Imagine you are an astronomer studying a newly discovered solar system that has both rocky inner planets and gaseous outer planets. Based on what you know about the formation of our Solar System, what can you infer about the conditions that led to the creation of these planets?

The fact that the system has rocky inner planets and gaseous outer planets suggests that the protoplanetary disk was warmer close to the star and colder farther out, so only heavy elements could condense near the star while ices could form farther away. Because the inner planets are small and dense, the material there was mostly metal and rock, showing that the inner disk had little gas left to accrete. In the outer parts, temperatures were low enough that hydrogen, helium, and ices could stick together, allowing giant planets to gather huge envelopes of gas. A concrete example is our own Solar System, where Earth formed from rocky material inside the frost line while Jupiter and Saturn grew from icy cores that captured thick gas atmospheres. Thus, the new system likely formed from a disk with a temperature gradient that produced distinct inner and outer planet types.