Do the seasons explain why Earth is teeming with life?
The axis on which Earth rotates is tilted by 23.5°—likely caused by an impact billions of years ago—which during our annual orbit around the Sun means that different parts of Earth receive sunlight for different lengths of time.
Cue Earth’s seasons, solstices, equinoxes … and life?
It’s long been thought that it’s the distance from its star that determines how likely it is for a planet to host life.
The theory goes that there’s a not-too-warm, not-too-cold, but just right “Goldilocks zone” within which liquid water can exist on the surface of a planet.
Now new NASA-funded research presented at the Goldschmidt Geochemistry Conference suggests that atmospheric oxygen—another key ingredient to life as we know it—may be more likely to occur on planets that orbit their stars on a tilted axis.
“Worlds that are modestly tilted on their axes may be more likely to evolve complex life,” said Stephanie Olson from Purdue University and lead researcher of the study. “This helps us narrow the search for complex, perhaps even intelligent life in the Universe.”
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The findings—which could help scientists refine NASA’s search for more advanced life on distant exoplanets—comes from a sophisticated model of the conditions required for life on Earth to be able to produce oxygen.
Oxygen is thought to be key to life because it’s essential in respiration, the chemical process that drives the metabolisms of most complex life forms—such as plants and animals.
The model allowed the researchers to see how changing conditions on a planet might alter the amount of oxygen produced by photosynthetic life. It follows a paper published last week that also focused on photosynthesis and suggested that Earth-like conditions on potentially habitable planets may be much rarer than previously thought.
The researchers found that as Earth’s rotation slowed and the length of days stretched—because of its tilt— its continents grew, surface pressure increased, and ocean circulation patterns changed in way that may have increased the production of oxygen.
“The most interesting result came when we modelled orbital obliquity—how the planet tilts as it circles around its star,” said Megan Barnett, a graduate student at the University of Chicago who was involved in the study. “Greater tilting increased photosynthetic oxygen production in the ocean in our model, in part by increasing the efficiency with which biological ingredients are recycled,” said Barnett. The effect was similar to doubling the amount of nutrients that sustain life.
“Small tilts or extreme seasonality on planets with Uranus-like tilts may limit the proliferation of life,” said Olson. Uranus tilts almost 98º, so almost on its side.“But modest tilt of a planet on its axis may increase the likelihood that it develops oxygenated atmospheres that could serve as beacons of microbial life and fuel the metabolisms of large organisms.”
The conclusion? Though exoplanet researchers are excited about the possibility of the upcoming James Webb Space Telescope (JWST) study the atmospheres of exoplanets, its findings may be misleading. In fact, worlds that are tilted on their axes may be more likely to evolve complex life.
Wishing you clear skies and wide eyes.