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Seth Shostak

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Last Chance to Be Special?

Posted: 12/ 5/2011 2:39 pm

Scientists connected with NASA's Kepler space telescope have announced the discovery of a planet orbiting a Sun-like star that may resemble our own world. Its name is Kepler-22b.

No, it's not Earth's identical twin -- it's not a doppelganger of our planetary home. Kepler-22b is somewhat more than twice the diameter of Earth. But it orbits in the so-called "habitable zone" of its home star, where temperatures will be to biology's liking. Of the many candidate worlds uncovered by Kepler, this is the one with the best shot for having a thick atmosphere and a landscape lacquered by watery oceans.

Cutting to the chase, of all the many new planets found in the last 16 years, Kepler-22b may be the most likely to have inhabitants. And if it is, prepare to ratchet down your self-image.

Despite the non-stop presence of mostly mindless aliens on television and in the movies, real extraterrestrials remain elusive. We've yet to uncover convincing evidence of any biology on worlds beyond our own. That includes furrow-faced Klingons, but also less imposing, bacteria-style microbes. As of today, you -- and the flora and fauna that veneer the Earth -- are the only known life in the cosmos. You are incredibly exceptional.

But Kepler's relentless discoveries could ensure that your generation is the last one making that statement with a straight face.

Two decades ago, astronomers were uncertain whether solar systems -- small fraternities of planets and moons -- were common hangers-on to stars. But data from the first 18 months of Kepler telescope operations have turned up 2,200 candidate detections -- indications of possible planets. Not all of these candidates will prove to be real planets, but most will. That includes Kepler-22b.

Discovering thousands of planets sounds impressive. But it's only a prologue to a far larger story. Extrapolating the results from searches of the past 16 years, it's safe to say that the vast majority of stars are ringed by planets. Indeed, the best guess is that the tally of planets in our own galaxy is approximately a trillion.

Kepler results also suggest that most of these plentiful orbs are small -- not the bloated, Jupiter-like objects that were so often bagged by planet hunters in the past. Of the Kepler candidates reported so far, roughly 40 percent are worlds no more than twice the diameter of Earth. Because of the way in which the space-based telescope collects its evidence, that percentage will surely grow with time. Small, rocky planets -- the type most interesting to scientists looking for life -- rule the cosmic roost.

The fraction of these newly discovered worlds thought to be habitable is modest -- perhaps a few out of every hundred. The percentage of worlds that are similar to our own is smaller still. Nonetheless, it's already reasonable to imagine that Kepler-22b has a billion siblings in our galaxy: a billion other Earth-like worlds threading the vast tracts of the Milky Way.

Of course, location is not everything. A habitable planet -- even one that's similar to Earth -- is not necessarily inhabited, any more than a city filled with bars is necessarily fraught with fist fights. But the smart money would say that it is. Similarly, worlds where life could spring up are obviously abundant. So the contention that biology is also abundant is hardly rash.

In light of these results, it's difficult to imagine that Earthly life is the only life. That would be more than bragging; it would be tantamount to declaring our planet a miracle.

To settle these matters, scientists are keen to scrutinize Kepler-22b (and other habitable worlds) more carefully, looking for such telltale biological markers as atmospheric oxygen or methane. Unfortunately, the sorts of space-based telescopes that could uncover signs of metabolism elsewhere are biding their time in development labs, not in space.

But those who seek signs of intelligent life aren't waiting. A preliminary search for radio signals from Kepler-22b using the Allen Telescope Array in Northern California is already underway. So far, they've heard only silence. But it's early days: There are billions of radio channels still to be examined.

So here's how it is: As of now, we're still special. We can still claim to be the crown of creation and the only living entities in a boundless universe. But Kepler-22b is one more step down a road that may soon take us to a new place, and a new mind-set. If the continuing search for biology beyond Earth bears fruit, our descendants will be forever different: no longer extraordinarily special, but also not hauntingly alone.