The role that Earth's oceans have on our planet's habitability is undeniable, but now scientists think that exoplanetary oceans are essential for alien life to evolve.
In a new study published by the journal Astrobiology, University of East Anglia (UEA), UK, researchers have come to the conclusion that, to make a planet habitable, a large liquid ocean is needed to stabilize its atmosphere.
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"We know that many planets are completely uninhabitable because they are either too close or too far from their sun," said David Stevens of UEA's School of Maths. "A planet's habitable zone is based on its distance from the sun and temperatures at which it is possible for the planet to have liquid water.
"But until now, most habitability models have neglected the impact of oceans on climate."
The habitable zone surrounding any star is the distance at which it's not too hot and not too cold to support liquid water on a planetary surface. Liquid water is vital for the evolution of life as we know it.
Earth orbits within our sun's habitable zone, unsurprisingly, whereas Mars is located on the outside edge and Venus on the inside edge. The life-giving contrast between Earth, Mars and Venus couldn't be more stark; Mars is a frozen, dry wasteland with dramatic surface temperature variations, and Venus is a choked, broiling world with searing surface temperatures. But Earth is stable, a factor that has allowed life to thrive for billions of years.
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Although a planet's distance from its star is important, whether or not it has an ocean appears to be a huge factor. In fact, the presence of an ocean is the ultimate planetary "climate control" for any planet, according to new computer models created by Stevens' team.
"Oceans have an immense capacity to control climate," he said in a UEA news release. "They are beneficial because they cause the surface temperature to respond very slowly to seasonal changes in solar heating. And they help ensure that temperature swings across a planet are kept to tolerable levels."
Although Mars is located on the outside edge of the sun's habitable zone, planetary scientists believe the red planet once possessed large bodies of water when the planet's atmosphere was thicker. The presence of liquid water on the surface of ancient Mars is exciting - after all, on Earth, where there's water there's usually life. But the presence of possible Martian oceans may have stabilized the atmosphere, making it less prone to wild temperature fluctuations and more comfortable for life to gain a foothold. Modern Mars endures air temperature fluctuations of over 100 degrees Celsius (212 degrees Fahrenheit).
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In the computer model, the researchers found that the heat transported by a global ocean has a "major impact on the temperature distribution across a planet," said Stevens. This, he argues, would make larger regions of an ocean-supporting exoplanet habitable.
"Oceans help to make a planet's climate more stable so factoring them into climate models is vital for knowing whether the planet could develop and sustain life," added Stevens. "This new model will help us to understand what the climates of other planets might be like with more accurate detail than ever before."
This study once again proves that while finding an exoplanet orbiting within its star's habitable zone is important, the real "holy grail" for finding a truly habitable world would be to look for rocky exoplanets possessing global oceans, worlds that may be more abundant than we thought.