But this planet is different.

It's different mostly because it's small. Nearly all the earlier discoveries were of massive worlds, lumbering giants comparable to Jupiter or Saturn. Such behemoths are likely to be buried in thick and toxic atmospheres, and seem ill-suited for supporting life. Mind you, it's not that nature prefers the creation of such brawny planets; it's only that the wobble technique used to find them strongly favors the heavyweights.

However, by measuring the motions of bantam stars, such as the red dwarf Gliese 581, it's possible to uncover lighter-weight worlds, since detectability depends on the ratio of stellar to planetary mass. Gliese 581c, as the new find is called, is the smallest yet discovered around a normal star, a mere 50% larger across than Earth. This diminutive size suggests (but does not prove) that it's a rocky world, like Venus, Earth or Mars.

In another stroke of luck, it turns out that this planet is likely to be - like Baby Bear's porridge - at just the right temperature. Unlike Earth, it hugs Gliese 581 with a tight grip. It's five times closer to its runty star than Mercury is to our Sun. On the other hand, Gliese 581 is only a few percent as luminous as the Sun. These two factors roughly cancel, and a simple calculation suggests an average temperature similar to the temperate zones of Earth.