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The Three Planets of Kepler-138

12 Jul 2015, 23:00 UTC
The Three Planets of Kepler-138
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Figure 1: Artist’s impression of a rocky planet.Located roughly 200 light years from Earth, Kepler-138 is a red dwarf star that is much fainter and cooler than the Sun. Observations by NASA’s Kepler space telescope have led to the detection of three planets circling Kepler-138. The three planets were detected using the transit technique and are identified as Kepler-138 b, Kepler-138 c and Kepler-138 d. When a planet passes in front of (i.e. transits) its parent star, it causes a slight drop in the observed brightness of the star, allowing the size of the planet to be measured. Additionally, the interval between consecutive transits is the planet’s orbital period. The transit technique allows the sizes and orbital periods of the planets around Kepler-138 to be determined.In a multi-planet system, gravitational perturbations between neighbouring planets can cause the transit timing for each planet to vary. This effect is particularly sensitive to planets that are closely spaced or near orbital resonances. The three planets around Kepler-138 are in orbital resonances and exhibit observable transit timing variations (TTV). Kepler-138b and Kepler-138c orbit near the 4:3 resonance (i.e. Kepler-138 b completes 4 orbits for every 3 orbits of Kepler-138 c); while Kepler-138 c and ...

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