In search of ET

Astronomical advances mean that over the next decades we may discover if other life exists
December 15, 2010
Out of this world: an artist’s impression of an exoplanet based on data from Nasa

Are we alone in the cosmos? Evidence for any extraterrestrial organisms, even mere bugs or bacteria, would be of huge scientific importance. But what fuels popular imagination is the prospect of advanced life—the “aliens” familiar from science fiction. Speculations about other worlds are nothing new. Great astronomers such as Johannes Kepler in the 17th century and William Herschel a century later imagined that the Moon and planets might be inhabited. In the late 19th century, Jules Verne and HG Wells further popularised such ideas. But the space age damped such speculations. The prime target, Mars, was revealed as a frigid desert with a thin atmosphere. There could be vestigial life there, but nobody expects a complex biosphere anywhere in the solar system.

Yet prospects are brighter if we widen our gaze. We know that many other stars have a retinue of planets circling them, just as the Earth, Mars and Jupiter circle around our own star, the Sun. These “extra-solar” planets, called exoplanets, have mainly been inferred indirectly, by detecting the wobble that their gravitational pull induces in their parent star. Several hundred nearby stars are already known to have planets. One star has at least five. But this evidence pertains mainly to “giant” planets the size of Saturn or Jupiter. Earthlike planets—hundreds of times smaller than Jupiter—cannot be detected this way.

But there’s another technique. A star would appear to dim slightly if it had a planet that transited in front of it, blocking out a fraction of the light reaching us. (A classic phenomenon in our solar system is a transit of Venus, when Venus appears as a dark spot that moves across the face of the Sun.) The Kepler spacecraft, launched last year by Nasa, carries a telescope that has been pointing at the same patch of sky for 18 months. It monitors the brightness of 100,000 stars—and does this every half hour with a precision better than 0.01 per cent. The signature of an orbiting planet would be a slight diminution in a star’s brightness, lasting for a few hours, which repeated regularly. An announcement is expected in February that Kepler has enough data to tell how common such planets are. We’ll be especially interested in planets like ours, orbiting other Sun-like stars at a distance such that water, if present, neither boils nor stays frozen.

But we’d like to see planets directly—not just their shadows. And that’s hard. Viewed from an interstellar distance, our Earth would appear, as the astronomer Carl Sagan described it, as a “pale blue dot” very close to a star (our Sun) that outshines it by a factor of billions. The shade of blue would change, depending on whether the Pacific ocean or the Eurasian land mass was facing the observer. The task is like looking for a firefly next to a searchlight from thousands of miles away. It will be 20 years before sufficiently powerful telescopes are deployed, on the ground or in space. But once such planets can be detected, a lot can be learned about them—whether they have continents and oceans, the length of their “day,” their climate.

Should we expect life on any extra-solar planets? We know far too little to lay confident odds. Indeed, the origin of life on Earth remains a mystery. We don’t know what led from amino acids to the first replicating systems, and to the intricate protein chemistry of monocellular life. The crucial step may have involved a fluke so rare that it happened only once in the galaxy. Or the process might be inevitable given the “right” environment.

The cosmos could teem with life; on the other hand, our Earth could be unique among the billions of planets that surely exist. And even if “simple” life were widespread, advanced life might be rare. Astronomical advances in the coming decades will render the night sky vastly more interesting: each star will be perceived not as a featureless point of light, but as the hub of a planetary system whose key properties we’ll know. And we may learn something even more fascinating: whether biological evolution is unique to the “pale blue dot” that is our home, or whether Darwin’s writ runs through a wider universe. Were our biosphere unique it would disappoint some, but in compensation, we need be less “cosmically modest”—our Earth, tiny though it is in a galactic perspective, could be uniquely significant.