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In the three-way horse race to prove that biology is not just a terrestrial aberration, there’s one steed that many people ignore: sampling the air of distant planets to see if they contain the exhaust gases of life, or in the jargon of astrobiologists, biosignatures.

Everyone knows the other horses: (1) Finding biology in the solar system by hurling rockets to Mars or some of the moons of Jupiter or Saturn, and (2) expanding our SETI experiments to eavesdrop on radio or laser signals from clever beings on distant worlds.

The former has the advantage that, if microbes have sprung up elsewhere in the solar system, we might be able to bring them back, dead or alive. We’d have aliens on Earth, an idea that would electrify astrobiologists.

The latter horse - SETI - endeavors to discover the most interesting variety of extraterrestrial, namely those that are technically advanced. But a difficulty with SETI is that we need to be aiming our telescopes in the direction of signals while they wash over our planet, neither before nor after. There’s a synchronicity problem.

Bugle call, and the third horse - a contender not hobbled by the inconvenience of synchronicity. Consider: the Earth’s atmosphere is roughly 21 percent oxygen, a consequence of billions of years of photosynthetic activity. Oxygen is an exhaust gas of greenery. You can blame the rusted out-body of that clunker in your driveway on plants.

The neat thing about this is that the oxygen signature in Earth’s air has been present for roughly two billion years. For all that time, oxygen has been broadcasting its existence into space. It could be easily detected by any advanced extraterrestrials with enlightened astronomy budgets, even from vast distances. There’s no synchronicity problem, because this signal lasts for eons.

That’s nag number three’s tempting appeal, and Daniel Angerhausen, a researcher at NASA’s Goddard Spaceflight Center, is betting on it. He’s used telescopes on the ground, in the air, and in orbit to look for the spectral signatures of not just oxygen, but other biology “tells,” such as methane.

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