Fantasy Alien Exo Planet isolated galaxy spacegettyComplex life here on earth is largely thought to be possible due to the geophysics of earths plate tectonics. That is, how our planets tectonic plates enable large amounts of carbon dioxide to be continually removed from earths atmosphere and recycled back into its crust.In contrast, Venus-like planets on the inner edge of extrasolar habitable zones are thought to have what is known as stagnant lid geophysics in which there is no systematic carbon recycling of the planets atmosphere. The result is an early onset of a hellish runaway greenhouse.Yet a paper accepted for publication in the journal Astronomy & Astrophysics posits that planets on the inner edge of a solar systems habitable zone might remain habitable for up to two billion years longer than previously thought. That is, if life had evolved on the planet early enough to avoid a runaway greenhouse.If so, that might mean that planets which heretofore would never have been seen suitable for the evolution of complex life might become tenable for the evolution of intelligence.We explore a new and unconventional way to identify life on exoplanets: by searching for low CO2 levels in atmospheres where wed normally expect much higher concentrations, Dennis Hning, a planetary scientist at Germanys Potsdam-Institute for Climate Impact Research, and the papers lead author, told me via email.Planets close to their stars typically experience runaway greenhouse effects, leading to CO2-rich atmospheres, Hning said. However, life could counteract the rising temperatures by enhancing weather, removing CO2, and extending habitability for billions of years, he said.MORE FOR YOUTiming Is EverythingIn our paper, we compared the onset time of the runaway greenhouse between planets of the same composition with and without a biosphere, said Hning. We found that the biotic planet remained habitable for one to two billion years longer compared to the abiotic planet, he said.This study is the first to directly link the atmospheric CO2 signature of planets near the inner edge of the habitable zone to the presence or absence of a biosphere, Hning and colleagues note in the paper. We find that planets with low atmospheric CO2 in this region are more likely to be inhabited, as biological processes efficiently regulate CO2 levels through enhanced weathering and thereby postpone the runaway greenhouse and the accompanying dramatic rise of atmospheric CO2, they write.Biology Helps Regulate ClimateWe raise and explore the hypothesis that low atmospheric CO2 levels on planets near their stars can serve as an indirect biosignature, said Hning. On earth, the biosphere effectively helps to regulate the climate because vegetation strongly contributes to weathering by which CO2 eventually gets bound in rocks, which works against heating up the planet, he said. We find that life can delay the runaway greenhouse by up to 1-2 billion years compared to abiotic planets, said Hning.Complex EvolutionA longer habitable period gives life more time to evolve complexity, said Hning. Developing complex or even intelligent life is not a linear process, he said. This extended timespan could increase the chances of a planet developing intelligent life, said Hning.As for how to look for such planets?I would look first for rocky planets at least 4 billion years or older; systems where the host star shows signs of aging by becoming brighter with age, Ludmila Carone, a planetary scientist at the Space Research Institute of the Austrian Academy of Sciences, and the papers second author, told me via email.I would then check the location of the planet with respect to the habitable zone of their star, Carone said. As the star ages, the habitable zone is pushed outwards, she said. Such planets receive just a bit too much energy over time, so the surface warms up such that their water oceans - if they ever existed in the first place - start to evaporate, she said.The Bottom Line?This study shows that a biosphere can compensate for a stagnant lid and lengthen the duration of habitable conditions, Stephen Kane, a planetary geophysicist at the University of California, Riverside, who was not involved in the study, told me via email. This study demonstrates the intricate interdependence of a biosphere with the environment in which it forms, said Kane.For future telescopes, context will be key.Its not sufficient to detect single molecules, said Carone. Methane as a biomarker can be "simply" blocked by thick clouds or scrubbed out of the atmosphere with too much humidity, she said. Ultimately, we may need a multi-telescope strategy to truly identify habitable exoplanets, said Carone.Complex PlanetsThe exciting thing about rocky planets is their complexity, which demands highly interdisciplinary approaches, said Hning. Bridging geophysics, atmospheric science, astronomy, and even biology is a challenging endeavor, but I believe that its the key to unlocking the next breakthroughs in finding extraterrestrial life, he said.