From Brendan Ashe
Your look at more sophisticated attempts to define the habitable zone around stars – and Earth’s delicately balanced position within the solar system – leads me to wonder how this new approach affects the Drake equation, which aims to estimate the number of alien civilisations out there (8 June, p 40).
If, over x billion years, a planet has to orbit within a shifting Goldilocks zone, as its sun slowly increases its heat output, then the chance of life elsewhere must become a longer shot. A planet now has to be born and then remain within this shifting band long enough for life and then evolution to have a chance. The solar system’s habitable zone has moved considerably, and Earth’s achievement in having remained within it seems commensurately more impressive.
Hampton, Middlesex, UK
