Spotting signs of life in an alien planet's atmosphere may be tougher than scientists had thought.

One prominent such "biosignature" target, ozone, may get trapped near the equators of Proxima b, TRAPPIST-1d and other potentially habitable worlds that orbit close to their host stars, making the gas hard to detect from afar, a new study suggests.

"Absence of traces of ozone in future observations does not have to mean there is no oxygen at all," study lead author Ludmila Carone, of the Max Planck Institute for Astronomy in Heidelberg, Germany, said in a statement. "It might be found in different places than on Earth, or it might be very well hidden." [Exoplanet Discovery: The 7 Earth-Sized Planets of TRAPPIST-1 in Pictures]

Ozone is an unstable molecule that consists of three oxygen atoms. Here on Earth, the stuff is generally produced in the atmosphere after ultraviolet (UV) radiation from the sun splits "normal" diatomic oxygen (O2).

The vast majority of Earth's O2 is generated by living organisms — plants and photosynthetic microbes — so ozone serves as a sort of secondary biomarker, at least for Earth-like life.

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Earth's atmospheric flows distribute most ozone relatively evenly into our planet's famous ozone layer, which helps shield life from harmful UV radiation. So hypothetical aliens studying Earth from afar with powerful telescopes would have a good chance of detecting the gas.