Eventually, researchers hope to develop a more detailed Planetary Habitability Index that factors in the idea that life might exist under exotic conditions unknown on Earth.

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In its solar system there is another planet, Gliese-581d, that is also of interest in the search for life, according to the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo.

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"There is no agreement by the astrobiology community on how to measure habitability, " said Abel Mendez, director of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo, who is working on several planetary indexing systems.

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To compare alien planets, Dr. Mendez at the Planetary Habitability Laboratory and his colleagues are creating an Earth Similarity Index that ranks worlds by their resemblance to our own planet, on a scale from zero to one, based on their mass, radius, density and surface temperature.

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