David Stevenson’s Commentary on the habitable zone as a guide for the search for life in the universe is, as always, trenchant (Physics Today, November 2018, page 10). Certainly, we should not focus all of our astrobiology efforts into searches for Earth-like life, lest we miss the variety of life and habitats that may exist elsewhere.
Many proponents of the habitable zone concept never argued otherwise. Rather, they find its value to be not in how it can help us exclude “unhabitable” planets from search efforts but in how it can help us chase the only lead we have in the hunt. Ideally, we would explore all potential habitats for life. But in a funding-constrained environment, it makes sense to allocate resources according to our best guess for where life can be found, with nonzero but smaller efforts spent on unlikely habitats and larger efforts on planets with “naked oceans.”
Until “look everywhere” is a funded strategy, spending most of our time in the habitable zone will have to do.
