Planets that are in the habitable zone

The Kepler spacecraft has discovered that one in five of stars similar to the Sun of the Earth’s solar system have planets similar to Earth. Scientists claim there are billions of planets that have the proper conditions for the support of life. A planet similar in size to the Earth in another star system that revolves around a star similar to the Sun must be in the Habitable Zone for it to be considered a planet with life on it. NASA’s Kepler mission discovered two star systems that have planets in the Habitable Zone. They were named Kepler-22 and Kepler-11. 

One of the criteria for a planet to be considered in the Habitable Zone is its distance from its host star. If it revolves too close to the star, then the water on the planet is probably too hot to last very long because it boils away. If it is very far away, then the water must be frozen because it is too cold. 

Stars are composed mostly of hydrogen and helium. The other elements present in a star might determine the probability of a Habitable Zone in its system. Scientists determine the possibility of a Habitable Zone by looking at their brightness and color. Theoretical astrophysicist and astrobiologist Patrick Young of Arizona State University and his colleagues used computer models to study the variations in the levels of the elements carbon, oxygen, sodium, aluminum, magnesium, silicon, calcium and titanium; and their possible effect on stars similar to the Sun. They found that oxygen, sodium, aluminum, silicon, magnesium and calcium have a significant but small effect on a star’s evolution. 

Twenty years ago the Habitable Zone was defined by researcher James Kasting of Penn State University. The area where planets can support life uses the astronomical unit, which is the distance from the Sun to the Earth. The old zone discovered by Kasting was 095 AU to 1.67 AU. The new zone, to which Kasting contributed, is 0.99 AU to 1.7 AU. The zone appears to eliminate Venus as life supporting because it is just 0.7 AU from the Sun, but it does include Mars because it is 1.5 AU from the Sun. But it is just theory.

A planet that might have the proper conditions for life was recently discovered. Gliese 581d is probably in the range of the Habitable Zone. It also was determined that it could have enough water on its surface to support life.

An exoplanet is a planet that orbits a star other than the Sun. The Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo is working on a catalog of the habitable exoplanets. It has a table of the Habitable Zone Composition of the planets of the solar system. The terrestrial and ocean planets have values between -1 and +1. The planets in this range are Mercury, venus, Earth, and Mars. Jupiter’s moon Europa and two of Saturn’s moons Titan and Enceladus are also in the -1 to +1 range.

Kepler-62e is one of the exoplanets most likely to have complex life like animals and plants because its 0.83 passes the required 0.8 Earth Similarity Index (ESI). It orbits the star Kepler-62.  The ESI is based on a 1.00 for Earth. It also is in the Habitable Zone for life and is not too close to the inner edge of the zone where it is likely to be too hot. Gliese-667Cc also passes with 0.82 ESI and not having an orbit too close to the inner edge. The star it revolves around is Gliese-667C. Although just a candidate exoplanet with an unconfirmed orbit, Gliese-581g is predicted to have the same ESI as Gliese-667Cc (0.82).

Any exoplanet with an ESI between 0.5 and 0.8 could be suitable for simple life. The planets that have been found with an ESI in that range include Tau Ceti-e (0.77), Gliese-667Cf (0.76), Kepler-22b (0.75), Gliese-163c (0.74), Kepler-61b (0.73), HD-40307g (0.72), Kepler-62f (0.67), Gliese-667Ce (0.60), and Gliese-581d (0.53). Of these, Gliese-581d, Gliese-667Ce, Kepler-62f, HD-40307g (unconfirmed), Kepler-22b, and Gliese-667Cf are not too close to the inner zone or outer zone to support simple life. The orbit of Gliese-667Ce is very close to the same distance from the star Gliese-667C as Mars is from the Sun. Similarly, the orbit of Kepler-22b is very close to the same distance from the star Kepler-22 as the Earth is from the Sun and Kepler-62f is close to the same distance from the star Kepler-62 as Mars is from the Sun.          

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