Recall that jovian planets share these physical properties: Large masses - 14. 5 to 318 mearth. Large diameters - 3. 88 to 11. 2 dearth. Large volumes - 58 to 1400 vearth. Small average densities - 0. 71 to 1. 67 g/cm^3. Like water = 1 g/cm^3, not rock and metal. Lightest element is hydrogen - nucleus has only 1 proton. Second lightest element is helium - nucleus has 2 protons + 2 neutrons. Conclusion: h + he must make up the 99% of these planets. Note: sun also is 99% h and he. Sunlight re ecting from the jovian planets has absorption lines caused by their atmospheric gases (kirchhoff"s 3rd law, p. 118) Id the pattern of absorptions with the patterns of many hydrogen molecules: h2, h2o, Note: we don"t see helium because it does not form molecules and its atoms absorb light only at higher temperatures. Key reason: the jovian planets are far from the sun.