Back when astrogeophysicist Christopher McKay got his doctorate in 1982, the hunt for extraterrestrial life was confined to the solar system. The obvious places to look were the planets and moons that seemed most likely to be habitable: Mars, two moons of Saturn (Enceladus and Titan), and a moon of Jupiter called Europa.

That started to change in 1995, says McKay, a senior scientist at the NASA Ames Research Center, when astronomers began finding planets orbiting distant stars. These so-called exoplanets now number nearly 1,800, with one of the most Earth-like, the planet Kepler 186-f that orbits a red dwarf star known as Kepler 186, announced just this past April.

In a recent issue of the Proceedings of the National Academy of Sciences, McKay summarizes how the search for habitable planets needs to go beyond simply looking in the "Goldilocks zone"—the orbital distance where it's not too hot, not too cold, but just right for biology.

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National Geographic spoke with McKay about how scientists can tell if an exoplanet is likely to be habitable, based on what is known about the range of environments that can support life on Earth.

In your PNAS paper, you talk about a "checklist for speculating on the possibilities of life on these distant worlds." What's on the list?

The first thing is temperature [i.e., temperature allowing for water in liquid form]. The astronomers know this—it's what defines the "habitable zone." But the next question you need to ask is whether water is actually present.

How do you determine whether a planet not only can have water, but does have water?

We need to have some measurement of the atmosphere to confirm that this isn't a planet that's lost all its water. You don't need much: One of the lessons of life on Earth is that a little water goes a long way. It's nice to have a Pacific Ocean, but you don't need it.

Once you know there's still some water on the planet, what do you need to know next?

Energy sources. Life on Earth uses only two types of energy for metabolism: sunlight and redox chemistry ... One or the other has to be present, and if you're in the habitable zone of a star, you at least have enough light to support photosynthesis.

So you have the right temperature, water, and sunlight. What else do you need?

The next criterion is sort of a negative: Make sure there's nothing that will kill you, such as radiation.

That could be a real problem with a planet like Kepler 186-f, right? It orbits a red dwarf star, and those tend to have a lot of solar flares.

It's true that we humans are cream puffs when it comes to radiation. You know, a little excess sunlight and we get sunburns and skin cancer. But microbes, which are likely to be the first life-forms we find, are much, much tougher with respect to both UV and ionizing radiation.

You also list nitrogen as essential for habitability.

Yes, because life is almost certainly going to use amino acids, and it needs nitrogen to build them. So that's a key requirement.

OK, temperature, water, sunlight, nitrogen, and nothing that will kill off life. Anything else?

Yes: oxygen. It's not evidence of life directly—it's not the same as seeing the life-forms themselves. It's like seeing tire tracks when you're lost in the desert. It's not the car. It's not proof you're about to be rescued. But it's certainly damn interesting. And if the oxygen level is high enough, our experience on Earth leads us to suggest that that should enable complex life, plants and animals. And that's very cool.

So can we go out and search today for the things on your list?

Well, not with any [equipment] that's in space right now. None of the telescopes on the ground right now will do it either, I don't think. But there's no reason why everything on this list couldn't be checked off for Kepler 186-f and other Earth-size planets in the habitable zones of their stars within the next decade.

How about Mars? Is it even still worth looking for life there?

I have to admit my hope is dimming with the results that have come back from Curiosity and other missions. But it's not gone yet, partly because Mars is so close by that it [would] take a lot of negativity before I give up. It's like searching for your keys under the lamppost—you look there because that's where the light is good.

Even if we find life on Mars, though, there's another problem: The first assumption I would make is that, yeah, that's life, but it's directly related to us [because Earth and Mars are so close, any life found on Mars might have originated on Earth and been carried over on a meteorite—or vice versa, that life on Earth might have been carried over from Mars]. You'd have to prove that it isn't. If we find it 500 light-years away, on the other hand, we know it's not related.

You're still working on the search for life inside the solar system, though.

Yes, I'm working on a Europa mission concept, I'm working on an Enceladus mission concept, and I'm working on some Titan mission concepts. I'm working on Mars data coming back right now, and I'm working on future Mars missions. And now I've got a student who's going to be looking at Kepler 186-f, too. So I'm involved with all five of those worlds. I'm like a parent with many children. I love them all, and I resist saying that one is better than the other.