It is a common misconception that the change in seasons is caused by the Earth getting closer or farther from the Sun. This is understandable because, in daily life, you feel warmer when you get closer to a hot thing like a stove or an old light bulb. By extrapolating this everyday experience to the planetary scale, people have a shaky but intuitive reasoning for the common misconception.
However, in Earth’s case, the facts do not fit the intuition. The real cause of Earth’s seasons is its axial tilt. Earth’s relative distance to the Sun month-to-month does not influence temperature and precipitation much because Earth’s orbit is close to a perfect circle. That is to say, it has low eccentricity.
All stable planetary orbits are ellipses with eccentricities ranging from 0, a perfect circle, to 1, where any value above 0.1 is considered highly eccentric. For example, the eccentricity of Earth’s orbit is quite low at around 0.02, whereas Mercury’s eccentricity of around 0.2 is the highest for a planet in the solar system.
A comparison between a system with 0 and high eccentricity, not-to-scale with the planets in the research paper. Image made by the author in Microsoft PowerPoint with default image tools and Creative Commons clipart.
It’s hard for astronomers to evaluate the impact of high eccentricity on planetary habitability in our solar system because Mercury’s climate is predominantly shaped by the fact that it is very close to the Sun. However, when it comes to studying planets outside the solar system, the eccentricity of their orbits may affect how livable they are on a seasonal basis. So, a team of scientists in the UK wanted to see how an extremely high eccentricity would impact an Earth-like planet’s habitability. To test this, they ran 2 simulations on planets that were copies of the Earth, except one had an eccentricity of exactly 0 and the other had an eccentricity of 0.4, which is unusually high.
To do this, they used a tool called The Whole Atmosphere Community Climate Model Version 6 or WACCM6. This detailed 3D Earth-Sun simulation combines dynamic systems like atmospheric chemistry, sea ice, and river flow with fixed variables like orbital parameters. The scientists configured their model to pre-industrial Earth conditions so that anthropogenic climate change wasn’t a factor.
When deciding how to determine the habitability of the simulated Earths, the team landed on surface temperature and cumulative precipitation over land. For temperature, the lower limit of what they considered habitable was 273Kelvin (0°C or 32°F), the freezing point of water, although they acknowledged that some animals can live in colder conditions. They used 30 centimeters (12 inches) per year of precipitation as the distinguishing line between deserts and regions capable of sustaining greater biodiversity, which, while not strictly necessary for life, the team considered a minimum for habitability in this study.
The scientists found the average surface temperature of the more eccentric planet was 2K (2°C or 4°F) higher than that of the planet with the circular orbit. However, both were well above freezing at 290K (17°C or 62°F) and 288K (15°C or 58°F), respectively, so there were more notable differences between the two. The more eccentric planet had 0.1% less sea ice, 5% less land snow coverage, and 25% more land area receiving 30 cm or more precipitation per year. The team suggested these findings meant that planets with highly eccentric orbits may, in their words, “have greater land habitability than their circular counterparts.”
They also found that extremes defined the eccentric planet. For example, this planet had extended dry periods followed by months of torrential precipitation. The light intensity differences from the Sun between seasons were so extreme that seasonal changes impacted how protective the ozone layer was, subjecting the surface to occasional spikes in harmful ultraviolet rays.
When the scientists compared the planets on a monthly basis, they found that the more eccentric planet had a better surface habitability rating than the circular one 80% of the time. The team noted that neither planet had any uninhabitable times, but the extreme conditions of the eccentric one might have strained ecosystems globally. However, they did not analyze the ecological trade-off between stability and overall better average habitability metrics.
The team acknowledged that they need astronomers to collect more detailed exoplanet data in order to compare their conclusions to real-life examples. Currently, only 89 potentially Earth-like exoplanets have measured eccentricities, and of those, only 16 are highly eccentric. The team explained that this is mainly because exoplanet hunting is inherently biased toward objects with circular orbits. But it could be that somewhere in the Galaxy, some alien microbes or a future human outpost enjoy a scorching, irradiated summer and a subzero, icy winter.
