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WHEN predicting which planets beyond Earth could support life, astronomers usually follow the water. Exoplanets with rocky surfaces are declared habitable if they orbit far enough from their star to potentially host oceans.

But as our planetary collection grows, and telescopes for studying them improve, some astrobiologists say it is time to narrow the hunt. Christopher McKay at NASA鈥檚 Ames Research Center in California has come up with an expanded 鈥渃hecklist鈥� for habitability, which also considers factors like a planet鈥檚 light levels, radiation exposure and atmospheric composition (PNAS, ).

鈥淚f we go through that checklist and, bang-bang-bang-bang, we鈥檝e got it all, that is incredibly exciting,鈥� says McKay. 鈥淭hen we have a compelling case for a planet with life.鈥�

聯If we go through that checklist and, bang-bang-bang-bang, we鈥檝e got it all, that is incredibly exciting聰

Some items can be inferred just from knowing a planet鈥檚 probable composition and distance from its host star. Others will require photographing the planets and examining their atmospheres. Spacecraft with those capabilities are now being developed.

鈥淭hat doesn鈥檛 mean we are positioned to do it yet, but the discussion has gotten very detailed and very specific,鈥� says Adam Burrows at Princeton University.

The list may one day pinpoint worlds that can be home to life even if conditions there would kill humans outright. For instance, McKay points to Chlamydomonas nivalis, or watermelon snow, a red-coloured alga that only grows in freezing water. And researchers have grown microbes in the lab at temperatures as high as 122 掳C.

Even planets with barely any water could host life, says McKay. Cyanobacteria, for example, live on rocks in the Atacama desert in Chile, which gets only a few days of rain and fog each year.

Any life on another world would need sufficient light or geothermal energy to drive its vital processes. But this may not be a lot 鈥� deep-sea plants called red macroalgae can grow while receiving just 1 per cent of the sunlight hitting the ocean鈥檚 surface.

And while high levels of ultraviolet radiation may be a worry for complex life, microbes are much hardier. The most extreme example is Deinococcus radiodurans, which can survive in the sort of conditions you might find inside a nuclear reactor.

However, we know of one place that could rewrite McKay鈥檚 list. Saturn鈥檚 moon Titan has liquids on its surface and an atmosphere. Its seas are filled with methane and ethane, and its atmosphere is a choking haze of nitrogen and methane. That doesn鈥檛 seem so hospitable, but Titan has shown evidence of complex molecules that may be building blocks for life.

鈥淭itan is a little reminder that there are perhaps more things in heaven and Earth than we can imagine, as Hamlet said. It鈥檚 a cautionary tale,鈥� says McKay. 鈥淚f we discover something new, we鈥檒l have to rewrite this chapter.鈥�

This article appeared in print under the headline 鈥淓xtreme checklist for alien hunters鈥�