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The Circumstellar Habitable Zone
What is the habitual zone, sometimes referred to as the "Goldilocks Zone"? It is an ecosphere, another term for the circumstellar habitable zone (CHZ), that is thought to exist around all stars, not just the Sun. If a planet is outside of this habitable zone, then it may not be capable of sustaining life. Outside of this band, liquid water will not be able to exist. Scientists believe that water is the most important factor in creating life, at least carbon based life. Its presence plays a big role as a solvent in biochemical reactions and this role cannot be minimized.
To see information about the conditions of life that form outside of Earth, see: So, There Is Water In Space... Now What?
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The Galactic Habitable Zone
The idea behind the galactic habitable zone is that the center of the galaxy acts in a similar way to how the Sun acts in the CHZ. The favorable area would have to be close enough to the galactic center so that it could pick up enough heavy materials to form planets and complex life. At the same time, it would have to be far enough away to have protection from life threatening radiation, which would damage any carbon-based life forms. One of the flaws of the galactic habitable zone theory is that it assumes that only carbon-based life can be created. This leaves out the possibility of life originating from some other form chemistry unknown on Earth. Current estimates of the size of the galactic habitable zone is between 23,000 to 29,000 light years.
For more information about the habitable zone and exoplanets see: Information on the Discoveries of Extrasolar Planets or Exoplanets
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Earth and the Habitable Zone
Earth is in the middle of the Solar System‘s habitable zone, with Venus and Mars on either side. The exact limits of the Sun‘s CHZ is unclear. However, the best estimate to date of the Sun's habitable zone for an Earth-sized planet is between 120 million and 240 million km (75 million and 150 million miles). Nevertheless, this is one reason why scientists are interested in finding life on Mars; it would extend the size of the habitable zone.
The habitable zone clearly depends on the size, and luminosity of the star. In general, hot, luminous stars, in other words, those with spectral types G3–G9, F, A, B, and O (our Sun is a G2V), all have wide habitable zones. For example, to enjoy Earth-like temperatures around Sirius, a star that is about 26 times more luminous than the Sun, a planet, like Earth, would have to orbit at about the distance Jupiter is from the Sun. On the other hand, if the Sun was replaced by a star with lesser luminosity, like Epsilon Indi, (it is a K4.5 spectral classification and has one-tenth the luminosity of the Sun), the Earth would need to be in a Mercury like orbit to receive the same level of warmth.
See also: Super-Earths - Can They Sustain Life?
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The Kepler Mission and Exoplanets
The Kepler spacecraft was launched in March 7, 2009. Its mission is to look for planets like Earth that may contain water and life. These are exoplanets. At this time there is very good evidence for the existence of three types of exoplanets; gas giants, ice giants, and hot-super-Earths in short-period orbits. Kepler is looking for just such planets to see if there is a possibility of life. The astronomical challenge is to locate terrestrial like planets, which appear to be about one half to twice the size of the Earth, and appear in the habitable zone where liquid water and maybe life might exist. To date, Kepler has found over 700 planet candidates and verified 7 to meet those conditions.
More information about the Kepler space mission can be found at Kepler Space Mission: Search for Extraterrestrial Planets
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The habitable zone, or ecosphere, is the volume around a star that has the right conditions to let life form. Liquid water is the condition that must exist in order for life to form as we know it. So a planet cannot be too close to a star or the heat would boil away the water, and it cannot be too far away as the cold would prevent liquid water from forming. There are two main theories of the habitable zone. One involves the circumstellar habitable zone and the other, the galactic habitable zone. One deals with the area around a star, and the other deals with the area around the center of the galaxy. Finally, astronomers are looking closely at the Kepler mission to discover Earth like planets around distant stars.
How are exoplanets discovered? Read about it at What Are Exoplanets, How Are They Detected, And What Do They Mean
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The Universe Today: http://www.universetoday.com/32622/habitable-zone/
Circumstellar Habitable Zone:
Internet Encyclopedia of Science: http://www.daviddarling.info/encyclopedia/H/habzone.html
Galactic Habitable Zone:
Internet Encyclopedia of Science http://www.daviddarling.info/encyclopedia/G/galactic_habitable_zone.html
Earth and the Habitable Zone:
Image: Internet Encyclopedia of Science http://www.daviddarling.info/encyclopedia/H/habzone.html
The Kepler Space Mission:
For more information about the habitable zone and exoplanets See: Project Pan-STARRS and the Outer Solar System