Only a few decades back, there were serious scientists who thought that planets might be miraculous. Not miracles like a burning bush or a docile teenager, but highly improbable objects. These researchers figured that the conditions necessary for making small, cold worlds could be rare---perhaps extremely rare. Most stars were believed to live their luminous lives alone, bereft of planetary accompaniment.
Well, those thoughts have been banished. In the last 15 years, hard-working astronomers have found many hundreds of so-called exoplanets around nearby stars, and NASA’s Kepler telescope is set to uncover thousands more. (If you don’t know this already, you’ve probably reached this site by mistake. But you've come this far already, so keep reading.) Kepler's principal task is to find habitable exoplanets---worlds with solid surfaces at the right distance from their host star to sport temperatures amenable to the presence of watery oceans and protective atmospheres---planets that might be very much like Earth (depending on some other factors that are harder to measure from light-years away, like geology and chemistry).
Kepler has already found about five dozen candidate objects that, while somewhat larger than our own, seem to meet these criteria. As this space-based telescope continues to peer into the heavens, more such planets will emerge from the data. Indeed, it seems a good bet to guess that at least a few percent of all stars are blessed with “habitable” worlds. That would tally to billions of life-friendly sites, just in our galaxy. This has already prompted SETI scientists to swing their antennas in the directions Kepler's most promising candidate planets, hoping to pick up the ABCs and MTVs of alien worlds. After all, these systems are arguably the best targets that SETI (the Search for Extraterrestrial Intelligence) has ever had. It’s like discovering a prolific fishing hole.
But there’s a fly in the ointment: While eavesdropping on a small bunch of star systems known to have terrestrial-style worlds is better than taking your chances with random targets, it’s not actually that much better. The reason is simple. The oldest confirmed fossils on Earth are about 3.5 billion years old, and there’s indirect, if sketchy, evidence for life going back 4 billion years. That’s roughly 90 percent of the age of the Earth, which is to say that biology bedecked our planet very early. Life seems to have been an easy chemistry experiment. So that’s yet more encouragement, as it hints that many of those habitable worlds will actually be inhabited. There could be life on billions of planets in the Milky Way. Yet on Earth, it’s only in the last few instants of geologic time that biology has spat out Homosapiens, which is, as far as we know, the first terrestrial species to successfully build a radio transmitter. So it seems that alien SETI researchers---the Klingons and Vulcans and whoever else is out there---could have spun their telescopes in our direction for billions of years without getting any signal (no intelligent life indeed). Yes, they might know that Earth was a kind and gentle world, blessed by air and oceans. And yes, they might have detected the oxygen in our atmosphere, and concluded that our planet has life. But intelligent life? They wouldn’t know, unless they’ve been monitoring the Earth very, very recently. There’s one more thing to consider, namely how long we will continue to broadcast our presence. It’s hard to say, given our apparent vulnerability to self-destruction via advanced weaponry, environmental catastrophe, or some other horror. But let’s give ourselves the benefit of the beneficent doubt and assume that humans hang around a million years---approximately as long as the average species
does. Even if we transmit all that time, there’s still only one chance in 5,000 that someone examining our planet at a random moment in cosmic history will find a sign of intelligence on Earth. I might point out that these depressingly small odds are, in reality, likely to be even smaller. So the bottom line is that Earth-like worlds are not enough to significantly increase SETI’s chances of success until, and unless, we find many, many thousands of them. And that’s a project for the next generation of planet-hunting hardware. Yes, finding habitable exoplanets is tremendously interesting, and a big psychological boon in our quest for cosmic company. But just because we’ve located a few oases in the desert doesn’t mean we’ll soon discover the Bedouins.
Seth Shostak is Senior Astronomer at the SETI Institute in California, and the host of the weekly radio show and podcast, "Big Picture Science."