Starlight shows life the right path.

Once upon a time, before the birth of the sun, a nearby star-forming region contained several simple organic compounds. Some of those molecules existed in mirror-image forms. At first, there were equal amounts of the right-handed and left-handed versions.

Then some mysterious process upset the balance. One of the forms flourished while the other became rare. That seemingly innocuous turn of events may have had a profound influence on the evolution of life.

At least that's the scenario that scientists are increasingly invoking to explain a puzzling fact about life on Earth.

Although amino acids, the building blocks of proteins, and sugars, which form the backbone of DNA, can occur in both right-handed and left-handed forms, terrestrial organisms use only left-handed amino acids and right-handed sugars. A study of amino acids in the Murchison meteorite suggested that the inequality between the mirror-image forms existed before life began on Earth (SN: 2/22/97, p. 118). Astronomers have now found a possible explanation for this mystery.

Light from a star-forming cloud in the Orion nebula possesses a property that could lead to the imbalance, researchers report. Specifically, some of the light is polarized in such a way that it could enhance the population of one of the mirror-image forms of organic compounds--including those that might later be incorporated into newborn stars and planets, serving as raw material for the earliest life.

Jeremy Bailey of the Anglo-Australian Observatory in Epping, Australia, and his collaborators describe their observations in the July 31 Science.

Since dust veiling the Orion cloud absorbs both visible and ultraviolet light, the astronomers studied the infrared light emitted by the region. Using the Anglo-Australian Telescope in Coonabarabran, Australia, the team found that 17 percent of the near-infrared light is circularly polarized.

In extrapolating their observations to ultraviolet light, the high-energy radiation that can create the imbalance, Bailey and his colleagues had to assume that its spectrum in the star-forming cloud was relatively narrow. Their laboratory studies suggest that the ultraviolet light should have a circular polarization "comparable" to that of the infrared, the researchers say, but the exact percentage may differ.

A circularly polarized light wave has an electric field that rotates either clockwise or counterclockwise about the wave's direction of motion. Ultraviolet light with these types of polarization are absorbed unequally by right-handed and left-handed molecules and can break down or destroy one form more easily than the other, creating an asymmetry in the relative populations of the two forms.

Whether the imbalance generated in a star-forming cloud can be preserved many millions of year later on the surface of a chemically active planet remains to be shown, comments Jonathan I. Lunine of the University of Arizona in Tucson. Moreover, the Orion cloud may not be the type of stellar nursery in which stars like the sun can form. "But the paper's importance lies in demonstrating one natural astrophysical factor that can initiate the asymmetry," he says.