What is a Habitable World?

In 2007 for the very first time, Earth scientists have discovered evidence of a world that could, possibly, meet all of the conditions required for life. Whether life exists on Gliese 581 c or not will likely not be known for years to come—but it could. What makes this super-Earth a potentially habitable world, and what do astronomers and astrobiologists look for when scanning the heavens?

What is a Habitable World?

A habitable world is one that life could evolve on or exist if transplanted there, according to our own definitions of life. Removing for a moment the idea that life could exist in a fashion incomprehensible to us, Earth scientists have long considered the possibility of life on other planets. The question “are there extraterrestrial beings out there”, asked by scientists, authors, philosophers and theologians for centuries, has evolved itself to become “what sort of conditions would have to exist for life to be possible on a planet other than Earth?”

For a planet to be habitable it must be terrestrial, for starters. The word terrestrial literally means “earth like” but specifically means a rocky planet, or a least, not a gas giant. In addition a planet must support liquid water, thought to be instrumental in the evolution of life, as well as an energy source. While some say that the existence of an energy source is the only real requirement for life to exist, the technical criteria for a habitable planet is a bit more specific and demanding. It is also important to note that this discussion is not of planets that could be habitable if made so by scientific endeavour, but a planet that already possesses the necessary conditions for life to form/ exist on its own merit.

The Energy Source

An energy source, or star, is required for a planet to be habitable, but it must be the right type of energy source. Stars in the main sequence of the HRN are a good start, but not all would qualify. Most theories state that a star must be between 7,000 K and 4,000 K—any star too hot or too cold would not be able to support life. A star must also live a few billion years at least to give life the time it needs to evolve. The star must emit the right kind of ultraviolet radiation to initiate important atmospheric conditions on a planet, but not too much or it would destroy life as it began. And, to top it all off, the planet must be orbiting it at just the right distance, called the habitable zone, to have the liquid water it needs and not be in tidal lock, which is when one side of the planet always faces the star.

More Criteria

The mass of the planet is important as well. Too low, and the planet can’t contain enough of an atmosphere; too high and the resultant gravity may crush life rather than support it. In addition to being found in a habitable zone of a “habitable friendly” star and having the right mass, a planet has to have the right chemical composition in its atmosphere. Liquid water is thought to play an important role in this as well, which is another reason it is so critical to habitable planets.

Final Thoughts

Gliese 581 c was discovered in 2007 orbiting Gliese 581, a star system that is relavitily close at 20 light years or so. It possesses much of the required criteria, although it is thought to be in tidal lock. The terminator, or twilight zone, between the dark and light sides, however, could potentially support life. Gliese 581 c will be a high-visibility target in all studies regarding extraterrestrial life form now on.

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