Humanity’s Next Home May Be Underwater

Planets with high obliquity—an axis of rotation that's extremely tilted relative to their star—are typically reckoned to be miserable places where temperatures flip-flop between freezing and boiling. Now, MIT researchers find that life aquatic may yet thrive on highly tilted planets. Aquaplanets, or worlds covered entirely by oceans, can actually be quite cozy, even those that are totally sideways, orbiting their star like a rotisserie chicken.

The findings, which hail from researchers at MIT's Department of Physics and Earth and Planetary Science, were publishe​d today in the journal Icarus. The study may have far-reaching implications on the search for extraterrestrial life.

Conventional wisdom asserts that the more extreme a planet's tilt, the less hospitable that planet would be. Tilted sideways, a planet's north pole would experience incessant daylight for six months, followed by half a year of utter darkness.

"The expectation was that such a planet would not be habitable: It would basically boil, and freeze, which would be really tough for life," study co-author David Ferreira said in ​a press release.

Instead, Ferreira and his colleagues found that planets with a global ocean experience peachy, spring-like weather year-round. To reach that conclusion, the researchers developed a climate model that simulates a high-obliquity aquaplanet—Earth-sized, a similar distance from its star, and covered entirely with water. Their simulations included planets tilted to 23 degrees (an Earth-like tilt), 54, and 90 degrees, with ocean depths ranging from 10 to 3000 meters.

The researchers found that water worlds—even those tilted a full 90 degrees—maintain temperatures of 60 degrees Fahrenheit or so. The reason has to do with water's unusual capacity to absorb and release heat very slowly. A highly tilted aquaplanet soaks up loads of solar energy during the summer, enough to keep releasing heat back into the atmosphere all winter long. (This is the same reason Earth's coastlines experience milder winters than landlocked regions.)

"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 said. "It's really surprising that the temperatures at the poles are still habitable."

The only catch, it seems, is that the ocean needs to be deep enough. Simulations with very shallow global oceans—say, ten meters deep—were far less climactically stable. As soon as a bit of ice forms on these worlds, it triggers a runaway "snowball" effect. Covered in ice, an aquaplanet becomes a giant solar reflector, loses its capacity to warm back up, and locks itself into an endless winter.

This study complements other recent planetary modeling work finding that a slew land-bearing exoplanets—those that sit a bit too far from their star— may be doomed to climatic instability and hellish winters. If current trends continue, scuba gear may end up being the haute couture of humanity's future home. And hey, underwater cities looked pretty cool in Bioshock.