This is a really fun wet-felted project. It would be a nice science fair project to make with a child. I've had success doing this with groups of 15 children, each making thier own planet. The key to working with larger groups is to give a few reminders to work slowly and gently with the wool when they first start to felt.
Materials: Styrofoam planet kit (I bought mine at a Michael's store.) It comes with 10 styrofoam balls sized to suit the planets, and two styrofoam rings - one for Saturn, one for a stand which goes under the Sun. The kit also has wires to assemble the project.) You will also need wool roving in a variety of colors with which to cover the planets, the rings and the sun.
Equipment: Warm water to which a squirt of liquid soap has been added. Access to a sink. Terry cloth towel, 12" length from the leg of pantyhose.
Method: Before you begin, look at the directions on the side of the box the kit comes in. It will give the order in which the planets are arranged and some color ideas. I also looked up some images of the planets on the internet. Armed with this information, choose a color and pick up one of the styrofoam balls and wrap the wool roving around it. The styrofoam has a texture which makes it fairly easy to make the fiber stick. Wrap in the manner of winding a ball of yarn. Two or three thin layers is sufficient. Dip this shape into the warm, soapy water. Gently, gently toss this ball from hand to hand for about a minute. This is the only tricky part of the project. If you are too vigorous at this point, you will get a lumpy "brain" texture instead of the nice, smooth surface you want to achieve.
When you feel the wool has started to felt, you can begin to roll the ball between your palms, in the manner of rolling cookie dough. When you are pleased with the smoothness of the surface, take it to the sink and rinse using alternating hot and cold rinses. This rinses out the soap and causes the wool to "full" (that is what the slight hardening of the felt is called). Blot the wet planet on a towel and allow to dry. Here are before-and-after pictures of some Mars:
Mercury, Venus, Mars, Uranus, Neptune and Pluto (I still consider it to be a planet, thankyouverymuch) are made like this, in solid colors of your choice.
I created Earth (haha) with blue and green wool. The larger planets - Saturn, Jupiter along with the Sun, require an extra step. Wrap the styrofoam with the color or colors of your choice. Looking at color images on the internet gave me some good ideas for colors and patterns which would best depict the individual plantes. After the wool is placed on the styrofoam, use a portion of pantyhose leg to secure the wool on the ball. This is much more easily done with two people. One person stretches the section of pantyhose (which has been knotted to form a pouch) leg wide open and the other person carefully places the wool-covered styrofoam ball, endeavoring to keep the wool in place as it goes in. Make a knot to enclose the ball, taking care that the knot is not too tight to undo later.
Dip this nylon-covered ball in soapy water and rub with your hands for three or four minutes. After this, you will notice that some of the fiber starts to "migrate" out of the nylon. At this point, untie the nylon stocking and carefully peel it away from the wool. Dip the now uncovered ball into the soapy water and continue to rub until you are happy with the surface texture. At the sink, rinse with alternate hot and cold water as above. More before and after Jupiter pictures:
I just happened to have a lovely stripey roving to make the rings for Saturn. I realize that they aren't exactly in the correct placement that the real rings are in, I'm happy with the effect. The wool is wrapped round and round, as is the stand that holds the Sun. They are felted in the same manner as the planets.
In the spirit of full disclosure, I lost the styrofoam ball which was supposed to be Venus. I used pink roving to make a solid wool sphere. I was a little careless when I felted it. The result was the dreaded "brain". The rippled look is still O.K. for a planet.
Consult the box for the final assembly instructions. If you have any trouble remembering the order of the planets, use this mneumonic device - My Very Educated Mother Just Sat Upon Nine Pizzas.
P.S. Jupiter turned out to be my favorite.