Want to Find Aliens? Look for More than Just Earth-Like Planets
They talk about producing a “Planetary Habitability Index” based more on the known extremes of life as found here on Earth. I like that idea as I feel quite confident that anywhere there is liquid water, temperature gradients and an assortment of carbon and minerals, life of some sort will form. I strongly suspect life is ubiquitous in our universe, though intelligent life (like there is any on Earth ;-)) might be extraordinarily rare.