Just as anthropologists sought "the missing link" between apes and humans, astronomers are embarking on a quest for a missing link in planetary evolution. Only instead of dusty fields and worn shovels, their laboratory is the universe, and their tool of choice is NASA's new Space Infrared Telescope Facility.

Launched on Aug. 25, NASA's fourth and final Great Observatory will soon set its high-tech infrared eyes on, among other celestial objects, the dusty discs surrounding stars where planets are born.

While other ground- and space-based telescopes have spied these swirling "circumstellar" discs, both young and old, they have missed middle-aged discs for various reasons. The Space Infrared Telescope Facility's unprecedented sensitivity and resolution will allow it to fill in this gap -- and in the process answer fundamental questions regarding how planets, including those resembling Earth, may form.

"With the Space Infrared Telescope Facility, we anticipate seeing many planetary discs at all stages of development," says Dr. Karl Stapelfeldt of JPL, a scientist with the mission. "By studying how they change over time, we may be able to determine what conditions favor planet formation."

Circumstellar discs are a natural step in the evolution of stars. Stars begin life as dense cocoons of gas and dust, then as pressure and gravity kick in, they begin to coalesce, and a flat ring of gas and dust takes shape around them. As stars continue to age, they suck material from this disc into their core. Eventually, a state of equilibrium is reached, leaving a more mature star encircled by a stable disc of debris.

It is around this time, about 10 million years into the lifetime of the star, that astronomers believe planets arise. Dust particles in the discs are thought to collide to form larger bodies, which ultimately sweep out gaps in the discs, much like those lying between the rings of Saturn.

"You can think of planets as wrecking balls that either clear away debris or gather it up as if it were mud," says Dr. George Rieke, principal investigator on one of the three science instruments onboard the observatory.

Infrared telescopes can sense the glow of the cosmic dust that makes up these discs; however, they cannot detect planets directly. Planets have less surface area than their equivalent in dust grains and thus give off less infrared light. This is the same reason coffee is ground up before brewing: the larger combined surface area of the coffee grains results in a more robust pot of coffee.

Past observations of circumstellar discs generally fall into two categories: young, opaque discs (called protoplanetary discs) with more than enough mass to match our own solar system's planetary bodies; or older, transparent discs (called debris discs) with masses equal to a few moons, and doughnut-like holes at their center. Middle-aged discs linking these two developmental stages have gone undetected.

One of the questions astronomers hope to address with the Space Infrared Telescope Facility is: What happened to all the mass observed in the younger discs? Somewhere in their evolution, mass is either eaten up by the star, ejected by the star -- or transformed into planets that lie in the doughnut holes of the discs. By analyzing the composition and structure of the "missing link" discs, astronomers hope to solve this riddle, and better understand how planetary systems like our own evolved.