Author: Seth Shostak
The Kepler Mission has a simple objective: find small planets around other stars - worlds like Earth that could possibly spawn life. It promises to be one of the most exciting astronomy projects of the coming decade.
In the last half-dozen years, astronomers have made a remarkable discovery: roughly 5 - 10% of stars similar to the Sun have planets. They have observed this fraction of stars to periodically wobble in response to the orbital motion of unseen worlds. However, every one of the planets detected by this method is hefty, typically the size of Jupiter or larger. This preponderance of massive worlds is an inevitable consequence of the detection scheme: only large planets induce measurable wobbles.
Giant planets are fine, but the holy grail of planet detection is to find Earth-sized worlds and, better still, find those that are in the "habitable zone" of their star - at distances that would allow oceans to exist. The Kepler Mission will be able to do that, not by searching for wobbles, but by very accurately measuring the light from stars.
"It's the first step in man's quest to find life elsewhere,"
As astronomy buffs are well aware, every few years we see Mercury or Venus cross the Sun's disk. During these transits, the planet blocks some of the light from the Sun - not much, mind you: typically 100 parts per million for Venus and less for diminutive Mercury. The Kepler telescope, with its 0.95 meter Schmidt optics and massive CCD detector array (sporting nearly 100 million pixels) will look for such transits around other stars. The amount of blocked light, and the length of the transit (typically many hours), will clue astronomers to both the size and orbit of any detected worlds.
Of course, to see a transit requires two things: (1) the orbit of the planet needs to be correctly lined up with the viewer, and (2) the observation needs to occur during the transit! Since we don't know anything about the sought-for planets' orbits, the researchers are arranging for the Kepler telescope to stake out a whole slew of stars, and monitor their individual brightness continuously for four years. In addition, careful attention to design means that brightness variations as small as 20 parts per million will be noted by Kepler.* Earth-bound telescopes - which perforce must peer through our churning atmosphere - could never achieve such accuracy. Kepler will be a space-based instrument. The telescope will be blasted into a heliocentric orbit sometime in 2007, to follow Earth around the Sun and keep its well-calibrated eye on a 100 square degree chunk of sky in the constellation of Cygnus. In that large patch of celestial real estate (roughly the same size as two Big Dippers), 100 thousand stars of 14th magnitude or brighter will be watched. It's a $300 million dollar project recently approved as a NASA Discovery Mission.
"You might think that the Hubble Telescope could do this," notes Dave Koch, an astrophysicist who's been working on Kepler preparations for nearly a decade. "But it can't. Hubble could only measure brightness variations to a part in a hundred - far too poor to find transits - and besides, even its wide-field cameras only image tiny patches of sky. And I haven't even mentioned the fact that a lot of astronomers would get upset if you bumped all other observing off Hubble for four years!"
What will Kepler find? That's the exciting part, because we don't know. Borucki has estimated that if Earth-sized planets are common, the mission will uncover roughly 50 of them in orbits comparable to our own. Thousands of closer-in and/or larger planets could be found. Even bulky moons around some of the Jupiter-sized planets discovered with the wobble technique might block enough starlight to make their presence known. All such worlds are potential abodes for life.
Jon Jenkins, a SETI Institute co-investigator for Kepler, is responsible for designing the algorithms and software required to keep tabs on its hit list of 100 thousand stars. He calls Kepler "One of the most exciting projects in which I could imagine being involved." He's been involved with mission preparations since 1995, but it's the future that excites Jenkins.
"The discovery of the New World was accomplished in what? A few decades? Well, the discovery of entirely new worlds around other stars will also take place in that length of time. And it will happen only once. How many Earth-sized planets are there? Finding out how many are in this small patch of celestial real estate will help us answer that incredibly interesting question."