The Kepler catalog database now holds over 200 Earth-size planet
candidates and over 900 that are smaller than twice the Earth’s
size, which makes for a 197 percent increase in this type of planet
candidates, with planets larger than 2 Earth radii increasing at
about 52 percent.
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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