from Earth – as it is locked in Earth’s orbit at only 238,000 miles away – but many planets have these satellites. For instance, Jupiter has 63 moons, Saturn has 47, and Mercury & 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.
Many moons are large enough to support interesting geography and attributes of their own. While Earth’s moon is largely uniform, except for a mottling of craters, Europa (a moon of Jupiter) has the potential for underground oceans, and Titan (a moon of Saturn) has an atmosphere even thicker than Earth’s atmosphere.
The four largest moons orbiting Jupiter – Ganymede, Io, Europa, and Callisto – were discovered by Galileo. They are named after characters associated in relationships with the Roman god Jupiter, although many of these characters come from the original Greek mythology surrounding Zeus (Jupiter’s counterpart).
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 intensity of light 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.
As we look further into our galaxy and beyond, the field of astrophysics continues to leap forward in exciting ways. Stars and the galaxies that contain them are incredible collections of energy and creation. The new James Webb Space Telescope launches by NASA is able to take images of galaxies that are billions of light years away.
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 an 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.
Solar eclipses are actually a great coincidence. The sun and the moon just happen to be proportionally aligned, so that the moon perfectly covers the sun during certain periods of time.
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.