“Life is not a miracle. It is a natural phenomenon, and can be expected to appear whenever there is a planet whose conditions duplicate those of the Earth.”
Such were the words of Harold Urey, physical chemist famed for his contributions to our understanding of organic matter. Indeed, ever since humanity’s search for extraterrestrial organisms began, we have found thousands of planets which may have the right criteria to support life, and astronomers predict that there are several billion planets situated in their circumstellar habitable zone – also known as the ‘Goldilocks Zone’.
Although many have heard of the Drake Equation, a formula estimating the number of intelligent civilisations currently alive in the universe, the more relevant measure in our endeavour to find alien life is the Earth Similarity Index (ESI). This scale, which takes into account several factors including radius, density, escape velocity, and surface temperature, seeks to quantify how similar any given planet or moon’s physical characteristics are to our Earth. Whilst some exoplanets have ranked remarkably high, such as Kepler-438b with an ESI of 0.89 out of 1.00, and Gliese 667 Cc with an ESI of 0.84, most of these are several hundred light years away – well out of our reach for modern technological standards.
What about inside our very own solar system? Well, there are three candidates which have been regarded as serious prospects for extraterrestrial life in recent years: Jupiter’s moons Europa and Ganymede, and Saturn’s moon Enceladus. In fact, just last month, NASA announced the exciting news that it had requested $255 million in funding for an exploration mission to Europa. However, according to the ESI, none of these moons rank any higher than 0.3 on the scale; so why are they deemed to hold such potential for life? The answer lies in oceans, geysers, and hydrothermal vents. More here Possibility of Alien Life is Greatest on Europa, Enceladus, & Ganymede.