The first extra solar, Earth-like planet was discovered by the Stephane Udry University of Geneva's Observatory in Switzerland in April 24th, 2007. This terrestrial planet is called GLIESE 581C and has a mass at least five to eight earth masses. There is another terrestrial planet in the GLIESE 581 system. The second planet GLIESE 581C lies in front of the theoretical “habitable zone” with an orbital period of 12.9 earth days while the third planet, GLIESE 581D, lies outside the habitable zone, having an orbital period of 83.6 earth days. Both these planets have very eccentric orbits which makes their orbital periods short compared to Venus, Earth and Mars in our solar system. These are the first two terrestrial planets discovered outside our solar system, which are earthlike in size and orbital distance from the parent star.
Scientists believe GLIESE 581c is too close to its star for water to exist on it, and GLIESE 581d may be too far away to support life, giving these two planets a “goldilocks” syndrome. The star in the GLIESE system is a red dwarf main sequence M star on the HR diagram. An M star has the ability to tidally lock its planets in the habitable zone. As a result of this tidal effect, one side of the planet would always face the star, and the other side would remain in darkness, giving rise to massive temperature differences. The tidal effect would never allow the sun-facing hemisphere to cool down, since the star’s rays would always be hitting the same side of the planet. This causes one side of the planet to be too hot to sustain water, which is probably the case for GLIESE 581c.
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