The first Earth-sized exoplanet orbiting within the habitable zone of
another star has been confirmed by observations with both the W. M. Keck
Observatory and the Gemini Observatory. The initial discovery, made by
NASA's Kepler Space Telescope, is one of a handful of smaller planets
found by Kepler and verified using large ground-based telescopes. It
also confirms that Earth-sized planets do exist in the habitable zone of
other stars.
The diagram compares the planets of the inner solar system to
Kepler-186, a five-planet system about 500 light-years from Earth in the
constellation Cygnus. The five
planets of Kepler-186 orbit a star classified as a M1 dwarf, measuring
half the size and mass of the sun. The Kepler-186 system is home to
Kepler-186f, the first validated Earth-size planet orbiting a distant
star in the habitable zone—a range of distances from a star where liquid
water might pool on the surface of an orbiting planet. The discovery of
Kepler-186f confirms that Earth-size planets exist in the habitable
zone of other stars and signals a significant step closer to finding a
world similar to Earth. Kepler-186f is less than ten percent larger than
Earth in size, but its mass and composition are not known. Kepler-186f
orbits its star once every 130-days and receives one-third the heat
energy that Earth does from the sun, placing it near the outer edge of
the habitable zone. The inner four companion planets all measure less
than fifty percent the size of Earth. Kepler-186b, Kepler-186c,
Kepler-186d, and Kepler-186e, orbit every three, seven, 13, and 22 days,
respectively, making them very hot and inhospitable for life as we know
it. The Kepler space telescope, which simultaneously and continuously
measured the brightness of more than 150,000 stars, is NASA's first
mission capable of detecting Earth-size planets around stars like our
sun. Kepler does not directly image the planets it detects. The space
telescope infers their existence by the amount of starlight blocked when
the orbiting planet passes in front of a distant star from the vantage
point of the observer. The artistic concept of Kepler-186f is the result
of scientists and artists collaborating to help imagine the appearance
of these distant
The first Earth-sized exoplanet orbiting within the habitable zone of
another star has been confirmed by observations with both the W. M. Keck
Observatory and the Gemini Observatory. The initial discovery, made by
NASA's Kepler Space Telescope, is one of a handful of smaller planets
found by Kepler and verified using large ground-based telescopes. It
also confirms that Earth-sized planets do exist in the habitable zone of
other stars.
"What makes this finding
particularly compelling is that this Earth-sized planet, one of five
orbiting this star, which is cooler than the Sun, resides in a temperate
region where water could exist in liquid form," says Elisa Quintana of
the SETI Institute and NASA Ames Research Center who led the paper
published in the current issue of the journal Science. The
region in which this planet orbits its star is called the habitable
zone, as it is thought that life would most likely form on planets with
liquid water.
Steve Howell, Kepler's Project Scientist and a
co-author on the paper, adds that neither Kepler (nor any telescope) is
currently able to directly spot an exoplanet of this size and proximity
to its host star. "However, what we can do is eliminate essentially all
other possibilities so that the validity of these planets is really the
only viable option."
With such a small host star, the team
employed a technique that eliminated the possibility that either a
background star or a stellar companion could be mimicking what Kepler
detected. To do this, the team obtained extremely high spatial
resolution observations from the eight-meter Gemini North telescope on
Mauna Kea in Hawai`i using a technique called speckle imaging, as well
as adaptive optics (AO) observations from the ten-meter Keck II
telescope, Gemini's neighbor on Mauna Kea. Together, these data allowed
the team to rule out sources close enough to the star's line-of-sight to
confound the Kepler evidence, and conclude that Kepler's detected
signal has to be from a small planet transiting its host star.
"The
Keck and Gemini data are two key pieces of this puzzle," says Quintana.
"Without these complementary observations we wouldn't have been able to
confirm this Earth-sized planet."
The Gemini "speckle" data
directly imaged the system to within about 400 million miles (about 4
AU, approximately equal to the orbit of Jupiter in our solar system) of
the host star and confirmed that there were no other stellar size
objects orbiting within this radius from the star. Augmenting this, the
Keck AO observations probed a larger region around the star but to
fainter limits. According to Quintana,
"These Earth-sized planets
are extremely hard to detect and confirm, and now that we've found one,
we want to search for more. Gemini and Keck will no doubt play a large
role in these endeavors."
The host star, Kepler-186, is an
M1-type dwarf star relatively close to our solar system, at about 500
light years and is in the constellation of Cygnus. The star is very dim,
being over half a million times fainter than the faintest stars we can
see with the naked eye. Five small planets have been found orbiting this
star, four of which are in very short-period orbits and are very hot.
The planet designated Kepler-186f, however, is earth-sized and orbits
within the star's habitable zone. The Kepler evidence for this planetary
system comes from the detection of planetary transits. These transits
can be thought of as tiny eclipses of the host star by a planet (or
planets) as seen from Earth. When such planets block part of the star's
light, its total brightness diminishes. Kepler detects that as a
variation in the star's total light output and evidence for planets. So
far more than 3,800 possible planets have been detected by this
technique with Kepler.
The Gemini data utilized the Differential
Speckle Survey Instrument (DSSI) on the Gemini North telescope. DSSI is a
visiting instrument developed by a team led by Howell who adds, "DSSI
on Gemini Rocks! With this combination, we can probe down into this star
system to a distance of about 4 times that between Earth and the Sun.
It's simply remarkable that we can look inside other solar systems."
DSSI works on a principle that utilizes multiple short exposures of an
object to capture and remove the noise introduced by atmospheric
turbulence producing images with extreme detail.
Observations
with the W.M. Keck Observatory used the Natural Guide Star Adaptive
Optics system with the NIRC2 camera on the Keck II telescope. NIRC2 (the
Near-Infrared Camera, second generation) works in combination with the
Keck II adaptive optics system to obtain very sharp images at
near-infrared wavelengths, achieving spatial resolutions comparable to
or better than those achieved by the Hubble Space
Telescope at
optical wavelengths. NIRC2 is probably best known for helping to provide
definitive proof of a central massive black hole at the center of our
galaxy. Astronomers also use NIRC2 to map surface features of solar
system bodies, detect planets orbiting other stars, and study detailed
morphology of distant galaxies.
"The observations from Keck and
Gemini, combined with other data and numerical calculations, allowed us
to be 99.98% confident that Kepler-186f is real," says Thomas Barclay, a
Kepler scientist and also a co-author on the paper. "Kepler started
this story, and Gemini and Keck helped close it," adds Barclay.
Journal Reference:
- Elisa V. Quintana, Thomas Barclay, Sean N. Raymond, Jason F. Rowe, Emeline Bolmont, Douglas A. Caldwell, Steve B. Howell, Stephen R. Kane, Daniel Huber, Justin R. Crepp, Jack J. Lissauer, David R. Ciardi, Jeffrey L. Coughlin, Mark E. Everett, Christopher E. Henze, Elliott Horch, Howard Isaacson, Eric B. Ford, Fred C. Adams, Martin Still, Roger C. Hunter, Billy Quarles, Franck Selsis. An Earth-Sized Planet in the Habitable Zone of a Cool Star. Science, 2014 DOI: 10.1126/science.1249403
Courtesy: ScienceDaily
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