Zonas de habitabilidade de alguns planetas e suas estrelas (Sol=1)

Was announced on April 25 this year (2007), the discovery of two planets orbiting the star Gliese 581 (Gl 581), in the constellation Libra. This star is not visible to the naked eye. The discoveries of extrasolar planets have become increasingly common in modern astronomy, and are no longer, in general, news that attracts attention of many people. But one of the newly discovered planets around Gl 581 is the one whose mass is the closest to the mass of the Earth among all the previously discovered to date. Moreover, it is located within the habitable zone of that star.

Gl 581 is a red dwarf star, and stars of this type are of particular interest in the search for extrasolar planets. The small mass of a red dwarf star makes the detection of light planets easier than in more massive stars, and the the habitable zone of these stars was close until 0.01 a.u. ¹ .

Planetary system Gl 581 has three planets discovered. The first and closer, called Gliese 581 b, discovered in 2005, is close to the mass of Neptune and a complete turn around the star at 5.36 days.

The second planet in order of distance from the star is the one that really call our attention in this system, Gliese 581 c. Its orbit lies within the habitable zone, a region within which a planet would be able to harbor liquid water. The mass of Gliese 581 c is the smallest of all extrasolar planets have been discovered so far (we are in April, 2007), estimated at 5.03 Earth masses ² , and its radius is about 1.5 of Earth’s radius ³ . This makes Gliese 581 c is the planet most similar to Earth ever found, and the most promising to harbor life as we know it on Earth. The third planet of this system is 0.25 a.u. of the star and its mass is 7.7 Earth masses.

Current techniques for detecting extrasolar planets enable us to know, in most cases, just very massive planets that are very close to its star. The planet Gliese 581 c is quite close to its star, as might be expected, but due to its low mass, could only be detected because Gl 581 is a red dwarf.

Note that the fact of a planet be in the habitable zone is not a guarantee that it has liquid water, but a guarantee that it can has liquid water. A planet in the habitable zone with a very small gravity could not keep the water on its surface for a long time, and it would tend to evaporate into space, over successive cycles of evaporation and rain.

Also a gravity that can hold water on the planet is no guarantee that life has evolved on the planet. An atmosphere causing a greenhouse effect much exaggerated, as with our neighbor Venus, is a factor that would preclude life as we know, even with water.

There is another detail in this system that makes it remarkable. Gl 581 has lower metallicity than the Sun ( Fe/H = – 0.25), unlike most of the stars that harbor planets. As say studies of planet formation, stars of low mass and low metallicity could hardly have giant planets around itself, but could easily have planets of low mass.

Although there are no guarantees that Gliese 581 c is a planet inhabited by living beings, it is gratifying to know that our techniques for detecting extrasolar planets already allow us to find planets in the habitable zone of their stars.

Below is a table with some data on the three planets of the planetary system Gl 581.

Obtained at http://obswww.unige.ch/exoplanets/gl581.html

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¹ a.u. means astronomical unity, and it is equal to the distance between Earth and Sun, about 150 millions km.

² One mass of the Earth: 5,98 x 10²⁴ kg.

³ Radius of the Earth: 6.378 km.