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Extrasolar planets

Inner edge of the habitable zone

Nature volume 504, pages 221–223 (2013)Cite this article

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A three-dimensional climate model indicates that the fraction of Sun-like stars that might harbour a rocky planet within their habitable zone could be smaller than previously estimated. See Letter p.268

The successful, if perhaps somewhat premature, conclusion of NASA's Kepler mission has spawned renewed interest in potentially habitable planets. According to recent research1, some 22% of Sun-like stars might host a rocky planet within their habitable zone — the region within which liquid water can exist on the planet's surface. With roughly 100 billion stars in the Galaxy, this implies that there could be as many as 22 billion other Earth-like planets. But this estimate depends crucially on how the zone's boundaries are defined. The habitable zone has recently been redefined1 as extending from 0.5 to 2 astronomical units around a Sun-like star, where 1 AU is the mean Earth–Sun distance. This is a curious choice, because the completely desiccated planet Venus orbits at 0.72 AU, suggesting that the habitable-zone inner edge is somewhere beyond that distance. On page 268 of this issue, Leconte et al.2 show that the inner edge probably lies closer to 0.95 AU, where the solar flux is about 1.1 times that at Earth's orbit (Box 1).

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Authors and Affiliations

  1. James F. Kasting and Chester E. Harman are in the Department of Geosciences, Penn State University, University Park, Pennsylvania 16802, USA.,

    James F. Kasting & Chester E. Harman

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  1. James F. Kasting
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  2. Chester E. Harman
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Correspondence to James F. Kasting.

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Kasting, J., Harman, C. Inner edge of the habitable zone. Nature 504, 221–223 (2013). https://doi.org/10.1038/504221a

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