Figure 3.
planet’s blocking of some light from the star.
Kepler measurements of the brightness decrease, and the frequency of transit, as well as
knowledge of the star’s size, are used to determine the size of the planet and the planet’s
distance from its star.
The Kepler mission has contributed more
than 950 exoplanet discoveries to the current
count of about 1700, in addition to the 38000
potential planets that await confirmation.
Several key milestones have been reached
by Kepler towards the goal of finding potentially habitable planets. A small number of
Earth-sized planets, such as Kepler-20e, have
been found, however they all orbit close to
their star, making them extremely hot and
therefore inhospitable to life. About a dozen
planets have been found to orbit in their star’s
habitable zone, however they are all larger
than Earth and most have a thick atmosphere
of gas like Jupiter. Kepler-186f is the first
“Goldilocks planet,” it has the right size, and
orbits at the right distance, to allow the exis-
April 2014
tence of water, thought to be a key ingredient
for supporting life.
Gemini’s Role
One way to verify claims of exoplanet detection is to look for a signature “wobble” in the
host star, as gravity from orbiting planets tug
on it. However, Earth-sized planets, like Kepler-186f, are too small to create a detectable
wobble with existing technology. Confirmation had to come in a new way.
To prove that the signals in the star’s light are
due to planets and not other nearby astrophysical sources, high-contrast images were
obtained from both Gemini North and Keck II
telescopes. Some common sources of apparent variability, which can trigger a false planetary detection, include a binary star system
in the background or foreground that introduces small variations in the total measured
light. Alternatively, if we are truly looking
at a binary star system, but a third star is so
GeminiFocus
Exclusion zones for
each of the planet
candidates in the
Kepler-186 system.
Prior to validation,
the star was known
as KOI (Kepler Object
of Interest) 571, and
Kepler-186f (KOI571.05) is shown in
the lower middle
panel. The shaded
regions indicate the
parameter space
that can be ruled
out for having
an astrophysical
source (like a binary
star system) that
is inducing light
variations that
could mimic a
planet signal. The
region shaded in
green is excluded by
Gemini speckle imaging,
the pink is from the
Keck-II adaptive optics
observations, the blue is
based on modeling the
transit light curve, and the
yellow is based on pixel
centroids data (measures
of the movement of the
pixels which can indicate
if the primary source is a
binary star system, not
shown in the Kepler-186f
panel). Observational
constraints rule out in all
parts of the save for the
white region, which then
goes through additional
statistical analysis. The
additional shaded region
provided by Gemini —
looking for sources very
close to the star — was
crucial in our validation of
Kepler-186f.
Credit: Thomas Barclay
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