As we observe more, the universe just gets more interesting. Recipes for planet formation - physicsworld.com Recipes for planet formation Nov 2, 2009 2 comments Observations of extrasolar planets are shaping our ideas about how planetary systems form and evolve. Michael R Meyer describes what's cooking elsewhere in our galaxy and beyond Variations on a theme Anyone who has ever used baking soda instead of baking powder when trying to make a cake knows a simple truth: ingredients matter. The same is true for planet formation. Planets are made from the materials that coalesce in a rotating disk around young stars essentially the "leftovers" from when the stars themselves formed through the gravitational collapse of rotating clouds of gas and dust. The planet-making disk should therefore initially have the same gas-to-dust ratio as the interstellar medium: about 100 to 1, by mass. Similarly, it seems logical that the elemental composition of the disk should match that of the star, reflecting the initial conditions at that particular spot in the galaxy.