It seems like every day that astronomers discover another possibly habitable world, like Proxima Centauri b, our closest exoplanent neighbor, and TRAPPIST-1f, one of seven recently discovered Earth-sized planets orbiting the same star. But don't prepare for first contact just yet. It will be exceedingly complicated to figure out whether there’s actually any life or potential for it on such planets, based on new research into our own evolving world.
To a distant observer peering through a telescope, even Earth would not have shown signs of life through most of its past. Despite the fact that our planet was teeming with mostly microscopic life for three billion years, levels of oxygen and methane -- gases often produced by metabolizing organisms -- would have been too low to be noticed from afar. This means that today's scientists on Earth might not be able to detect commonly assumed signs of extraterrestrial life, and they might give up on planets that are actually inhabited, according to a new study in the journal Astrobiology.
“There are huge swaths of time throughout Earth’s history during which it would’ve been difficult to see the presence of these metabolisms even though we know from the rock record that they were around. It’s a sobering thing,” said Christopher Reinhard, an Earth scientist at the Georgia Institute of Technology in Atlanta, and lead author of the study, who presented the research at a conference in Mesa, Arizona on April 27.
Scientists envision using oxygen, ozone and methane in a planet's atmosphere as key indicators of life. But there are problems with this approach. The gases are tough to detect with current technology, and their presence is suggestive, but not conclusive, evidence of living organisms. Even for an Earth-like planet, the search for life beyond our solar system turns out to be even tougher than previously thought.
“It’s becoming clear that you need more than one chemical signature, and at best you will just get a statistical estimate of whether there’s life,” said William Bains, a U.K.-based astrobiologist who works with planetary scientists at the Massachusetts Institute of Technology and was not involved in the study.
Oxygen has often been considered the primary sign of life for astrobiologists, since it’s hard to produce large quantities of oxygen gas without the presence of biological processes, like photosynthesis. But life in Earth's ocean predates detectable oxygen by more than a billion years, as the gas takes time to build up in the atmosphere. Ozone, which consists of three oxygen atoms, helps to shield nascent lifeforms on a planet's surface from ultraviolet radiation -- if there’s life down there at all. Methane is problematic as well, as it’s produced and consumed by biological processes in the ocean but can also be generated by geological processes.
Because oxygen and methane levels were so low for so long on Earth according to Reinhard’s models, until a few hundred million years ago a distant alien astronomer would’ve had few hints that life exists here. Earthling astronomers face the same problem when searching for life-friendly planets beyond the solar system. It’s not just that our favorite worlds in the “habitable zone” around their stars might be uninhabited; we could be prematurely ruling out many other worlds that actually do host life.
“We tend to be cautious about false positives, but false negatives are equally important,” said Stephanie Olson, an astrobiologist at the University of California, Riverside and a co-author of the study.
Context is everything. For example, astronomers might want to measure the colors of light of the host star, which indicate its age. This could help scientists infer if the planets have been around long enough to accumulate detectable oxygen or methane levels from life. They might decide to make younger planets a lower priority for follow-up study.
Scientists can also broaden their search beyond these three gases. The presence of oceans, indicated by water vapor in the atmosphere, would also be a strong indicator of the potential for life. If the planet had a core, then that would increase the chance of life-essential minerals being present. The tectonic and volcanic history of the planet would affect the chemistry of the atmosphere as well.
“We’re going to have to make choices about which planets to look at. But we’re starting to come up with a good series of metrics for planets that are or aren’t promising,” Reinhard said.
In particular, instruments on new space telescopes will need to cover wavelength ranges of light to probe at multiple biosignatures, including ozone, which would be easier to detect than oxygen, for example. But gathering extra data for a more complete and high-resolution picture of possibly inhabited planets doesn’t come cheap.
The complexity of signs of life may be difficult to surmount, but it’s also very exciting, said Morgan Cable, an astrochemist at the Jet Propulsion Laboratory in Pasadena, California. “We see this diversity in our own solar system, which may reflect the richness of exoplanets elsewhere. That gives me a lot of hope for finding interesting worlds that may be Earth-like or not, challenging our perceptions of life as we know it and life as we don’t.”