The star we call the Sun has a number of small objects circling around it. Many other stars in our Galaxy have objects orbiting them too and astronomers have recently discovered over four hundred of these other systems already. The largest members of the Sun's family are called planets, and one of these we call home. That planet, Earth, has many unique characteristics that enable life to exist on it. What are the other planets like? What we call a "planet" has been the subject of much debate recently with the latest definition creating a "dwarf planet" class of which Pluto is a member. A "planet" in our solar system is a celestial body that "(a) orbits the Sun; (b) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and (c) has cleared the neighborhood around its orbit." See the Kuiper Belt section in the next chapter for more on this.
We have learned more about our solar system in the past few decades than probably any other field of astronomy. The planets are no longer just objects up in our sky, but places we have been to and explored---worlds in their own right. To give an adequate coverage of each of the planets would fill up a whole book (or more)! Since this website is an introduction to all of astronomy, I will not explore each planet individually. Instead, I will focus on the common characteristics of the planets such as their atmospheres, magnetic fields and interiors.
Special attention will be given to the planets Venus and Mars because their orbits are similar to the Earth's orbit. Venus, Earth, and Mars were made from the same material at about the same distance from the Sun, so they should to be similar to one another. However, they are radically different from one another! Venus is extremely hot with a very thick, carbon dioxide atmosphere, Mars is cold with a very thin carbon dioxide atmosphere, and the Earth has moderate surface temperatures and a moderately-thick nitrogen atmosphere that also has a large amount of very reactive molecules of oxygen. At the end of the chapter I include a section about the large moons of the planets and the rings found around each of the jovian planets.
The next chapter covers the objects that give us clues of our origins: comets, meteorites, and asteroids. These objects are much smaller than planets and are made of left-over material that did not get incorporated into the planets. I will also give a brief description of what we know about how our solar system was formed in the next chapter.
This chapter covers aspects shared by all of the planets. There is a lot of high-quality information about each of the planets available on the web. Several people and organizations have put together very nice tours of the solar system with actual photos from space missions. Starting points for the best of these tours are given on the Planet Links web page (will display in another window). In addition most of the planet images or their captions are linked to the web sites from where I got the images. Usually, you will be able to find a higher resolution images on those sites. Vocabulary terms are in boldface in this chapter on planetary science.
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last updated: January 7, 2011