Cosmic soot.

Using an infrared telescope to peer far back in time, astronomers have made the first observations of complex organic molecules from an era when the universe was just 4 billion years old, less than a third of its current age.

The molecules, known as polyaromatic hydrocarbons, play a key role in star and planet formation, and they are among the building blocks of life. These chemicals form whenever carbon-based materials don't burn completely. They can be found, among other places, in the sooty exhaust from cars and airplanes and in charcoal-broiled hamburgers.

Lin Yan and George Helou of the California Institute of Technology in Pasadena and their colleagues used an infrared spectrometer on the Spitzer Space Telescope to find the hydrocarbons in distant galaxies that undergo spurts of intense star formation. Such galaxies are bathed in dust, which hides their visible light but causes them to glow brightly at infrared wavelengths.

The atoms in the complex hydrocarbons are present in an amount indicating that more than a single generation of stars produced them, says Helou. Therefore, the presence of the hydrocarbons in the ancient galaxies "tells us that by the time we see these galaxies, several generations of stars have already been formed," he says. That, in turn, suggests that planets and life had early opportunities to emerge in the universe, he adds.

The researchers describe their findings in the Aug. 1 Astrophysical Journal.