Why are the four inner planets in our solar system dense and rocky while the outer planets are gaseous? a) in the young universe hydrogen and helium where the only elements. b) intense radiation from our sun blasted much of the hydrogen and helium into the outer reaches of our solar system, while heavier elements remained closer. c) planets form from particles in a disk of gas and dust, colliding and sticking together as they orbit the star. d) a small contending planet, about one-tenth the size of earth, must have collided with earth about 4.45 billion years ago.