Protoplanetary disks are collections of gravitationally bound gas and dust where stars and planets are formed.

(a) what are the 4 main types of materials that compose protoplanetary disks? and what are their typical relative abundances?

(b) why did the terrestrial planets form close to the sun, and gas giants form far away from the sun?

(c) how do we reconcile the existence of "hot jupiters," which are commonly found around distant stars, with nebular theory?