Kepler Looks for Earths

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.
Whenever the technology improves and the detection of smaller objects becomes possible, they do, in fact, pop out of the data. For example, a European space telescope, Corot, recently spotted an object in the general size range of Earth. It is not "Earth-like," however, because it is very close to the star and completes an orbit in 20.5 hours.
The $572 million Kepler mission seeks to find Earth-size planets in Earth-like orbits -- the habitable, or "Goldilocks," zone that's just right for life -- around sunlike stars. No such planet has been seen to date.
The Kepler telescope, first proposed in 1992, failed on three occasions to get the green light from NASA because of concerns that the instruments were not sufficiently sensitive to detect something as small as a starlight-dimming, Earth-like planet, Edward J. Weiler, associate administrator for NASA's Science Mission Directorate, said in a telephone briefing yesterday.
But Weiler said patience paid off. The technology improved. In addition to its main mirror and lens, Kepler is equipped with a digital camera capable of obtaining a 95 million-pixel image.
Any planets identified by Kepler will be studied more closely by ground-based telescopes or the James Webb Space Telescope, scheduled to be launched in 2013. Those telescopes may be able to tweeze from the starlight information about the atmospheres of the planets. Detecting such gases as oxygen, carbon dioxide, water vapor or methane in an extraterrestrial atmosphere would be highly suggestive of the presence of life.
It's also possible that Kepler will discover things that no one has imagined.
"Every time we've made a prediction in terms of what we would find with planets, we were wrong," William Borucki, lead scientist on the Kepler mission, said in an interview last year. "We do not know whether we will see many, many Earths, or none at all, or something in between."
But Weiler made a bold prediction yesterday.
"It'll either be our kids or our kids' kids, but in this century humans will prove for the first time that life exists somewhere else in the universe," he said. "I'm not claiming they'll be Klingons or Romulans" -- to name some "Star Trek" characters -- "but plant life at least."
The successful launch of Kepler provided good news for NASA in the wake of the recent failure of an Earth-observing satellite called the Orbiting Carbon Observatory. It crashed into the Indian Ocean because of a malfunction shortly after launch.
"After the tragic loss of OCO just a week earlier, this was really important to get another success under our belts," Weiler said.