A hypothetical exoplanet with alternating climate. When he dives into the habitable zone (green), he thaws up. The surface is frozen for most of the year. Living things could possibly overwinter under the ice sheet. (c) NASA / JPL Caltech
In the search for life in the Milky Way, astrobiologists should consider not only earth twins, but also more exotic worlds. Monsters of giant planets or planets with eccentric orbits could also be alive with creatures, according to Stephen Kane and Dawn Gelino. "We may experience some surprises if we want to pinpoint what we consider habitable? consider ?, says Kane. Since astronomers discovered the first planet in a strange solar system in 1995, they are wondering if some of these distant worlds could be alive. So far, they have relied on the concept of so-called habitable or "habitable"? Zone ? the area within a solar system where liquid water can exist on the surface. In order for the planet not to be too hot and not too cold, it needs to be just the right distance from its sun. Based on a popular fairy tale in the USA, the researchers also call the habitable zone Goldilocks Zone.

But this concept should be expanded, write Kane and Gelino. According to the classical concept, the habitable zone in the solar system only includes a relatively narrow belt, in which alone the earth revolves and in some models also Mars. The two researchers believe, however, that even planets, which are only temporarily in the habitable zone, could be suitable for life.

Some of the exoplanets discovered in recent years move on elongated orbits whose distance from their sun is variable. "Such planets may not spend the entire time in the habitable zone, " says Kane. "There may be worlds that heat up for short periods between long, cold winters. On others, there could be short episodes with very hot conditions. So, even if such planets are very different from Earth, they do not have to be dead, the researchers say. On Earth, too, there are organisms such as bacteria, spores or lichens that can survive long, cold periods and then come to life again.

Even large moons orbiting giant gas planets could house organisms. "There are tons of giant planets out there, and each of them could have moons, just like the giant planets of our solar system, " says the researcher. If a gas planet circles in the habitable zone, then in principle its moons would have to be life-friendly. display

Although they have now found a number of interesting planets, the researchers can only say more about the life-friendliness, if they know the composition of the atmosphere. How drastically two worlds can develop with the same initial conditions is shown not least by the example of Venus and Earth. While every corner of the earth is inhabited by organisms, a hellish 400 degrees Celsius prevails on the Venus surface.

Stephen Kane & Dawn Gelino (NASA Exoplanet Science Institute, Pasadena, California) et al: Astrobiology, submitted science.de - Ute Kehse

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