'Rotisserie' Planets Could sustain Alien Life — If Wet Enough

Planets revolving on their sides were long believed to have climates too dangerous for life as we know it on Earth, but now, researchers have discovered that some of these "rotisserie" worlds might be more hospitable.

If these alien planets, which rotate sideways like a pig on a spit, are enclosed in oceans, they could in fact have a mild, springlike climate all year, researchers say, possibly expanding the number of possibly habitable planets where life may be discovered.

In the past two decades or so, astrophysicists have confirmed the presence of more than 1,800 worlds circling distant stars, raising the probability that some of these exoplanets might be home to extra-terrestrial life. The hunt for aliens often focuses on planets that look like Earth, the only world known by humans to host life.

One key factor affecting whether a planet might be inhabitable is its obliquity, the degree to which the axis on which the planet spins tilts. This effect the amount of sunlight any point on the planet experiences over the course of a year. The larger a world's obliquity, the more dangerous its seasons are.

Earth has a comparatively low axial tilt of about 23.5 degrees. Though, scientists suspect that exoplanets may show a range of obliquities, resembling anything from a vertical spinning top to a horizontal rotisserie.

Researchers had thought that the more extreme the tilt, the less inhabitable a world would be.

"The anticipation was that such a planet would not be habitable — it would normally boil, and freeze, which would be really hard for life," lead writer of the exoplanet study David Ferreira, a climate researcher at the University of Reading in England, said in a statement.

Though, Ferreira and his colleagues' new discoveries challenge those expectations, showing that such much tilted planets may stay habitable if covered completely by oceans. "In the hunt for habitable exoplanets, we're saying, don't discount high-obliquity ones as unsuitable for life," Ferreira added in a statement.

To see what life might be like on inhabitable planets with extreme tilts, scientists simulated Earth-size planets enclosed completely in water circling their stars at the same distance as Earth orbits the sun. The 3D models simulated circulation among the atmosphere, ocean and sea ice on "aquaplanets" with oceans 1.8 miles (3 kilometers) deep and "swamp" planets with comparatively shallow oceans that were 33 feet (10 meters), 165 feet (50 m) or 655 feet (200 m) deep.

The investigators replicated planets at three obliquities. The first was 23.5 degrees, like Earth's. The next was 54 degrees, the point at which the poles get more annual sunlight on average than the equator. The last was 90 degrees, the point at which a planet is basically lying on its side — the poles would each point at the star for a quarter of the year, and then away for another quarter, changing between extremes of light and darkness.

Ferreira and his associates discovered that a global ocean would absorb sufficient solar energy from the star and discharge it back into the atmosphere for such a world to preserve a rather mild, spring like climate year round.

"We discovered that the ocean stores heat during summer and gives it back in winter, so the weather is still pretty mild, even in the heart of the cold polar night," Ferreira said
Even a shallow global ocean only about 165 feet (50 m) deep would be sufficient to keep a high-obliquity planet at an average of about 60 degrees Fahrenheit (15.5 degrees Celsius) year-round.

"The most astonishing result here is how little ocean is required to sustain a mild climate at the poles, even in the heart of winter and summer times," Ferreira told Space.com. "We were not expecting how proficient an ocean, even a shallow one, would be at moderating temperature."

Past study had proposed that great alterations in temperature between the dark and light sides of a high-obliquity planet might lead to powerful weather. Though, the researchers did not see a strong change in the intensity of weather on such worlds compared to the weather on Earth. In its place, "at high obliquity, more active weather occurs in summer, while winters are quiet times in terms of storm activity," Ferreira said. "That is the opposite of what happens on Earth."

In the future, the scientists may simulate worlds covered with both land and water. Ferreira and his associates — John Marshall, Paul O'Gorman and Sara Seager, all at MIT — detailed their discoveries in the Nov. 15 issue of the journal Icarus.



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