The Goldilocks Zone
Ken Tapping, 20th March, 2013
Goldilocks was a rather badly-brought up little girl who walked into the home of The Three Bears while they were out, and tried out their beds and their breakfast porridge to find out which of the Mother, Father and Baby Bears’ stuff most suited her. This unwanted visitation was not appreciated by the bears when they came home.
This picky little girl, for whom everything had to be just right, now has her name applied to the “habitable zone” for planets. The “Goldilocks Zone” is the range of distances a planet can be from its star to be in the right temperature for liquid water to be present on its surface, which is an essential condition for “life as we know it”. At sea level on Earth this corresponds to a temperature range of 0 to 100 degrees Celsius, respectively the freezing and boiling points of water. However it looks as though our definition of the Goldilocks Zone is a bit too narrow to be really useful.
Over recent years we have been finding creatures on our world living happily under conditions that would kill us very quickly. We now call them extremophiles. We find living things in the near-boiling water in hot springs and communities of animals around hydrothermal vents (black smokers) on the sea floor, where jets of mineral-laden water emerge from the sea floor with temperatures of hundreds of degrees. This superheated water is forced to remain liquid by the high pressure imposed by kilometres of overlying water. Now we know of life forms living kilometres underground, in hot rock, under extremely high pressures. It seems that if the pressure is high enough to keep water in a liquid form, there is likely to be something swimming around in it, even at temperatures of hundreds of degrees. Even on hostile-seeming worlds, there could be conditions somewhere on it where water might be present as a liquid.
This means we cannot define a Goldilocks zone without taking into account other things. For example, if Mars were bigger, it might not have become the almost airless, frigid desert it is today. A more massive world would have stronger gravity, which would have been able to better prevent its atmosphere drifting off into space. If it had a strong enough magnetic field it could have deflected the solar wind, stopping it from scouring its upper atmosphere away. Today, with a denser atmosphere, with the greenhouse effect warming the planet, Mars could have oceans of liquid water, with living creatures swimming in it.
In principle a dense, high-pressure atmosphere can keep water liquid at high temperatures. However, if that atmosphere is too rich in greenhouse gases, the temperature of that planet can run away until the world is too hot for any conceivable form of life. That’s what seems to have happened to Venus.
This all suggests that simply calculating a habitable zone for planets based upon distance from a star is too pessimistic, and the quest for life “out there” will be more fascinating than we might have thought.
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