Life at other stars: a matter of climate.

Among the glittering denizens of the heavens, which stars are most likely to support life? Researchers had previously concluded that stars at least 2 billion years old, with a surface temperature and mass similar to those of the sun, might form planets capable of fostering life. A new study suggests that a group of stars with slightly lower mass and surface temperature has an equally good chance of creating life-sustaining planets.

Results of the study, which uses a computer model to determine the climate of planets near a variety of stars, could help guide NASA's Search for Extraterrestrial Life (SETI) (SN: 11/7/92, p. 317), says James E Kasting of Pennsylvania State University in University Park. He and his colleagues, Daniel P. Whitmire of the University of Southwestern Louisiana in Lafayette and Ray T. Reynolds of NASA's Ames Research Center in Mountain View, Calif., report their work in the January ICARUS.

The researchers restricted their study to possible planets that would contain liquid water -- an ingredient deemed essential for life- and that would have an atmosphere similar to Earth's. They also assumed that stars capable of forming planets would space those bodies logarithmically, as in the solar system.

In determining the "continuously habitable zone" around a particular star class the region in which climate is temperate and stable long enough to sustain life the team took into account the intensity and variation of radiation emitted by different star types. A planet forming too close to a given star loses water due to heating and photodisassociation, while a planet too far away will be frozen. Because more massive stars burn more intensely, their habitable zone begins farther out, notes Kasting.

The study supports previous findings that sun-like stars, classified as G stars, are good candidates for producing life. The team discovered that K stars, which have 70 percent of the sun's mass, may make equally good candidates. The team suggests that it may be wise, as SETI progresses, to look for telltale radio signals among nearby K stars rather than more distant G stars.

David R. Soderblom of the Space Telescope Science Institute in Baltimore says he has included some K stars in a list of stars for the SETI survey, But it is difficult to determine whether agiven K star is old enough to have supported the evolution of multicellular organisms. Soderblom says that with improved star-dating techniques on the horizon, the new report may convince him to add more K stars to the SETI survey,