After a series of rocket burns, Kepler separated from its booster engine shortly before 9 p.m. PT, prompting a round of applause in Mission Control.
The Kepler Telescope won't be able to detect any advanced civilizations that might exist. But hopefully it will tell us how common "Planet Earths" are in our galaxy. In the past few years, astronomers have found evidence of more than 300 planets orbiting distant stars, most of them much larger than Earth, and more like Jupiter than our habitable world. Some are far too hot to sustain life. Others are frozen. Some are in very close, fast orbits around their parent stars, with a "year" equaling just a few of our days.
A variety of techniques have been used to pick up telltale signs of these planets. Using the Doppler Effect (similar to a train whistle that seems to change pitch as it moves past you at a railroad crossing), astronomers have been able to detect "wobbling stars." The wobble is caused by the interaction of gravity between a star and an orbiting planet. This reveals itself as a tiny shift in the star's light spectrum. Using this information, the size of the planet and distance from its sun can be calculated.
The results so far are encouraging, indicating that planets are common in the galaxy. But finding an Earth-sized world similar to our own requires a sensitive instrument placed above the Earth's atmosphere.
The French-European COROT Telescope, launched in 2006, is already on the job, and may be first to confirm a potentially habitable Earthlike world. COROT has already found a number of "hot super Earths." The smallest of these is less than twice the size of Earth, but too hot to sustain life.
Kepler has far greater capabilities than COROT. The method that Kepler will use can be described with a simple example: If somebody walks past a window in your house, in daylight, the amount of light entering the room will be decreased briefly. Using the same reasoning, when a planet moves in front of its sun, as seen from Earth, the starlight, measured by sensitive instruments, will briefly be reduced (for two to 16 hours). Kepler will be able to detect the change in brightness from these "transits," down to 20 parts per million.
There are, however, other circumstances that can cause such a dip. So it is necessary to keep watching that star, patiently waiting for the same event to happen again. If it does, then a prediction can be made about when a third occurrence will take place. NASA says four transits are necessary to indicate a planet. Then, a ground telescope must make the final confirmation, looking for the "wobble effect" on the parent star.
Much information can be deduced, including the planet's distance from its sun, its size, mass, and temperature.
Kepler will aim its unblinking eye continuously at 100,000 stars that are thought to be likely candidates for "planet Earths," those circling in a habitable zone around their suns. The mission is scheduled to last three and a half years, long enough to confirm the presence of planets that orbit their stars around once per Earth-year. Kepler is designed to answer the question, How common are Earthlike planets in the galaxy? But even though there are 100,000 possibilities, nobody knows if Kepler will find even one "Planet Earth." The mission (constrained by budget limitations) may not be long enough to make any significant discoveries. There is an option, however, to extend it to six years.
A planet must orbit its star at the proper angle so that it temporarily obscures the light of its sun as seen from Earth. NASA calculates the odds on this happening at 1 half of 1 percent, for an Earthlike planet orbiting a star like our own.
NASA has also crunched the numbers on the probability of finding Earthlike planets. NASA expects to turn up 50 of them, assuming they are in one-year orbits and around the same size as Earth. The number goes up to 640 if most are approximately 2.2 times the size of Earth. It's almost a foregone conclusion that Kepler will also detect many larger planets inhospitable to life.
There is also an intriguing possibility. Ground-based telescopes may be able to tell us eventually whether any Earthlike worlds contain oxygen or methane in their atmospheres, telltale signs of possible life.
But even if no "Planet Earths" are found by Kepler, the information will be useful. It will tell scientists that Earthlike worlds are rarer than they thought, and a more ambitious search will be needed in the future to find them.
QUICK FACTS ABOUT THE KEPLER MISSION
MISSION NAMED FOR: Johannes Kepler (1571-1630), who formulated the basic laws of planetary motion. This year marks the 400th anniversary of Kepler's discoveries.
MISSION COST: Approx. $600 million
SCHEDULED LAUNCH: March 6 at 10:49 p.m. ET, 7:49 p.m. PT
DETECTOR: Photometer (advanced version of your own digital camera!) with 0.95-meter aperture, 95 megapixels
PRIMARY MIRROR: 1.4-meter diameter
ORBIT: Kepler will not be in an Earth orbit, but an Earth-trailing orbit, circling the sun so that the Earth does not obscure its view.
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