What Clues Indicate a Planet is Livable?

The article, 'Earth: The Lone Pale Blue Dot?' details specific indicators that another planet could harbor life. Searching for extra-terrestrial life would be an important mission for future space expeditions. So what are the clues that a planet could be hospitable to life? The answer from this NASA site:


"To determine if a planet is livable, scientists will look for carbon dioxide and water vapor, signposts that a planet has an atmosphere and oceans, respectively. Atmospheres not only provide air to breathe, but also act like blankets to keep a planet warm and help buffer potential residents from damaging ultraviolet and cosmic rays. Oceans help regulate a planet's temperature and provide liquid water, an essential ingredient for life on Earth."


It is believed that some molecules indicate life already exists. Oxygen, ozone and methane would suggest this. Methane is associated with swamps on earth and oxygen is expelled by plants on this planet. However detection of molecules like these can also be false positives for life. Saturn's moon Titan has an atmosphere containing methane but no living organisms.

Between oxygen and methane the former is the better indicator of life but both found together would be very convincing according to Dr. Victoria Meadows of NASA's Spitzer Science Center, in Pasadena, California. Dr. Meadows said, "for example, if we found carbon dioxide, oxygen and water vapor, in addition to methane, then we'd be pretty convinced that we were looking at an environment like our own."

Molecules can also indicate an environment that is hostile to life. Sulfur dioxide is an example and suggests a dead planet.