Building a terrestrial planet requires raw materials that weren't available in the early history of the universe. The Big Bang filled space with hydrogen and helium. Chemical elements like silicon and oxygen - key components of rocks - had to be cooked up over time by stars. But how long did that take? How many of such heavy elements do you need to form planets?
Spectra From Different Earth-Like Planets - Of the four terrestrial planets in our Solar System )Mercury, Venus, Earth and Mars), the last three possess atmospheres. The spectra imaged are the type we would expect when searching for Earth-like planets in other solar systems