The nascent Sun did not shine on planets; rather, it illuminated a thick disk of material from which Earth and its solar-system companions later condensed. Figure 1 is an artist’s rendition of what the disk might have looked like. Comets, in their wide-ranging travels through our solar system, gather samples preserved from the disk that ultimately gave rise to Earth. For that reason, much excitement has attended recent missions in which probes smashed into comets and the resulting debris was closely analyzed.
Still, little is known of the structural and evolutionary details of circumstellar, or protoplanetary, disks. In March of this year, two teams of astronomers independently reported observations that could complement the cometary studies: detailed IR spectra of water vapor and organic molecules from regions of protoplanetary disks where terrestrial planets might someday form. The spectra, taken by instruments aboard the Spitzer Space Telescope and at the W. M....
