Curious children are masters of the direct question from where do babies come from? to why are flamingos pink? Their boundless curiosity also extends to the heavens. At a recent school presentation about my work with NASA and JPL, a sixth grader asked me why some planets are made of rocks while others are made of gas. Upon probing, I learned the boy was fascinated with the planet Saturn and its exotic rings; he wanted to know more about this huge world and the other gas giants. Wow, I thought, what a great astronomical question. It demonstrated to me that many members of the younger generation seem more interested in exploring the universe around them than do many adults who are supposed to be better educated maybe thats why youth and the future go hand-in-hand. Ok, in answer to the sixth-graders question, why are some planets made of rock and others made of gas? Planets form by a process known as accretion this is a vast lumping together of dust, rock and ice that occurs within a giant spinning disk that eventually becomes a solar system. This process eventually sorts out an array of heavenly objects from stars, planets, asteroids, comets, dwarf planets and brown dwarves, etc. into orbits. But there are exceptions. Some planets may be knocked out of orbit and become rogue worlds that wander off into the infinite void. In the case of our solar system and there are now a growing number of extrasolar exceptions the smaller, warmer rocky planets formed close to the Sun while the cold gas giants formed far away. But in all cases of planet formation, gravity is the powerhouse of accretion. Lets look at gas giants like Jupiter, Saturn, Uranus and Neptune: These familiar gas giants formed around ice and rock cores far from our Sun. As these objects swept up even more ice and dust, their growing gravity trapped lighter-than-air hydrogen and helium atoms that were plentiful in the spinning debris disk of the proto-solar system. Slowly a gaseous world was built, layer by layer of gas and nebular debris, and of increasing pressures. Comets and other objects were also swept up by baby gas giants. The long process of accretion eventually formed the giant planets we know today. Gas giants are said to lack solid surfaces but that isnt entirely true. Deep below the dense, ultra-high pressure atmospheres of gas giants, these worlds have rocky or solid metallic cores. But youd have to pass through layers of gas, liquid metal and frigid slush to get to these surfaces. So in the familiar rocky planet sense, there isnt a surface to land a spacecraft on. Whats in the Sky: On April 21 the Lyrid meteor shower is at its best. Moonlight will probably dampen the view Lou Varricchio, M.Sc., is a former NASA senior science writer. He is currently involved with the NASA-JPL Solar System Ambassador outreach program in Vermont.