The solar system comprises eight planets that orbit the sun. One age-appropriate science project involves building a model of the solar system. Students can build a model of the solar system using Styrofoam balls and copper wire (both can be purchased at craft supply stores). Paint the balls in the color of the planets they represent (green, white and blue for Earth, blue for Neptune, brownish yellow for Jupiter) and attach them to each other with copper wire so that they look like they are in orbit.
Individual planets are interesting as well. One second-grade science project involves making a large model of a planet by painting the planet's features on a large inflatable ball. Have pupils choose a planet, look up pictures of the planet on the Web, then paint their ball the same colors as the planet. Once the pupils have painted their ball, they can give a brief description of which colors represent which features on the planet's surface.
All planets, including Earth, have their own unique atmospheric conditions. One project on planetary weather that you can assign to second-grade pupils involves having them research weather conditions on different planets and present their findings to the class. Pupils can rely on easy-to-read sources like educational websites designed for young children.
One controversy in astronomy is the possibility that life existed at some point on other planets in our solar system. For example, some geographic features found on Mars may reflect the presence of water, which is necessary to life as we know it, in the past. Pupils can research the possibility of life on other planets using resources from the National Aeronautics and Space Administration. Once their research is complete, pupils can present their findings to the class or write a one- or two-paragraph summary of their findings. Have pupils state an opinion on whether there is, or has been, life on other planets.