With Earth as Only Model for Life,
Scientists Seek Life on Other Planets

Alien encounters and science fiction permeate pop culture, but what would it really mean if scientists found life beyond Earth? If even a single-celled organism on another planet was discovered, for many, this would be the last thread of evidence proving that life is simply chemistry.

In a recent lecture at the AAAS, Bruce Jakosky from the University of Colorado at Boulder discussed the social and scientific ramifications of finding life on another planet, and the scientific principles behind determining the viability of extraterrestrial life, a field known as astrobiology.

"We likely are doing it for the exploration value—a search for knowledge in order to find out what is in the world around us," Jakosky says. "By learning about the world around us, we are learning about what it means to be human."

Jakosky's lecture was part of a monthly series sponsored by the AAAS Program of Dialogue on Science, Ethics and Religion (DoSER). The Program builds on AAAS's long-standing commitment to relate scientific knowledge and technological development to the purposes and concerns of society at large. The lecture series provides an opportunity for scientists to engage with theologians, ethicists, philosophers and the public concerning the societal implications of their work.

While teaching undergraduate astronomy early on in his career, Jakosky said he was stirred to explore the nature of science and how science should better fit into activities that engage the public.

"I noticed that the classroom was always packed and students were really interested on the days that we discussed the possibility of life on other planets," Jakosky said.

The search for alien life does not allow for the application of traditional scientific methods, however. On Earth, researchers observe their surroundings, develop a hypothesis and then perform an experiment to test the hypothesis.

"Think of Galileo dropping two differently-massed balls off of the leaning tower of Pisa."

With astrobiology, however, there is a problem Jakosky calls "the statistics of one." In order to seek signs of extraterrestrial life, scientists look at the one example of life that is known (Earth) and try to generalize from it. Although researchers can look for the ingredients for life, they cannot be certain if these traits are specific only to life on Earth, or if the same characteristics would be required of life anywhere in the universe.

Instead of relying on classical approaches in their search for extraterrestrial life, researchers must depend on geophysical evidence that has accumulated over time and on exploration science.

Known as "historical narratives," geophysical evidence is used by researchers to make conjectures about past events. Few events are as dramatic as the one that wiped out the dinosaurs 65 million years ago, but scientists can use geological measurements to test for climate changes and for asteroids that have collided with Earth. Scientists cannot test historical narratives, yet they can look for clues to reveal what may have happened.

"In fact," Jakosky says, "the big questions in science, such as the fate of the universe, the origin of galaxies and the origin of life on Earth, can only be addressed through historical narratives."

From what is already known about the Earth and its living inhabitants, life requires liquid water, a source of energy, organic molecules, and a mix of biogenic elements such as Carbon, Hydrogen, Oxygen and Nitrogen. Consequently, Jakosky says, the search for extraterrestrial life is narrowed to a search for the conditions that are necessary for life to exist.

According to NASA's Web site, the search for extraterrestrial life is a major part of the new vision of the federal agency's Space Sciences program. In answer to the question, "Are we alone?" NASA responds:

"We seek evidence of life in our solar system and of life-sustaining, Earth-like worlds around nearby stars. We have embarked on an ambitious program to explore the planet Mars. Our spacecraft have discovered evidence that large amounts of liquid water once flowed on the planet, and that today, it may be frozen beneath the surface."

Exploring Mars or Europa for signs of life is a logical step to take if all life-forms need the same key ingredients. However, Jakosky returns to the problem of "the statistics of one": Since Earth is the only case-study for life, it is possible that life on other planets may not form from the same elements as terrestrial beings.

"Whatever the outcome of such a search, it will tell us much about our understanding of the origin of life on Earth and the likelihood of life being widespread throughout the galaxy," Jakosky said.

Jakosky's presentation was followed by a response from Jim Miller, a senior program associate in the DoSER program, who holds a Ph.D. in theology with a focus on science and theology. Miller proposed that a constructive though asymmetrical relationship exists between the historical narratives generated by scientists and religious narratives.

Jakosky received his Ph.D. in 1982 from the California Institute of Technology. His research interests focus on the surfaces and atmospheres of planets, evolution of atmospheres, and possible life on other planets. Jakosky is currently involved in the Mars Global Surveyor mission as an Interdisciplinary Scientist for Surface-Atmosphere interactions, and is head of the University of Colorado effort in astrobiology, as part of the larger NASA Astrobiology Institute initiative.

For more information on the DoSER program, visit its Web site, www.aaas.org/spp/dser.

—Carol Hoy

2 April 2003