A.J. Rushby et al/Astrobiology 2013; Penfold/Istockphoto, Banderlog/Istockphoto, Enjoynz/Istockphoto, Bikerboy82/istockphoto, S. Egts
To determine whether a planet could support life, astronomers first look at whether it falls within its star’s habitability zone, the Goldilocks distance that is not too hot or too cold. But that range can change as a star evolves. A recent estimate of the lifetimes of the habitability zones of Earth and various exoplanets suggests Earth could become unable to support life as soon as 1.75 billion years from now, when the sun brightens before dying out.
A.J. Rushby et al. Habitable zone lifetimes of exoplanets around main sequence stars. Astrobiology. Vol. 13, September 2013, p. 833. doi: 10.1089/ast.2012.0938
R. Cowen. The hunt for habitable planets. Science News. Vol. 174, December 20, 2008, p. 16.