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Antarctic algae are pretty chilled out

When deciding which planets beyond Earth could host life, astronomers usually follow the water. Exoplanets with rocky surfaces are declared habitable if they orbit far enough from their star to be warm, while potentially supporting oceans and seas.

But as our planetary collection grows, and our telescopes for studying them improve, some astrobiologists say it is time to narrow the search. Taking what we know about the extremes that life can endure, at NASA鈥檚 Ames Research Center in California has come up with an expanded checklist for habitability.

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� The list may one day pinpoint not only benign, Earth-like environments, but also worlds that may host other forms of life in conditions that would kill humans outright.

Some items on the checklist can be inferred just from knowing a planet鈥檚 size, mass and distance from its host star. Others will require directly photographing the planets and probing the contents of their atmospheres. Spacecraft with those capabilities, like the proposed Starshade mission and the upcoming James Webb Space Telescope, are already 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 and perhaps more real,鈥� says at Princeton University. 鈥淚n the next few decades we may have a plan for determining whether life exists elsewhere.鈥�

When the right instruments come online, here are some of the wildest things we may find thriving on exoplanets, according to McKay鈥檚 checklist:

1. Frozen or boiling bugs
The habitable zone around a star is largely based on where an orbiting planet would have a relatively mild temperature range, so water can be a warm, pleasant liquid. But McKay points to microbes on Earth that can survive well below freezing or above boiling temperatures. In 2013, researchers at McGill University in Montreal, Canada, that were able to reproduce at -15 掳C. And researchers have at temperatures as high as 122 掳C. An icy world could be covered in something akin to Chlamydomonas nivalis, or watermelon snow 鈥� a red-coloured alga that only grows in freezing water.

2. Arid algae
It is also possible for a planet to host life even if it has barely any water, says McKay. Cyanobacteria live under and inside rocks in the Atacama Desert in Chile, which gets only a few days of rain or fog every year. And lichens grow in the Antarctic Dry Valleys, only occasionally getting small sips of water from melting snow. 鈥淵ou don鈥檛 need a Pacific Ocean,鈥� says McKay. 鈥淎 planet like the fictional world in Dune would be habitable, though you may not have .鈥�

3. Deep, dark seaweed
Any life on another world would need sufficient starlight or geothermal energy sources to drive its vital processes. Luckily you don鈥檛 need a lot of sunlight to drive photosynthesis 鈥� sea plants called red macroalgae can grow in deep water with just one per cent of the sunlight we receive on Earth鈥檚 surface.

4. Radiation-proof bacteria
Anyone who has been sunburned knows the dangers of too much ultraviolet radiation. Humans and other complex organisms are sensitive to UV light and other radiation, like the cosmic rays that stream down from space. But microbes are much hardier. The most extreme example is Deinococcus radiodurans, the world鈥檚 toughest bacterium, which can survive in the sort of conditions you might find inside a nuclear reactor.

5. Oxygen haters, nitrogen lovers
It is generally agreed that abundant oxygen or ozone in the atmosphere would be a sure sign that complex life already exists on a planet, although it鈥檚 not the only one. But some bacteria, like the Actinomyces genus found in soils and compost, cannot function when there鈥檚 oxygen around. Other bacteria don鈥檛 use oxygen for growth but will tolerate its presence. Instead of lots of oxygen, we may first need to find worlds rich in nitrogen. Nothing we know of can survive without nitrogen, which is essential for building proteins and DNA. 鈥淚t鈥檚 hard to imagine a kind of life that isn鈥檛 going to need lots of nitrogen,鈥� says McKay.

However, there is one place in our solar system that could change this list entirely. Saturn鈥檚 moon Titan has an atmosphere, liquids on the surface and even a weather cycle. But instead of water, its liquids are methane and ethane, and its atmosphere is a choking haze of nitrogen and methane. But Titan has also shown evidence that it has 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 will have to rewrite this chapter.鈥�

Journal reference: PNAS, DOI: 10.1073/pnas.1304212111