Phase curves and secondary eclipses of gaseous exoplanets are diagnostic of atmospheric composition and meteorology, and the long observational baseline and high photometric precision from the Kepler Mission make its dataset well-suited for exploring phase curve variability, which provides additional insights into atmospheric dynamics.Observations of the hot Jupiter Kepler-76b span more than 1,000 days, providing an ideal dataset to search for atmospheric variability. In this study, we find that Kepler-76b's secondary eclipse, with a depth of 87 ± 6 parts-per-million (ppm), corresponds to an effective temperature of 2,830 +50 −30 K. Our results also show clear indications of variability in Kepler-76b's atmospheric emission and reflectivity, with the phase curve amplitude typically 50.5 ± 1.3 ppm but varying between 35 and 70 ppm over tens of days. As is common for hot Jupiters, Kepler-76b's phase curve shows a discernible offset of (9 ± 1.3) • eastward of the sub-stellar point and varying in concert with the amplitude. These variations may arise from the advance and retreat of thermal structures and cloud formations in Kepler-76b's atmosphere; the resulting thermal perturbations may couple with the super-rotation expected to transport aerosols, giving rise to a feed-arXiv:1905.07781v1 [astro-ph.EP] 19 May 2019 2 Jackson, Adams, Sandidge, Kreyche, & Briggs back loop. Looking forward, the TESS Mission can provide new insight into planetary atmospheres, with good prospects to observe both secondary eclipses and phase curves among targets from the mission. TESS's increased sensitivity in red wavelengths as compared to Kepler means that it will probably probe different aspects of planetary atmospheres.

Previous studies have shown that planets that rotate retrograde (backwards with respect to their orbital motion) generally experience less severe obliquity variations than those that rotate prograde (the same direction as their orbital motion). Here we examine retrograde-rotating planets on eccentric orbits and find a previously unknown secular spin-orbit resonance that can drive significant obliquity variations. This resonance occurs when the frequency of the planet's rotation axis precession becomes commensurate with an orbital eigenfrequency of the planetary system. The planet's eccentricity enables a participating orbital frequency through an interaction in which the apsidal precession of the planet's orbit causes a cyclic nutation of the planet's orbital angular momentum vector. The resulting orbital frequency follows the relationship f = 2˙ −Ω, where˙ andΩ are the rates of the planet's changing longitude of periapsis and ascending node, respectively. We test this mechanism by simulating cases of a simple Earth-Jupiter system, and confirm the predicted resonance. Over the course of 100 Myr, the test Earths with rotation axis precession rates near the predicted resonant frequency experienced pronounced obliquity variations of order 10 • -30 • . These variations can be significant, and suggest that while retrograde rotation is a stabilizing influence most of the time, retrograde rotators can experience large obliquity variations if they are on eccentric orbits and enter this spin-orbit resonance.

We introduce our new code, SMERCURY-T, which is based on existing codes SMERCURY and Mercury-T. The result is a mixed-variable symplectic N-body integrator that can compute the orbital and spin evolution of a planet within a multiplanet system under the influence of tidal spin torques from its star. We validate our implementation by comparing our experimental results to that of a secular model. As we demonstrate in a series of experiments, SMERCURY-T allows for the study of secular spin–orbit resonance crossings and captures for planets within complex multiplanet systems. These processes can drive a planet’s spin state to evolve along vastly different pathways on its road toward tidal equilibrium, as tidal spin torques dampen the planet’s spin rate and evolve its obliquity. Additionally, we show the results of a scenario that exemplifies the crossing of a chaotic region that exists as the overlap of two spin–orbit resonances. The test planet experiences violent and chaotic swings in its obliquity until its eventual escape from resonance as it tidally evolves. All of these processes are and have been important over the obliquity evolution of many bodies within the solar system and beyond and have implications for planetary climate and habitability. SMERCURY-T is a powerful and versatile tool that allows for further study of these phenomena.

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