Although water is one of the key ingredients scientists look for when considering if a planet is habitable or not, other factors, such as the planet's distance from its star and its tilt, play an important part in the likelihood of a planet containing life.
However, the definition of habitable planet just got a little looser thanks to new research done by MIT scientists that shows that planets covered completely in water within their star's habitable zone could still contain life, even if the tilt of its axis is completely off as long as the planet's water depth was at least 165 feet deep.
When we consider potentially habitable planets, our usual assumption is that these planets live within the habitable zone of their stars. This is that place in orbit that's not too close and not too far from the star so that liquid water can form on the planet's surface. However, other forces play a part in life appearing on a planet, including size. A planet that's too big has too much gaseous pressure. A planet that's too small has too little gravity to keep an atmosphere. So most often, what we look for are planets the same size as Earth.
Of course, the presence of water, particularly in oceans, is also important. Oceans help regulate the temperature of planets, at least as long as there's enough water in them.
The MIT scientists started with these concepts and used an Earth-sized planet in their simulation at the exact same distance from its star as the Earth is from the sun. They did, however, change two key factors: water covered their planet and its axis was tilted so far that it rotated on its side.
They found that the planet was still habitable, even at the poles, in spite of the planet's odd tilt. The poles face the sun directly in the summer, but their temperatures only reached 104 degrees Fahrenheit during that time, which is certainly hot, but not completely inhospitable to life. During the winter, the poles face away from the sun, but again, the temperature change was not that extreme: about 55 degrees Fahrenheit. This, too, is a perfectly habitable temperature.
Of course, this is all dependent on the oceans being at a certain depth. Simulations showed that this world was most hospitable if the oceans covered the planet and were at least 165 feet deep. However, at 66 feet, the planet would be too cold and probably ice over quickly, meaning that life wouldn't have time to form there.
"With deeper oceans, a collapse into a global snowball is possible, but a bit harder," says study lead author David Ferreira. "It feeds into the idea that if you have an extensive, big, deep ocean, your chances to find life or a climate that is habitable are higher."
The simulations used are similar to those used by climatologists to predict global warming, and shows how important oceans, even on Earth, play on weather and climate.
"Naturally, the question is how you would apply that knowledge to the planets that are in a different astronomical state than Earth," says Ferreira. "One would expect oceans in such planets would be a strong regulator on the climate as well, and a factor in habitability."
