Axial tilt and tidal locking also matter in a planet’s habitability.
The “habitable zone” around stars, where it’s warm enough for liquid water to exist on an Earth-like world’s surface, has long been the gold standard in assessing the potential for life on other worlds, but as our understanding of astrobiology deepens, scientists are looking for other clues to habitability.
The kind of days, nights and seasons that shape conditions on alien worlds can differ radically from Earth’s, even when a planet or moon is situated within the habitable zone.
One qualifier is a planet’s axial tilt, also known as its obliquity. The Earth spins at an angle of 23.5 degrees relative to the sun, meaning most sunlight hits the equator, while the poles are so cold they form ice caps. However, a planet tilted over by more than 55 degrees could potentially form an equatorial ice belt, as well as poles that would be incredibly hot during the summer and extraordinarily cold during the winter, and so life living in polar regions would have to adapt to both extreme heat and cold.
Or, an alien planet or moon might be trapped in a resonance with whatever body it is orbiting, such that the length of time it takes to make one rotation is exactly the same amount of time it takes to orbit its parent body. The consequences of this is that the planet or moon becomes “tidally locked,” and has one frozen side that always faces away from its star and one side that is in constant sunlight. Our moon, for example, is tidally locked to Earth, which is why we always see the familiar “Man in the Moon” on its nearside.
Such differences influence whether these worlds are warm enough to possess flowing water, or whether they are instead frozen snowballs experiencing global ice ages. Even planet Earth, the only world known to have life on it, has teetered toward frozen extremes in its geological history.
“The increase in complex life is thought to have happened both due to the rise in oxygen and to the evolutionary pressure created by a snowball state,” she says.
For instance, the freezing temperatures and other harsh conditions during a snowball state on Earth likely exerted a strong pressure on life to either adapt or die. This evolutionary pressure probably led to the development of complex organisms, who then competed with each other for resources, exerting still more pressure to adapt or die.
However, Checlair and her colleagues found that tidally-locked planets in habitable zones may be unlikely to enter a snowball state. The scientists detailed their findings in Astrophysical Journal.
To reach their conclusions, the researchers developed a global climate model of a tidally locked Earth-like planet in a habitable zone. They focused on how much light the planet absorbed from its star and how much bounced back into space with highly-reflective ice cover.
The scientists found that because of the way that ice accumulates across the surface of this planet, the snowball state would not just suddenly happen. Instead, their model suggested it would smoothly transition from partial to complete ice coverage and back. Furthermore, an active carbon cycle – carbon being a powerful greenhouse gas – could help tidally-locked planets avoid complete glaciation.
“It is not clear yet whether a snowball state is more detrimental or more beneficial to the possibility of life on habitable planets,” says Checlair. “It definitely has an effect on habitability, but further study is required to determine whether this effect is positive or negative.”
Axial tilt on alien worlds
In a different study, Brian Rose, a climate dynamicist at the University at Albany in New York, examined alien planets in habitable zones with a range of axial tilts. He and his colleagues wanted to see if worlds with high obliquities in habitable zones could possess stable, long-lived ice belts around their equators, as well as other consequences that would have major impacts on those planets. For instance, the polar regions of high-obliquity planets would experience constant sunlight for days during the summer and perpetual darkness for days during the winter, so that “all photosynthetic life would have to be well-adapted to this strongly seasonal regime,” he says.
Rose’s team developed a global climate model that could simulate many different obliquities. These models also simulated the way that snow, ice, water and land reflect light at different latitudes, and the way that atmospheric and ocean currents move heat from warm to cold regions of the planet.
The researchers discovered that it may be rare for any habitable world to possess an equatorial ice belt. They also found that potentially habitable planets with high obliquities of 55 degrees or more could swing from completely ice-free to completely ice-covered states. Rose and his colleagues also detailed their findings in a paper published in Astrophysical Journal.
“What’s exciting about these results is really the simplicity of the model, which lets us explore really wide variations in possible planetary characteristics in a simple and organized way,” Rose says.
The scientists found that on high-obliquity worlds, ice-free polar caps usually absorbed more light than the ice-covered equatorial regions reflected, which caused warming that easily destabilized the ice belts, says Rose. They found that the number of those worlds with polar ice caps should outnumber those with equatorial ice belts by three- to four-fold.
“Stable ice belts are possible but relatively rare, requiring a ‘just right’ combination of planetary characteristics,” Rose said.
According to Rose, potentially habitable planets with high obliquities might be prone “to violent climatic swings between global snowball and completely ice-free conditions.”
“Is this hypothetical planet more or less suitable for harboring life than Earth?” asks Rose. “I don’t have a good answer to that question. I think the community is still grappling with these concepts.”
Rose’s co-author Cecilia Bitz was partially supported by funding from the NASA Astrobiology Institute element of the NASA Astrobiology Program. Meanwhile, Checlair’s work was supported by NASA’s Habitable Worlds program. NASA Astrobiology provides resources for Habitable Worlds and other Research and Analysis programs within the NASA Science Mission Directorate (SMD) that solicit proposals relevant to astrobiology research