We might not be alone in the universe. Actually, it may be confirmed as a very crowded place by new computer models developed to help identify habitable planets. Estimates of places where life can exist have been based on the possibility of them having surface water. But software recently developed by the Aberdeen University allows researchers to identify planets with underground water kept liquid by heat from planets. Water is fundamental for life and planets too close to the sun lose water to the atmosphere through evaporation. On the other hand, planets located in distant reaches from their star have their surface water locked away as ice. Sean McMahon, who is carrying out the work, explained: "Traditionally people have said that if a planet is in this Goldilocks zone—not too hot and not too cold—then it can have liquid water on its surface and be habitable." But this concept might change when considering that planets can receive two sources of heat—heat direct from the star and heat generated deep inside the planet. It is easy to observe it in our own planet. As you go down through the crust (壳) of the Earth, the temperature gets higher and higher. Even when the surface is frozen, water can exist below ground. There could be immense quantities of water in fact—full of primitive life. Professor John Parnell, also from Aberdeen University said: "There is a significant habitat for microorganisms below the surface of the Earth, extending down several kilometres". "And some believe that the majority of life on Earth could even reside in this deep biosphere." So the Aberdeen team are developing models to predict which distant planets might harbour underground reservoirs of liquid water with the possibility of alien life. 小题1:What is considered as the symbol of life existence traditionally?
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