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A large proportion of planets in the so-called habitable zone 鈥� the area in orbit around a star where conditions are right for liquid water on a world鈥檚 surface, and thus potentially for life 鈥� weren鈥檛 always there. That might mean that we are vastly overestimating the number of worlds that could host life.

While researchers often think of the habitable zone of any given star as being relatively static, it actually changes as the star evolves and its brightness and temperature change. That means that worlds born well outside the habitable zone that started their lives either much too hot or too cold for liquid water on their surfaces could become more temperate later in their stars鈥� lifetimes.

at NASA鈥檚 Goddard Space Flight Center in Maryland and at Pennsylvania State University have dubbed these worlds belatedly habitable planets, as opposed to continuously habitable planets that spend their entire existences in the habitable zone. They calculated that, depending on how you define the habitable zone, 29 to 74 per cent of planets could be belatedly habitable.

That has major consequences for the possibility of water on these worlds. Those born closer to their star than the habitable zone may have all of their water boiled away before they enter the habitable zone, and for those born farther away any water is likely to take the form of difficult-to-melt global glaciers.

鈥淭he planets that are too cold are going to have a better chance of becoming habitable than the ones that are too hot,鈥� says Tuchow. 鈥淵ou don鈥檛 want a planet that鈥檚 been too close to the star for a long time and lost all of its water and then entered the habitable zone, because then no matter how long it鈥檚 there it won鈥檛 have much or any water.鈥�

We don鈥檛 know much about how life arose on Earth, so our understanding of the conditions necessary for life are vague, but it is clear that belatedly habitable planets have more obstacles for life than continuously habitable ones. 鈥淚f life cannot exist on these planets, it might have major implications for the abundance of life in the universe,鈥� he says.

Once we have more observations of planets we know to be belatedly habitable, such as the famous TRAPPIST-1 planets, we will be able to use models of stellar evolution to determine which worlds currently in the habitable zone are most likely to still have water, which we know is necessary for life on Earth. That will help researchers narrow down their search for signs of life elsewhere in the universe.

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