This story was updated at 12:15 p.m. ET.
MOUNTAIN VIEW, Calif. — NASA's planet-hunting Kepl
er spacecraft has confirmed the discovery of its f
irst alien world in its host star's habitable zone
— that just-right range of distances that could a
llow liquid water to exist — and found more than 1
,000 new explanet candidates, researchers announce
d today (Dec. 5).
The new finds bring the Kepler space telescope's
total haul to 2,326 potential planets in its first
16 months of operation.These discoveries, if conf
irmed, would quadruple the current tally of worlds
known to exist beyond our solar system, which rec
ently topped 700.
The potentially habitable alien world, a first fo
r Kepler, orbits a star very much like our own sun
. The discovery brings scientists one step closer
to finding a planet like our own — one which could
conceivably harbor life, scientists said.
"We're getting closer and closer to discovering t
he so-called 'Goldilocks planet,'" Pete Worden, di
rector of NASA's Ames Research Center in Moffett F
ield, Calif., said during a press conference today
. [Gallery: The Strangest Alien Planets]
The newfound planet in the habitable zone is call
ed Kepler-22b. It is located about 600 light-years
away, orbiting a sun-like star.
Kepler-22b's radius is 2.4 times that of Earth, a
nd the two planets have roughly similar temperatur
es. If the greenhouse effect operates there simila
rly to how it does on Earth, the average surface t
emperature on Kepler-22b would be 72 degrees Fahre
nheit (22 degrees Celsius).
Hunting down alien planets
The $600 million Kepler observatory launched in M
arch 2009 to hunt for Earth-size alien planets in
the habitable zone of their parent stars, where li
quid water, and perhaps even life, might be able t
o exist.
Kepler detects alien planets using what's called
the "transit method." It searches for tiny, tellta
le dips in a star's brightness caused when a plane
t transits — or crosses in front of — the star fro
m Earth's perspective, blocking a fraction of the
star's light.
The finds graduate from "candidates" to full-fled
ged planets after follow-up observations confirm t
hat they're not false alarms. This process, which
is usually done with large, ground-based telescope
s, can take about a year.
The Kepler team released data from its first 13 m
onths of operation back in February, announcing th
at the instrument had detected 1,235 planet candid
ates, including 54 in the habitable zone and 68 th
at are roughly Earth-size.
Of the total 2,326 candidate planets that Kepler
has found to date, 207 are approximately Earth-siz
e. More of them, 680, are a bit larger than our pl
anet, falling into the "super-Earth" category. The
total number of candidate planets in the habitabl
e zones of their stars is now 48.
To date, just over two dozen of these potential e
xoplanets have been confirmed, but Kepler scientis
ts have estimated that at least 80 percent of the
instrument's discoveries should end up being the r
eal deal.
More discoveries to come
The newfound 1,094 planet candidates are the frui
t of Kepler's labors during its first 16 months of
science work, from May 2009 to September 2010. An
d they won't be the last of the prolific instrumen
t's discoveries.
"This is a major milestone on the road to finding
Earth's twin," Douglas Hudgins, Kepler program sc
ientist at NASA headquarters in Washington, D.C.,
said in a statement.
Mission scientists still need to analyze data fro
m the last two years and on into the future. Keple
r will be making observations for a while yet to c
ome; its nominal mission is set to end in November
2012, but the Kepler team is preparing a proposal
to extend the instrument's operations for another
year or more.
Kepler's finds should only get more exciting as ti
me goes on, researchers say.
"We're pushing down to smaller planets and longer
orbital periods," said Natalie Batalha, Kepler de
puty science team lead at Ames.
To flag a potential planet, the instrument genera
lly needs to witness three transits. Planets that
make three transits in just a few months must be p
retty close to their parent stars; as a result, ma
ny of the alien worlds Kepler spotted early on hav
e been blisteringly hot places that aren't great c
andidates for harboring life as we know it.
Given more time, however, a wealth of more distan
tly orbiting — and perhaps more Earth-like — exopl
anets should open up to Kepler. If intelligent ali
ens were studying our solar system with their own
version of Kepler, after all, it would take them t
hree years to detect our home planet.
"We are getting very close," Batalha said. "We ar
e homing in on the truly Earth-size, habitable pla
nets."