An artist's interpretation of the Kepler observatory in space.
NASA's Kepler spacecraft hunting for Earth-like planets around other stars has found 706 candidates for potential alien worlds while gazing at more than 156,000 stars packed into a single patch of the sky.
If all 706 of these objects pass the stringent follow-up tests to determine if they are actually planets, and not false alarms, they could nearly triple the current number of known extrasolar planets. They were announced as part of a huge release of data from the mission's first 43 days by NASA's Kepler science team this week.
The Kepler space observatory monitors stars for subtle changes in their brightness, which could indicate the presence of alien planets passing in front of them as seen from Earth. Astronomers will use the newly-released data from Kepler to determine if orbiting planets are responsible for the variation in brightness of several hundred stars.
?"This is the most precise, nearly continuous, longest and largest data set of stellar photometry ever," said David Koch, the mission's deputy principal investigator at NASA's Ames Research Center in Moffett Field, Calif., in a statement. "The results will only get better as the duration of the data set grows with time."
By measuring tiny decreases in the brightness of stars when planets cross ? or transit ? in front of them, astronomers can determine the size of the planet. [The strangest alien planets.]
To date, astronomers have discovered more than 400 alien planets lurking around stars beyond our solar system. That includes six newfound worlds discovered by a French observatory that were announced earlier this week.
Zoo of parent stars
Kepler currently monitors a star field in the constellations Cygnus and Lyra. The stars make up a full range of temperatures, sizes and ages. Many of them are stable, but others pulsate.
Some of the stars show starspots, which are similar to sunspots, and a few even produce flares that are so powerful they would sterilize their nearest planets, should any exist.
In this particular star field, Kepler has identified 706 planetary candidates, of which the data for 306 of these were part of the public data release this week.
The 28 members of the Kepler science team are using ground-based telescopes, the Hubble Space Telescope and the Spitzer Space Telescope to perform follow-up observations on a specific set of 400 objects that were not publicly released to double-check if they are good candidates for alien planets.
Data from these follow-up observations will determine which of the objects of interest can be identified as planets. These findings will subsequently be released to the scientific community in February 2011.
Double-checking potential planets
Follow-up observations are necessary in order to distinguish candidates that are actual planets from false alarms, such as binary stars, which are two stars that orbit each other.
"For the most interesting objects, we go through a process of putting the data through a series of sieves," Charles Sobeck, Kepler's deputy project manager, told SPACE.com. "For final candidates that have passed all the tests, we then go to the expensive resources like Hubble and Spitzer."
The size of planetary candidates can also only be approximated until the size of the stars they orbit is determined from additional spectroscopic observations made by ground-based telescopes.
"I look forward to the scientific community analyzing the data and announcing new exoplanet results in the coming months," said Lia LaPiana, Kepler's program executive at NASA Headquarters in Washington, D.C., in a statement.
Search goes on
The Kepler observatory will continue conducting science operations until at least November 2012. It will also continue searching for Earth-like planets, including those that orbit stars in a warm, habitable zone where liquid water could exist on the surface of alien planets. ?
And, since transits of planets within this habitable zone of solar-like stars occur about once a year and require three transits for verification, it is expected to take at least three years to locate and verify any potential Earth-size planet.
"The Kepler observations will tell us whether there are many stars with planets that could harbor life, or whether we might be alone in our galaxy," said Kepler's science principal investigator William Borucki of NASA's Ames Research Center.
So far, Kepler's observations have produced a wealth of information, and it has surpassed the expectations of its mission scientists, Borucki said.
"We never thought we'd have this much this early, it's absolutely wonderful," Borucki told SPACE.com. "The instruments are working well, but we still have some work to do. We're certainly not finished with this kind of work, and each year, we go to more and more difficult targets. So, people have to be patient."
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