Cut out a circle about 1 foot in diameter from a thin piece of cardboard. The top of a pizza box is a good source for this cardboard. Draw a line from top to bottom showing the diameter of the circle. Then spin the circle, and draw a perpendicular line that intersects the first. The central point will be the location of the sun.
Draw the orbits of the planets using a compass to create perfect circles around the center. Make the first four orbits close to the Sun, more or less evenly spaced. These will be the orbits of the inner rocky planets. Leave a gap for the asteroids, and then draw the next four orbits closer to the circumference of the circle, again more or less evenly spaced. These will be the orbits of the outer gas planets.
Pierce the cardboard at the center and somewhere along the length of each orbit with the sharp point of the scissors. Make the holes big enough for the string to pass through them.
Cut out circles from construction paper to represent the sun and the planets. The sun should be largest, but not so large it crowds out others underneath the cardboard. The others should be large enough to write their names inside their diameters. The color of the sun should be yellow, the Earth blue and Mars red. The other color choices are up to you. If you want, you can draw multicolored stripes on the gas giants to properly represent their appearance. Somewhere on each planet, write its correct name.
Snip nine equal lengths of string. Tape each planet to one end of a string. Slip the other end of each string through the appropriate hole in the cardboard. The sun is in the center. Mercury is in first the first orbit, then Venus, followed by Earth and Mars is fourth. After the gap in the circles drawn on the cardboard, put Jupiter, followed by Saturn, Uranus and Neptune. Tape the ends of the strings to the top of the cardboard. Adjust so that all the planets will hang down at an equal distance from the cardboard.
Cut five more lengths of string. Tape four of their ends to the circle where the lines that intersect at the center reach the circumference. Tie the other four ends together, and tie the remaining string to this knot. You can use a tack or nail to secure the other end of this string to the ceiling.