Kepler-10c is an exoplanet orbiting the G-type star Kepler-10, located around
568 light-years away in Draco. Its discovery was announced by Kepler in May
2011, although it had been seen as a planetary candidate since January 2011,
when Kepler-10b was discovered. The team confirmed the observation using data
from NASA's Spitzer Space Telescope and a technique called Blender that ruled
out most false positives. Kepler-10c was the third transiting planet to be
confirmed statistically, after Kepler-9d and Kepler-11g. The Kepler team
considers the statistical method that led to the discovery of Kepler-10c as
what will be necessary to confirm many planets in Kepler's field of view.
Kepler-10c orbits its host star every forty-five days at a quarter of the
average distance between the Sun and Earth. It has a radius more than double
that of Earth, but a higher density, suggesting a mainly rocky composition
with around 5–20% ices by mass. For comparison, the Earth's oceans represent
only 0.02% of our planet's mass, with an additional amount potentially a few
times this stored in the mantle. Discovery and confirmation
In January 2011, the closely orbiting planet Kepler-10b was confirmed in the
orbit of the star Kepler-10 after measurements of its transiting behavior
and a radial velocity effect detected in Kepler-10's spectrum provided the
information needed to prove that it was indeed a planet. An additional,
longer-period dimming was detected in Kepler-10's spectrum, suggesting that a
second planet existed in the system; however, there remained the possibility
that this signal could have some other cause, and that the transit event was a
false positive. Attempts to measure the radial velocity effects of this object,
now named KOI 072.02, were fruitless; therefore, to rule out false positive
scenarios, the Kepler team used a technique called Blender.
The application of Blender was supplemented by use of the IRAC
instrument on the Spitzer Space Telescope, which was used on August 30
and November 15, 2010, to further define Kepler-10's light curve at the point
where KOI 072.02 appeared to transit it. It was found that the transiting object
did not produce a color, an aspect that is characteristic of stars. This
suggested even further that KOI 072.02 was a planet. In addition, the IRAC
instrument found no difference in the transit signal when comparing the star's
light curve in the infrared and in visible light; stars that are aligned
with Kepler-10 might appear visibly similar, but would appear different in
the infrared. The WIYN Observatory's 3.5m telescope
was used for speckle imaging on June 18, 2010; in addition, the PHARO camera on
the Palomar Observatory's 5m telescope was used for its adaptive optics
capabilities. These observations, combined with observations of
Kepler-10's spectrum taken from the W.M. Keck Observatory, ruled out the
possibility that a nearby star's light was corrupting the observed spectrum of
Kepler-10 and creating the results that had led astronomers to believe that a
second planet existed in Kepler-10's orbit. All of these possibilities, with
the exception of if such a star existed exactly behind or in front of Kepler-10,
were effectively ruled out; even with this, the Kepler team found that if a
star was indeed aligned with Kepler-10 as seen from Earth, such a star would
probably not be a giant star. With a greater degree of certainty
established, the Kepler team compared the models formed using Blender to the
photometric observations collected by the Kepler satellite. The Blender
technique allowed the Kepler team to rule out the majority of the
alternatives including, notably, that of triple star systems. Blender then
allowed the Kepler team to determine that although all models representing
hierarchical triple stars can resemble the light curve of Kepler-10, the
aforementioned follow-up observations would have detected them all. The only
possible blends remaining after ruling out hierarchical triple stars was that
of determining if the curve is caused by interference from a background star, or
if it is indeed caused by the orbit of a transiting planet.
Comparisons of KOI 072.02 to the 1235 other Kepler Objects of Interest in
Kepler's field of vision allowed astronomers to use models that led to
the confirmation of KOI 072.02 as a planet with a high degree of certainty.
KOI 072.02 was then renamed Kepler-10c. The planet's confirmation was announced
at the Boston meeting of the American Astronomical Society on May 23, 2011.
Kepler-10c was the first Kepler target to be observed using Spitzer with the
hope of detecting a shallow transit dip in a light curve. At the time of
Kepler-10c's discovery, Spitzer was the only facility capable of detecting
shallow transits in the Kepler data to an extent at which the data could be
meaningfully analyzed. The planet was also the third transiting planet that
was validated through an analysis of statistical data, after the planets
Kepler-9d and Kepler-11g. In Kepler-10c's confirmation paper, the
Kepler team discussed how a large fraction of planets in Kepler's field of
view would be confirmed in this statistical manner.
Host star Kepler-10 is a G-type star located 173
parsecs from Earth. It is 0.895 solar masses and 1.056 solar radii, making it
slightly less massive than the Sun, but approximately the same size.
With an effective temperature of 5627 K, Kepler-10 is cooler than the Sun. The
star is also metal-poor and far older: its metallicity is measured at [Fe/H] =
−0.15. Kepler-10 has a measured age of approximately 10.6 billion years.
Kepler-10 has an apparent magnitude of 11.2, which means that the star is
invisible to the naked eye from the perspective of an observer on Earth.
Characteristics Kepler-10c is the outermost of the two
known planets of Kepler-10, completing one orbit of the star every 45.29485
days at a distance of 0.2407 AU. The inner planet, Kepler-10b, is a rocky
planet that orbits every ~0.8 days at a distance of 0.01684 AU. Kepler-10c's
equilibrium temperature is estimated at 584 K, almost four times hotter than
Jupiter's. The planet's orbital inclination is 89.65º, or almost edge-on
with respect to Earth and to Kepler-10. Transits have been observed at points
where Kepler-10c has crossed in front of its host star.
Kepler-10c has a mass of 15–19 Earth masses. With a radius only 2.35 times
that of Earth, and a density higher than Earth's, it is unlikely to contain
significant amounts of hydrogen and helium gas. Outgassed or accreted
hydrogen-rich atmospheres would have been lost over the 10.6 billion-year
lifetime of the Kepler-10 system. Instead, the composition is likely to be
mainly rocky, with a water fraction of 5–20% by mass. The bulk of this water is
likely to be in the form of high-pressure "hot ice" phases.
See also 55 Cancri e
Kepler-10b Kepler-20b
References