World Builders™
World Builders™
Session One  --  Astronomy
Session One  --  Astronomy

     The Life Zone
     The Life Zone

The "Goldilocks Model"        Is Your World  Too Hot, Too Cold or Just Right for Life?

   So far as we know, life needs a liquid of some kind in order to work chemically. The life forms that we are familiar with are dependent on  water. Water is made of small, plentiful  atoms and water, often in the form of ice, is abundant in the universe.

      Perhaps there are life forms that use other liquids, such as liquid methane, but we don't know about them. We tend to think in terms of water-based life.

    It seems reasonable to believe that planets that are close to the sun are too hot to have liquid water on their surfaces. Planets that are far away would probably be too cold. There is a region in the middle where planets might have temperatures that would permit liquid water to exist on their surfaces. The space in which these planets could orbit is called the life zone, or zone of habitability.

     However, a planet in the life zone might not be suitable for life due to other variables. For instance, if there is a dense atmosphere and a strong greenhouse effect, as there is on Venus, temperatures at the surface could be very hot and not suitable for life as we know it. This diagram shows a planet that has a strong Greenhouse Effect
   If the planet is too small to hold an atmosphere, it will not have water on the surface no matter how comfortable the temperatures are. Water would evaporate, and the gas molecules would just wander away.

     Planets far from their sun, like Saturn in our solar system, receive little heat from the sun and are thought to be made mainly of frozen gases. However, local conditions might generate some heat.

     Gravity can exert forces that cause friction in rocks and ice. The friction can generate heat even on dark planets.


Go on to  Gravity Can Cause Stresses that Generate Heat for more information.

More technical help with calculating the habitable zone for your star can be found at

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© 1998, 2003 Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at