Space Investigators

Anne Marie Toccket, Research Unplugged intern
November 09, 2005

There were no signs in corn fields, no shadowy images of human features hiding in a planet's face. No little green men.

In fact, when confronted with the blunt question, "So, are there aliens?", James Kasting, host of the final installment of the Research Unplugged fall season, smiled and answered carefully.

man in gray suit moves hands

James Kasting answers audience questions about the possibility of life on other planets.

"We aren't talking about a mission to find intelligent life," he said. Instead Kasting, a distinguished professor of geosciences at Penn State and former NASA scientist, discussed the Terrestrial Planet Finder mission, the space agency's plan to launch two observatories equipped to pick up chemical traces of life on planets outside our solar system.

His talk, titled, "Space investigators: Is there life on other planets?" gave the audience a glimpse into this mission and into Kasting's broader interest in defining the "habitable zones" around stars.

The habitable zone is that area around a star that is neither too far—and therefore too cold—nor too near—and so too hot—to support the presence of liquid water on a planet's surface, Kasting explained. By first identifying the zones where life could arise, researchers substantially narrow their focus. Of the 155 (and counting!) extrasolar planets discovered since 1995, he said, about 20 fall within these limits.

Certain base-line conditions, in addition to a reasonably temperate climate, must be satisfied in order for a planet to host life, Kasting continued. First, it must have an atmosphere, containing both water vapor and carbon dioxide. It must have sufficient mass to hold onto that atmosphere via gravity, and its orbit must be relatively stable.

The Planet Finder probes would work by revealing the distinct chemical signatures of atmospheric gases such as carbon dioxide, water vapor, ozone, and oxygen in the colors of infrared radiation they pick up from a given planet, he continued.

What about the possibility of other, non-carbon-based, forms of life? one audience member asked.

"That's a great question," said Kasting. "That's one that is discussed at every origin-of-life meeting." While not impossible, he added, alternative life forms are not especially likely.

"There's a good reason that nature chose carbon to build life with," he said. "Carbon naturally makes long-chain molecules, which are suitable to the formation of complex organisms."

Silicon, the favorite alternative of science-fiction writers, is unwieldy by comparison. "When you mix silicon with oxygen, you get rock," Kasting shrugged.

Are there any planets out there that might be habitable for human life? someone asked at last.

"Since I read nothing but sci-fi until I went to college, this option is definitely of some interest to me," Kasting smiled. Then he reminded his listeners that living elsewhere would require not only the right set of surface conditions, but some giant leaps in human technology.

"As of today," he said, "it isn't possible on any kind of large scale."

James Kasting, Ph.D., is distinguished professor of geosciences in the College of Earth and Mineral Sciences. He can be reached at

Last Updated November 09, 2005