Until now, researchers believed a factor in the habitability potential of an exoplanet involved its obliquity, or its axial tilt relative to the parent star. They thought that if a planet tilted mostly on its side, it would be unable to support life—even if it resided in the habitable zone. However, a new model indicates that life could survive those conditions, provided it was covered in oceans. The study was led by David Ferreira, then at MIT, and the paper was published in Icarus.
Earth has a relatively low obliquity of 23.5˚. As the planet spins about its axis, the entire surface of the planet experiences exposure to daylight as well as getting a break from the sun at night (with some seasonal exceptions for locations closer to the poles). This allows the overall surface temperature to even out, like cooking on a rotisserie.
It was traditionally believed that if a planet had an obliquity closer to that of Uranus (98˚) and a pole directly facing the parent star, one side would be in perpetual direct sunlight for half of the year, while the other side would be ice cold. These extreme overheating/freezing cycles would create inhospitable conditions for life. However, Ferreira’s group found that the presence of oceans could balance things out.
“The expectation was that such a planet would not be habitable: It would basically boil, and freeze, which would be really tough for life,” Ferreira said in a press release. “We found that the ocean stores heat during summer and gives it back in winter, so the climate is still pretty mild, even in the heart of the cold polar night. So in the search for habitable exoplanets, we're saying, don't discount high-obliquity ones as unsuitable for life.”
Ferreira’s team made models for three planets in a star’s habitable zone, differing only in axial tilt. The planets had respective obliquities of 23˚, 54˚, and 90˚, and were subjected to simulations of variable ocean depths and atmospheric conditions. Even for the planet with the greatest obliquity, an ocean as shallow as 50 meters (164 feet) would be enough to create an average temperature of 15˚C (60˚F) over the course of the year.
“We were expecting that if you put an ocean on the planet, it might be a bit more habitable, but not to this point,” Ferreira continued. “It’s really surprising that the temperatures at the poles are still habitable.”
However, if the oceans were too shallow, the planet would not be able to keep itself warm. Simulations of water 10 meters (32 feet) deep resulted in a runaway snowball effect, where the ice would freeze over completely on the dark side. Later during the year when the ice was exposed to the star, it would reflect too much sunlight to be heated. Ice would begin to form during this time on the other side, resulting in a planet that is completely frozen over.
“Some people have thought that a planet with a very large obliquity could have ice just around the equator, and the poles would be warm,” Ferreira concluded. “But we find that there is no intermediate state. If there’s too little ocean, the planet may collapse into a snowball. Then it wouldn’t be habitable, obviously.”
This modeling is purely hypothetical, and does not represent any specific currently-identified exoplanet. Out of the 2,000 exoplanets currently identified, only about three or four have water-like densities and would possibly fit this bill. However, understanding that high obliquity is not an automatic deal breaker to a planet’s potential habitability is very important as the study of exoplanets continues to move forward.