Kepler Telescope to Set Earth-Like Planet Count Straight

The scoop: NASA's Kepler telescope is slated to pinpoint Earthly exoplanets zooming around distant stars, but what, exactly, is the point? Seth Shostak of the Search for Extraterrestrial Life (SETI) institute gives us the low-down.

For planets, just as for headaches, bigger isn't always better.

Large worlds tend to be unappealing gas balls, swathed in thick, noxious atmospheres of methane, ammonia and other ingredients that -- while useful in the kitchen -- are not the kind of stuff you'd really like to breathe in. Consider Jupiter and Saturn: they're lovely to look at, but awful places to live.

We assume that if there's other life out there, it's probably hunkered down on a small, rocky world like Earth, orbiting its star at a distance that allows liquid oceans to ebb and flow on its surface; in other words, Earth-like worlds. But until now, finding such bantam planets was beyond the capabilities of our telescopes.

The reason? Simple: Nearly all of the approximately 350 planets discovered in the past dozen years were found by measuring slight wobbles of their home stars. Unfortunately, a terrestrial-sized world hardly budges its stellar master. For example, the Earth provokes the Sun into wobbling at roughly the speed of an ant's walk. It's not easy to find such a subtle sashay.

NASA's new Kepler telescope, however, will circumvent this problem by hunting for planets in an entirely different fashion. It's set to search for an ever-so-slight dimming of stars that occurs when planets cross in front of them. Since this happens infrequently, the NASA spacecraft will stare at about 100,000 stars for nearly four years, waiting patiently for these tiny eclipses. Thanks to its space-based perch, Kepler can measure stellar dimming as small as 0.002 percent, leading its builders to hope that it will find as many as 50 Earth-like worlds -- that is to say, planets approximately the same size as Earth -- waltzing about their stars at roughly the same orbital distance as our home planet does.

The real importance of Kepler is not the many new planets it will trawl from the velvet voids of space. Rather, it's the fact that it will give us data on the frequency of planets. After Kepler's four-year stint in the ultimate staring contest, we can make a informed guess about which fraction of stars have planets that are both Earth-sized and Earth-like.

The optimists are betting that the fraction will be large -- one-fourth, or maybe even one-half. That would bode well for cosmic company, because it would indicate the presence of 100 billion Earth-like planets in our Milky Way galaxy alone. Of course, the optimists could be wrong. Indeed, it's conceivable that Kepler might strike out, finding absolutely no terrestrial-type worlds. If that unexpected scenario plays out, we'll be able to say that fewer than 5 percent of all Sun-like stars have Earth-like planets.

That would be a sobering result, but that's what science is all about: finding the facts, rather than sitting around endlessly arguing about what might be true.

So it boils down to this: your ancestors undoubtedly beheld the night sky and wondered how prevalent Earth-like worlds might be. But wondering is all they could do. In just a few years' time, thanks to Kepler, you will know.

Seth Shostak is a senior astronomer at the SETI institute. The views expressed are the author's alone and do not represent the official position of the Discovery Channel.

Article posted March 2, 2009.

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