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| [Image credit: NASA Ames Research Center/W. Stenzel] |
Washington: NASA is marking two milestones in the search for planets
like Earth; the successful completion of the Kepler Space Telescope's 3 1/2-
year prime mission and the beginning of an extended mission that could last as
long as four years.
Scientists have used
Kepler data to identify more than 2,300 planet candidates and confirm more than
100 planets. Kepler is teaching us the galaxy is teeming with planetary systems
and planets are prolific, and giving us hints that nature makes small planets
efficiently.
So far, hundreds of
Earth-size planet candidates have been found as well as candidates that orbit
in the habitable zone, the region in a planetary system where liquid water
might exist on the surface of a planet. None of the candidates is exactly like
Earth. With the completion of the prime mission, Kepler now has collected
enough data to begin finding true sun-Earth analogs -- Earth-size planets with
a one-year orbit around stars similar to the sun.
"The initial
discoveries of the Kepler mission indicate at least a third of the stars have
planets and the number of planets in our galaxy must number in the
billions," said William Borucki, Kepler principal investigator at NASA's
Ames Research Center in Moffett Field, Calif. "The planets of greatest
interest are other Earths and these could already be in the data awaiting
analysis. Kepler's most exciting results are yet to come."
NASA's Kepler Space
Telescope searches for planet candidates orbiting distant suns, or exoplanets,
by continuously measuring the brightness of more than 150,000 stars. When a
planet candidate passes, or transits, in front of the star from the
spacecraft's vantage point, light from the star is blocked. Different sized
planets block different amounts of starlight. The amount of starlight blocked
by a planet reveals its size relative to its star.
Kepler was launched
March 6, 2009. Its mission was to survey a portion of the galaxy to determine
what fraction of stars might harbor potentially habitable, Earth-sized planets.
Planets orbiting in or near habitable zones are of particular interest.
Kepler began the
search for small worlds like our own on May 12, 2009, after two months of
commissioning. Within months, five exoplanets, known as hot Jupiters because of
their enormous size and orbits close to their stars, were confirmed.
Results from Kepler
data continue to expand our understanding of planets and planetary systems.
Highlights from the prime mission include:
-- In August 2010,
scientists confirmed the discovery of the first planetary system with more than
one planet transiting the same star. The Kepler-9 system opened the door to
measurement of gravitational interactions between planets as observed by the
variations in their transit timing. This powerful new technique enables
astronomers, in many cases, to calculate the mass of planets directly from
Kepler data, without the need for follow-up observations from the ground.
-- In January 2011,
the Kepler team announced the discovery of the first unquestionably rocky
planet outside the solar system. Kepler-10b, measuring 1.4 times the size of
Earth, is the smallest confirmed planet with both a radius and mass
measurement. Kepler has continued to uncover smaller and smaller planets, some
almost as small as Mars, which tells us small rocky worlds may be common in the
galaxy.
-- In February 2011,
scientists announced Kepler had found a very crowded and compact planetary
system – a star with multiple transiting planets. Kepler-11 has six planets
larger than Earth, all orbiting closer to their star than Venus orbits our sun.
This and other subsequently identified compact multi-planet systems have
orbital spacing relative to their host sun and neighboring planets unlike
anything envisioned prior to the mission.
-- In September 2011,
Kepler data confirmed the existence of a world with a double sunset like the
one famously portrayed in the film "Star Wars" more than 35 years
ago. The discovery of Kepler-16b turned science fiction into science fact.
Since then, the discoveries of six additional worlds orbiting double stars
further demonstrated planets can form and persist in the environs of a
double-star system.
-- In December 2011,
NASA announced Kepler's discovery of the mission's first planet in a habitable
zone. Kepler-22b, about 2.4 times the size of Earth, is the smallest-radius
planet yet found to orbit a sun-like star in the habitable zone. This discovery
confirmed that we are getting continually closer to finding planets like our
own.
-- In February 2012,
the Kepler team announced more than 1,000 new transiting planet candidates for
a cumulative total of 2,321. The data continues the trend toward identifying
smaller planets at longer orbital periods, similar to Earth. The results
include hundreds of planetary systems.
-- Recently, citizen
scientists participating in Planet Hunters, a program led by Yale University
that enlists the public to comb through Kepler data for signs of transiting
planets, made their first planet discovery. The joint effort of amateur
astronomers and scientists led to the first reported case of a planet orbiting
a double star. The three bodies in turn are being orbited by a second distant
pair of stars.
"Kepler's bounty
of new planet discoveries, many quite different from anything found previously,
will continue to astound," said Jack Lissauer, planetary scientist at
Ames. "But to me, the most wonderful discovery of the mission has not been
individual planets, but the systems of two, three, even six planets crowded
close to their stars, and, like the planets orbiting about our sun, moving in
nearly the same plane. Like people, planets interact with their neighbors and
can be greatly affected by them. What are the neighborhoods of Earth-size
exoplanets like? This is the question I most hope Kepler will answer in the
years to come."
In April 2012, NASA
awarded Kepler an extended mission through as late as 2016. More time will
enable the continued the search for worlds like our own -- worlds that are not
too far and too close to their sun.
"The Earth isn't
unique, nor the center of the universe," said Geoff Marcy, professor of
astronomy at the University of California at Berkeley. "The diversity of
other worlds is greater than depicted in all the science fiction novels and
movies. Aristotle would be proud of us for answering some of the most profound
philosophical questions about our place in the universe."