There may be as many as 100 million habitable worlds in the Milky Way. But just what, exactly, are the requirements for life? And what are the environmental extremes that life can handle? A new checklist for the habitability of exoplanets attempts to answer these questions.
Astrobiologists have a really vague and sweeping way of classifying potentially habitable planets: If an exoplanet has a rocky surface, and if it's capable of holding on to liquid water, it's considered potentially habitable.
That's wholly inadequate, which is why NASA space scientist Christopher P. McKay decided to come up with some more specific parameters. Using life on Earth as a model, and taking inspiration from the remarkable way some of this life has adapted to the harshest of conditions, he came up with the following checklist — one that shows, quite encouragingly, just how extreme alien life can get.
Our current conceptions of the origins of life can't help us escape an inalienable fact: Life needs liquid water in order to thrive and survive. Here on Earth, it's the common ecological requirement for life. Consequently, the temperature of an exoplanet is the first parameter that astrobiologists must consider — both because of its influence on liquid water and because it can be directly estimated from orbital and climate models of exoplanetary systems. (Image: NASA.)
According to McKay, life can grow and reproduce at temperatures as low as -15 °C (-5 °F) and as high as 122 °C (250 °F). That is a very generous — and encouraging — range. And to ensure the proper state of water, exoplanets need to have a pressure greater than ~0.01 atmospheres.
The Availability of Water
But life also requires a minimum amount of liquid water to survive. As demonstrated in Earth's deserts, life needs very little water. McKay points out that photosynthetic cyanobacteria and lichens are capable of surviving some of the most water-deprived conditions; some of them live beneath translucent rocks in deserts, surviving on as little as a few days of rain or fog each year.
"You don't need a Pacific Ocean," notes McKay in a New Scientist article. "A planet like the fictional world in Dune would be habitable, though you may not have sand worms."
Consequently, McKay says that water availability only needs to be a few days per year of rain, fog, snow, or a relative humidity greater than ~80%. Again, very generous parameters.
Access to Energy
Energy for life can come from the heat generated by geothermal processes or light from an exoplanet's central star. But McKay says that life based on geothermally derived chemical energy would, by virtue of the scarce and low energy output, always remain small and globally insignificant. As for the minimum amount of light, studies show that life can thrive off remarkably low levels of exposure.
In fact, even at the orbit of Pluto, light levels exceed the minimum required levels by a factor of about 100 times! In addition, low luminosity red dwarfs that radiate in the infrared could support photosynthesis using a three- or four-photon mechanism photon instead of the two-photon system used in plants on Earth.
Not Too Much UV and Radiation
We also need to consider those things that are bad for life, like UV and radiation. But the doses that can be tolerated by many microorganisms is, in the words of McKay, "astonishingly high given natural levels of radiation in the environment." Analysis of Deinococcus radiodurans, a well-studied soil heterotroph, shows that life can evolve some rather extreme tolerances. What's more, exoplanets do not require a magnetic field to the habitable. McKay writes:
Any plausible field would not deflect cosmic rays because these particles are much too energetic. These particles are primarily stopped by the mass of the atmosphere or surface materials.
So again, good news in terms of broad-ranged habitability.
Life also requires a source of nitrogen; after carbon, it's the most important element needed for life. Studies have shown that microorganisms require a minimum of 1–5 × 10−3 atmospheres N2 for fixation. A number of energetic processes can convert N2 to nitrate (even in CO2 atmospheres), like aurorae, lightning, and volcanoes.
And course there's oxygen to consider. Multicellular life on Earth is dependant upon it — and in fact, the rise of complex life on Earth has been correlated with the rise of oxygen levels. Simple organisms don't require a lot, but if we're going to find complex life, McKay says that we'll need to look at environments with 0.01 atmospheres of oxygen. Also, planets with this concentration could indicate the presence of photosynthetic life.
From McKay's article, which now appears in PNAS:
Our understanding of life on exoplanets and exomoons must be based on what we know about life on Earth. Liquid water is the common ecological requirement for Earth life. Temperature on an exoplanet is the first parameter to consider both because of its influence on liquid water and because it can be directly estimated from orbital and climate models of exoplanetary systems. Life needs some water, but deserts show that even a little can be enough. Only a small amount of light from the central star is required to provide for photosynthesis. Some nitrogen must be present for life and the presence of oxygen would be a good indicator of photosynthesis and possibly complex life.
All this said, there is one place in our solar system that could change this list. From New Scientist:
Saturn's 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. "Titan is a little reminder that there are perhaps more things in heaven and Earth than we can imagine, as Hamlet said. It's a cautionary tale," says McKay. "If we discover something new, we will have to rewrite this chapter."
Read the entire study at PNAS: "Requirements and limits for life in the context of exoplanets."