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’s Ames Research Center in California has come up with an expanded “checklist” for habitability, which also considers factors like a planet’s light levels, radiation exposure and atmospheric composition (PNAS, DOI&colon; 10.1073/pnas.1304212111).
Some items can be inferred just from knowing a planet’s 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.
“That doesn’t mean we are positioned to do it yet, but the discussion has gotten very detailed and very specific,” 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 °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 – deep-sea plants called red macroalgae can grow while receiving just 1 per cent of the sunlight hitting the ocean’s 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’s list. Saturn’s 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’t seem so hospitable, but Titan has shown evidence of complex molecules that may be building blocks for life.
When this article was first published, it described Chlamydomonas nivalis as a red alga. It is in fact a green alga with a red pigment.