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What exactly does the word ‘habitable’ mean? The question comes to mind because of two things, the first being the media buzz over Gliese 581g, now widely described as the first potentially habitable planet we’ve found. The second is Paul Davies’ presentation yesterday at the International Astronautical Congress in Prague, where Davies was careful to differentiate between ‘habitable’ and ‘inhabited.’ More on the latter in a moment. Let’s look first at this outstanding find, two new planets in the Gliese 581 system discovered through the unflagging efforts of the Lick-Carnegie team.
“If these [habitable worlds] are rare, we shouldn’t have found one so quickly and so nearby. The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that’s a large number. There could be tens of billions of these systems in our galaxy.”
The paper is Vogt, Butler et al., “The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581,” accepted by The Astrophysical Journal and available as a preprint.