Twenty-two light years away in the direction of the constellation Scorpius lies a system of three stars known as Gliese 667. Today, scientists have announced that a close examination of one of those stars, Gliese 667C, has revealed a solar system simply loaded with planets, with six or possibly seven planets, three of which orbit within the star's habitable zone -- the not-too-close-not-too-far region that could potentially sustain liquid water.
In April, NASA announced the discovery of two planets in the habitable zone of another star, Kepler 62, but Gliese 667C is the first star known to tally up three. The planets are what's known as "super Earths" -- larger than our own planet, but nowhere near the size of even the smaller gas giants (Neptune and Uranus) of our own system.
A press release from the European Southern Observatory (ESO) said, "The new planets completely fill up the habitable zone of Gliese 667C, as there are no more stable orbits in which a planet could exist at the right distance to it. " The habitable zone of Gliese 667C is much smaller and closer in than our own star's, because Gliese 667C is a cooler star. A video that the ESO released shows what the orbits look like. You'll see that all but one are quite close to the star -- the ESO has drawn in where Mercury's orbit would be, relative to our own star, for scale (and Mercury has the smallest orbit of all the planets in our solar system, so this really gives an idea of just how tight these orbits are).
With each discovery of more and more exoplanets (perhaps as many as 17 billion Earth-sized ones in our galaxy alone), it feels like we are getting closer and closer to answering The Big Question: whether or not we're alone in the universe. Sara Seager, an astrobiologist at MIT, recently proposed an equation to buffer Drake's, which attempts to calculate the probability that there's intelligent life out there. Seager's equation takes a slightly different turn, asking not what the likelihood of intelligent life is, but what the likelihood is that there is detectable life out there (life of any kind, not necessarily intelligent).
That is the moment we live in -- one in which we can begin to ask not whether something is out there, but whether we might actually be able to find it. With the launch of the James Webb Space Telescope in 2018, we'll have a chance to look for "detectable biosignature gases" in the atmospheres of identified exoplanets, and that's when things will really start to get interesting.
Do more planets mean that life is out there? We don't yet know, and the reason we don't yet know is because we don't know the probability that life begins at all, even given the right conditions. But, because we apparently have an abundance of laboratories in our galaxy (some 17 billion), that should at some point turn out to be a knowable thing -- a point made beautifully by astrobiologist Caleb Scharf in Aeon recently. Scharf writes:
The special thing about our planet-rich universe is that it's tuned both for life and for finding out about life. If we were [in an] imaginary universe with only one planetary system, we would have no way of learning how frequently life arises. We live in a universe that allows us to get some measure of our own significance. There is nothing in our present understanding of the nature of life or the universe that says this was absolutely necessary, yet here it is.
It's not clear that we'll ever be able to solve this puzzle. To start with, we need to build that equation for abiogenesis, we have to dig deeper in the astrophysical dirt to find places in the universe that might harbour life, to follow nature's breadcrumbs, as it were. The strategy is straightforward: seek out more worlds that might share some of Earth's characteristics, and search their surfaces for the chemical signatures of life. It won't be easy but, unlike 20 years ago, we now know we have a galaxy's worth of planets to chase, and we know that if we persevere, the equation will eventually come into focus.
That this work is even possible has, in a very real sense, already changed our universe. Not because it's told us anything quantitatively new about life elsewhere, but rather because it's raised the stakes for evaluating our significance, our cosmic loneliness, to a whole new level. Not only are the fundamental properties of our universe aligned, and tuned to, the needs of life, they also promise success in our quest to discover life's frequency, origins, and perhaps the very causes of this tuning itself. And it didn't have to be like this at all. If you turn it over in your mind enough times, you realise that Albert Einstein was right when he said: 'The most incomprehensible thing about the universe is that it is comprehensible.'