This artist's rendering provided by NASA on Thursday, April 17, 2014 shows an Earth-sized planet dubbed Kepler-186f orbiting a star 500 light-years from Earth. Astronomers say the planet may hold water on its surface and is the best candidate yet of a habitable planet in the ongoing search for an Earth twin. (AP Photo/NASA Ames, SETI Institute, JPL-Caltech, T. Pyle)

Call it the cosmic version of fool’s gold. What was once considered a sure-fire sign of life on distant planets may not be so sure-fire after all, a new study suggests. Instead, it may simply be the artifact of a lifeless world and its equally lifeless moon.

“I like this paper. It’s a great concept,” says Sara Seager, a planetary scientist at the Massachusetts Institute of Technology in Cambridge. “I think the [scientific] community is gradually realizing that it might be impossible to be absolutely certain, based on its atmosphere, that an exoplanet hosts life.”

Life on planets orbiting other stars doesn’t have to literally broadcast its existence: Radio signals are just one way earthbound scientists might detect biological activity elsewhere in the universe, says Hanno Rein, a planetary scientist at the University of Toronto, Scarborough, in Canada. Signs of life are much more likely to be subtle, especially if the organisms are simple. One way to look for such clues is to search for chemical evidence, particularly in light passing through the atmospheres of the planets, Rein says. By comparing the spectrum of light passing through an exoplanet’s atmosphere with that of the unfiltered light emitted by its parent star, astronomers can identify substances present in the exoplanet’s air.

Thus far, Rein notes, scientists haven’t been able to agree upon a single chemical—oxygen, for example—that could be a conclusive sign of extraterrestrial life. But researchers generally agree that certain mixes of two or more chemicals in an exoplanet atmosphere could be a strong sign of life, Rein explains. Here’s the idea: A mixture of gases that would normally react until one is completely gone simply can’t exist over the long term, unless one or both of the gases is being constantly replenished—possibly by the biological activity of life forms. One easy-to-understand example, Rein says, is a mix of methane and oxygen. Left to themselves, those two substances react to form carbon dioxide. And if life on Earth weren’t continuously producing those two gases, chemical reactions between the two would eventually scrub the less prevalent one from the atmosphere, leaving only the other.

Now, Rein and his colleagues propose a scenario that could easily lead researchers looking for extraterrestrial life astray. What if, they say, a distant exoplanet has a moon with an atmosphere of its own? And furthermore, what if that “exomoon’s” atmosphere contains large amounts of a gas that would typically react with one from the exoplanet’s atmosphere if given a chance? Even if neither body hosted life, the combined light from the two objects might easily be mistaken as having passed through a single atmosphere of a body that hosts life, the researchers report online today in the Proceedings of the National Academy of Sciences.

In fact, with current instruments and observing techniques, astronomers might not even recognize that the distant exoplanet has a moon, Rein notes. The two bodies would be so close together that, as seen from Earth, their light would blend into one smudge. Consider, Rein says, the spectrum of light that distant astronomers might see if Earth—whose atmosphere is more than 20% oxygen—had a moon with a methane-rich atmosphere like Saturn’s moon Titan. In that instance, the single smudge of light would contain signs of both reactive gases.

The team’s scenario “is a really very interesting way to get a ‘false positive’ ” for extraterrestrial life, says Wesley Traub, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. Scientists could easily be fooled by such a blended spectrum, he notes, and it’s likely that they’d be fooled for quite a while. And if scientists ever did detect such a spectrum, “it’s not clear what the next step would be,” he suggests. Many of the techniques now used to detect exoplanets—such as observations of their gravitational effect on the movements of their parent stars, or mini-eclipses that occur regularly as they pass in front of the stars as seen from Earth—aren’t sensitive enough to detect the presence of an exomoon.

This story has been provided by AAAS, the non-profit science society, and its international journal, Science.