An Earth-Like Axial Tilt Might Be Necessary for Complex Life to Arise

Earth has numerous properties that make it an ideal home for life as we know it, including a robust magnetic field that deflects radiation, a temperate climate with liquid water, a large moon that stabilizes the planet’s rotation, and a modest axial tilt. That last item may be more important than we previously thought, according to a new study funded by NASA. The study suggests that a tilted axis leads to more oxygen production, and that means more complex life.

Scientists have been increasingly interested in what makes Earth a haven for life as more and more exoplanets are detected in the sky. If we’re going to go looking for alien life, which are the best exoplanets to investigate? Sure, those that have liquid water and aren’t being fried by radiation, but the role of oxygen cannot be overstated. All the complex life on Earth needs oxygen to survive.

After you breathe in O2, it helps your cells produce most of the energy they need to operate. In the earliest days of life on Earth, there was very little oxygen, and life was very simple. However, the explosion of photosynthetic organisms boosted oxygen levels and made multicellular life viable. The study, led by Stephanie Olson of Purdue University, used a sophisticated model of Earth to tease out what properties are most vital to fostering the formation of life. It turns out, axial tilt could have a huge impact.

They found, as predicted, that increasing day length, higher surface pressure, and ocean nutrient circulation all helped to boost oxygen in the atmosphere. The effects of axial tilt were unexpected, though. Greater tilting increased photosynthetic oxygen generation. Earth’s 23.5-degree tilt is what gives us our seasons, and that shifting temperature appears to drive more photosynthesis in oceans. Adding an Earth-like tilt could have the same effect as doubling the volume of nutrients in the oceans.

According to the authors, a small tilt like Mercury (2 degrees) or an extreme one like Neptune (98 degrees) could lead to lower oxygen production, and therefore, fewer opportunities for complex life to develop. Of course, this assumes the biochemistry on other planets is similar to Earth. All we can really say for certain is that oxygen is a very good electron acceptor, and that makes it valuable metabolically. That should be true everywhere, so more oxygen means more complex life. Probably. That’s not to say complex life has not arisen in an anaerobic environment that relies on other types of chemistry. This just gives us a place to start.

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