The most recent Kepler news release from February 28 reveals that the total count of Kepler planet candidates has reached 2321 and 1790 host stars, with 1091 planets emerging in the new analysis. The headline of the findings is profound: "A clear trend toward smaller planets at longer orbital periods is evident with each new catalog release. This suggests that Earth-size planets in the habitable zone are forthcoming if, indeed, such planets are abundant."
Ten planets in the habitable zone (out of a total of 46 planet candidates there) are near Earth in size, and the fraction of host stars with multiple candidates has grown from 17 to 20 percent.
In a March 8th analysis in Space Daily, John Rehing suggests that current Kepler findings might prove that Earthlike planets may be extremely rare. But 16 months of observation is insufficient to detect any precisely Earthlike planet, because the ground rule that only those earth-sized candidates with three transits observed means that a minimum of 24 months of observation will be required.
However, as new data comes in, Rehling says the barriers enforced by the geometric bias are pushed outward, and as more candidates are reported, more terrestrial worlds like the Earth, rather than giants like Jupiter, are revealed.
This release shows two favorable and profound trends. As the Kepler Mission team put it, "With each new catalog release a clear progression toward smaller planets at longer orbital periods is emerging. This suggests that Earth-size planets in the habitable zone are forthcoming if, indeed, such planets are abundant."
However, Rehling notes that the fine print of this latest release tend toward more pessimistic projections.Mainly, "We see more Earth sized planets which are very close to their stars, and therefore likely very hot; and, separately, we see more giant planets which are located farther out from their stars."
Overall, our solar system is typical in placing larger planets farther out than smaller planets. However, it is quantitatively atypical according to Rehling: "While Kepler shows us the happy result that there are almost certainly several planets for every star, it shows us that our solar system is distributed freakishly outwards, in comparison to more typical planetary systems."
The data also indicates that as Kepler's mission continues, it may not find precise Earth analogues, although this will depend in part upon luck. The worst ramification is that most Kepler candidates are located quite far from Earth, making possible follow-up science with spectroscopy and imaging extremely challenging.
We would be better able to make observations of earth-like planets, Rehling concludes, located closer to us, "at distances of tens of light years instead of hundreds. But if the abundance of earthlike planets is only a few percent, there will be comparatively fewer of these worlds in our neighborhood."
Any future effort to find and examine earthl-like planets in our corner of the Milky Way will be limited by the frequency of such planets, and this result serves to dim prospects somewhat, or to require considerably larger and more expensive telescopes than would be needed if the more optimistic projections proved out.
Out of the 156,000 stars being monitored by Kepler, we are effectively searching only 27 for perfect Earth analogues: If they are less abundant than 3%, we may very likely find none, unless the Kepler's mission is extended to allow several years more of data collection.
"I think the discoveries we're making are showing what could be done if we continue to extend it," said Charlie Sobeck, Kepler deputy project manager at NASA's Ames Research Center in Moffett Field, Calif. "So we're hopeful, but there's no guarantee."
When Kepler launched in 2009, the telescope's science mission was set to run through November 2012 — a lifetime of 3.5 years. But the instrument could operate for six years, or perhaps longer, if it receives more funding, team members have said. Its mission is to find roughly Earth-size planets in or near the habitable zones of their parent stars — a just-right range of distances that could support liquid water and, perhaps, life as we know it on the alien worlds.Kepler's overall goal is to help scientists determine just how common such planets may be throughout our galaxy.
It would cost about $20 million per year to keep the Kepler mission running at its current level of activity beyond November 2012, Sobeck added.
Kepler finds alien planets using what's called the transit method. The telescope detects the telltale dips in brightness caused when an alien planet crosses in front of, or transits, its star from Kepler's perspective. Kepler needs to witness three of these transits to firmly identify a planet candidate.
This technique has been extremely effective. In just its first four months of operation, Kepler discovered 1,235 exoplanet candidates. So far, two dozen of them have been confirmed by follow-up observations — including Kepler-16b, a world with two suns that was announced recently.Kepler team members have estimated that 80 percent or so of the telescope's candidates will probably end up being the real deal. If that's the case, Kepler's finds to date would more than double the number of known alien planets.
The Kepler main mission is to help scientists determine just how many potentially habitable, Earth-size alien planets may be out there. Of the first 1,235 planet candidates, 68 are roughly Earth-size and 54 appear to orbit in their stars' habitable zones. And five candidates meet both of those criteria.
"What we're seeing is this trend — the smaller the planet, the more of them there are," Sobeck told Space.com. "That's great news for the idea of finding Earth-like planets, or Earth-size planets. Once you have Earth-size planets, all it has to do is be in the right orbit, and it's habitable."
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