Q&A: The 5 Ingredients Needed for Life Beyond Earth
A NASA scientist lists the essentials that extraterrestrial life must have to exist.
Newly
discovered exoplanet GJ 504b, illustrated here, has about four times
the mass of Jupiter, making it the planet with the lowest mass directly
imaged around a sunlike star.
Illustration by NASA's Goddard Space Flight Center/S. Wiessinger
Published June 25, 2014
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.
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.
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