Gliese 581: one planet might indeed be habitable

Figure 1. Illustration of the habitable zone (HZ) boundaries as obtained by the two teams. The upper part of the figure shows the HZ of the Sun (at its present age). The red curve shows only the most extreme outer limit of the HZ. The actual outer boundary is indeed located somewhere between 1.7 and 2.4 AU. The green limits show the boundaries of the photosynthetic zone as computed with the model by von Bloh et al. The middle part of the figure shows the limits of the HZ of Gliese 581 computed with the atmospheric models from Selsis et al. The lower part illustrates the boundaries of the photosynthetic zone computed with the geophysical models from von Bloh et al. The boundaries are shown for several possible ages (5, 7, and 9 Gyr-old) of the Gliese 581 planetary system. Following the latest estimation, Gliese 581 would be 7 Gyr-old. The purple bars surrounding planets Gliese 581 c and d illustrate the variable distance to the star caused by the eccentricity of the orbits. Copyright Astronomy & Astrophysics.