The way a planet is tilted on its rotational axis with respect to its orbital plane around a star - what we know as 'axial tilt' - could be key to the emergence of complex life.
According to a new study, a modest axial tilt, like Earth's, helps increase the production of oxygen, which is vital for life as we know it - and planets with tilts that are too small or too large might not be able to produce enough oxygen for complex life to thrive.
"The bottom line is that worlds that are modestly tilted on their axes may be more likely to evolve complex life," said planetary scientist Stephanie Olson of Purdue University. "This helps us narrow the search for complex, perhaps even intelligent life in the Universe."
It's possible that life may emerge outside the parameters we know here on Earth, of course, but this pale blue dot is the only world which we know for a certainty harbors life. Therefore, it's expedient to model our searches accordingly.
When looking for habitable worlds elsewhere in the galaxy, the first things we look for are: is it relatively small and rocky, like Earth? And does it orbit the star at a distance called the habitable zone, the Goldilocks region of not too hot, not too cold, where temperatures allow liquid water on the surface?
Those questions are good, but the contributing factors to the emergence of life are likely a lot more complex.
The presence of a magnetic field, for instance, is thought to be pretty important, because it protects the planetary atmosphere from stellar winds. The eccentricity of the planet's orbit, and what kind of other planets are present in the system might also be key.
Olson and her team went a little more granular, looking at the presence and production of oxygen; specifically, the conditions on the planet that may impact the amount of oxygen produced by photosynthetic life.
Most organisms (although not all) on Earth require oxygen for respiration - we can't live without it. Yet early Earth was low in oxygen. Our atmosphere only became rich in oxygen about 2.4 to 2 billion years ago, a period known as the Great Oxidation Event. It was triggered by a boom in cyanobacteria, which pumped out vast amounts of oxygen as a metabolic waste product, enabling the rise of multicellular life.
Olson and her team sought to understand how the conditions arose in which cyanobacteria could thrive, using modelling.
"The model allows us to change things such as day length, the amount of atmosphere, or the distribution of land to see how marine environments and the oxygen-producing life in the oceans respond," Olson explained.
Their model showed that several factors could have influenced the transport of nutrients in the oceans in a way that contributed to the rise of oxygen-producing organisms like cyanobacteria.
Over time, Earth's rotation slowed, its days lengthened, and the continents formed and migrated. Each of these changes could have helped increase the oxygen content, the researchers found.
Then they factored in axial tilt. Earth's axis isn't exactly perpendicular to its orbital plane around the Sun; it's tilted at an angle of 23.5 degrees from the perpendicular - think of a desktop globe.
This tilt is why we have seasons - the tilt away from or towards the Sun influences seasonal variability. Seasonal temperature changes also influence the oceans, resulting in convective mixing and currents, and the availability of nutrients.
So perhaps it's not surprising that axial tilt had a significant effect on oxygen production in the team's 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," explained planetary scientist Megan Barnett of the University of Chicago.
"The effect was similar to doubling the amount of nutrients that sustain life."
But there's a limit. Uranus, for example, is tilted at 98 degrees from the perpendicular. Such an extreme tilt would result in seasonality that may be too extreme for life. A small tilt, also, might not produce enough seasonality to encourage the right level of nutrient availability. This suggests there may be a Goldilocks zone for axial tilt, too - neither too extreme, nor too small.
It's another parameter we can use to help narrow down planets elsewhere in the galaxy that are likely to harbor life as we know it.
"This work reveals how key factors, including a planet's seasonality, could increase or decrease the possibility of finding oxygen derived from life outside our Solar System," said biogeochemist Timothy Lyons of the University of California Riverside.
"These results are certain to help guide our searches for that life."
The research has been presented at the 2021 Goldschmidt Geochemistry Conference.