Alien "ID Chart" to Aid Search for Extraterrestrial Life

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"It's interesting that the only way we can tell what's going on in oceans or on the ground is by measuring waste gases in the atmosphere," said Wesley Traub of NASA's Jet Propulsion Laboratory in Pasadena, California.

The team's members mapped the atmospheric changes that occurred during Earth's 4.5 billion years of evolution.

They identified six different epochs—stages in the evolution of carbon-based life—so that they can compare the spectra of those atmospheres with those of any promising new planets.

"If an extrasolar planet is found with a spectrum similar to one of our models, we potentially could characterize that planet's geological state, its habitability, and the degree to which life has evolved on it," Traub said.

For example, the presence of very early life could be indicated by rising methane levels like those created when anaerobic bacteria, which grow in the absence of oxygen, first appeared on Earth some 3.5 billion years ago.

More advanced life could also be detected.

"Two good signs of life would be large amounts of oxygen [which made multicellular life possible on Earth] and nitric oxide, which is produced by life and only by life," Traub said.

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