GeminiFocus May 2014 | Page 7

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 5