Free Astronomy Magazine November-December 2019

NOVEMBER-DECEMBER 2019 EXOPLANETS It is clear that these scenarios are ex- tremely different − nevertheless, they are sometimes confused. It is under- standable that “habitable zone” does not automatically mean “habitability.” In fact, there are a number of factors that determine whether a world can be habitable or not. The fact of orbit- ing in the habitable zone of a star is only one of them, and it is not even an essential factor, because there can exist worlds capable of hosting life even outside the habitable zone of a star. Although the concept of habitability related to circumstellar space seems to belong to modern science, it has no very recent origin. The first mention of a “temperate zone of the Solar Sys- tem” dates back to 1853 and is found in the essay Of the plurality of worlds , by the eclec- tic English Reverend William Whewell. O n the left, a portrait of William Whewell, the philosopher- scientist who first mentioned in an essay (frontispiece below) the tem- perate zone of the Solar System. them at relatively short distances from the Sun and that there are probably billions of them in the whole Milky Way, a small percentage of which could host life as we know it. This last phrase, dutifully re- peated as a mantra in all astrobiology texts, leads us to the con- cept of “habitability,” a term that appears to be very precise, but that when applied to extrasolar planets takes on an elasticity that makes it, to say the least, vague. What do we mean by “hab- itable planet”? Per- haps a copy of the Earth, where humans could live without en- countering excessive adaptation difficul- ties? Or a planet on which at least some forms of terrestrial life, perhaps the sim- plest, could prolifer- ate? Or again, a planet that simply or- bits in the habitable zone of a star?