Kepler telescope embarks on quest for other Earths

WASHINGTON — How many Earths are out there?

That question is the driving force behind NASA's new space telescope, Kepler, which was launched from Cape Canaveral late Friday on what could prove to be a historic mission.

As it circles the sun, Kepler will aim itself at the constellation Cygnus, where it will unblinkingly observe a patch of sky roughly equal to the size of an outstretched hand. In that region of our galaxy are about 4 million stars. Kepler will pay close attention over several years to about 100,000 of them.

The telescope will look for dimming of starlight that might be caused by a planet passing in front of the star. This requires extraordinary sensitivity: The occultation of the light would be something like one part in 10,000 (imagine a 10,000-watt light bulb periodically dimming to 9,999 watts).

The dimming of the starlight will reveal the diameter of the transiting object. The telescope would ideally observe the object at least three times to get a confirmed measurement of its orbital period. The instrument is named for the German astronomer Johannes Kepler, who in 1609 published the laws of planetary motion, which define the relationship between the orbital period of a planet and its distance from the sun.

The detection would require a good bit of luck. Scientists estimate that 199 out of 200 hypothetical planets won't be in an orbital plane that aligns them so neatly with Kepler's watchful eye. For that reason, the telescope plays the odds and studies a relatively broad area of the sky. If all of those 100,000 stars have Earth-like planets, Kepler might find as many as 500 over the course of its 3 1/2-year mission.

But it might find none. The null result would surely incite a great deal of debate and, perhaps, consternation.

"If we find hundreds, we know Earths are plentiful and everywhere, and if we find very few or even none, it means that Earth is very, very rare," said Jon Morse, head of astrophysics at NASA.

He noted, however, that there are reasons to believe that rocky, Earth-like planets are common. Since 1995, astronomers have found more than 300 planets beyond our solar system. Almost all of them are gas giants like Jupiter, many of them in tight orbits around their stars. But this is known to be a skewed census of what's out there, because the technology of planet-hunting is most sensitive to the biggest planets in the closest, hottest orbits.