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Register a new userAtmospheres on Nonsynchronized Eccentric-tilted Exoplanets II: Thermal Light Curves
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Thermal light-curve analysis is a powerful approach to probe the thermal structures of exoplanetary atmospheres, which are greatly influenced by the planetary obliquity and eccentricity. Here we investigate the thermal light curves of eccentric-tilted exoplanets across various radiative timescales, eccentricities, obliquities, and viewing geometries using results of shallow-water simulations presented in Ohno $&$ Zhang (2019). We also achieve an analytical theory of the thermal light curve that can explain general trends in the light curves of tilted exoplanets. For tilted planets in circular orbits, the orbital phase of the flux peak is largely controlled by either the flux from the hot spot projected onto the orbital plane or the pole heated at the summer solstice, depending on the radiative timescale $tau_{rm rad}$, planetary day $P_{rm orb}$, and obliquity $theta$. We find that tilted planets potentially produce the flux peak after the secondary eclipse when obliquity is $theta$ > 90 deg for the hot regime $tau_{rm rad}<P_{rm rot}$, or $theta>18$ deg for the cool regime ${tau}_{rm rad} > P_{rm rot}$. For tilted planets in eccentric orbits, the shape of the light curve is considerably influenced by the heating at the periapse. The flux peak occurring after the secondary eclipse can be used to distinguish tilted planets from nontilted planets when the periapse takes place before the secondary eclipse. Our results could help to constrain exoplanet obliquities in future observations.
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Relatively long-period nonsynchronized planets---such as warm Jupiters---potentially retain the primordial rotation, eccentricity, and obliquity that might encapsulate information on planetary climate and formation processes. To date, there has not been a systematic study on climate patterns on these planets that will significantly influence their observations. Here we investigate the atmospheric dynamics of nonsynchronized, fast-rotating exoplanets across various radiative timescales, eccentricities, and obliquities using a shallow water model. The dynamical pattern can be demarcated into five regimes in terms of radiative timescale $tau_{rm rad}$ and obliquity ${theta}$. An atmosphere with $tau_{rm rad}$ shorter than a planetary day usually exhibits a strong day--night temperature contrast and a day-to-night flow pattern. In the intermediate $tau_{rm rad}$ regime between a planetary day and a year, the atmosphere is dominated by steady temperature and eastward jet patterns for ${theta}$ < 18 deg but shows a strong seasonal variation for ${theta}$ > 18 deg because the polar region undergoes an intense heating at around the summer solstice. If $tau_{rm rad}$ is longer than a year, seasonal variation is very weak. In this regime, eastward jets are developed for ${theta}$ < 54 deg and westward jets are developed for ${theta}$ > 54 deg. These dynamical regimes are also applicable to the planets in eccentric orbits. The large effects of exoplanetary obliquities on circulation patterns might offer observational signatures, which will be investigated in Paper II of this study.
Close-in exoplanets with highly eccentric orbits are subject to large variations in incoming stellar flux between periapse and apoapse. These variations may lead to large swings in atmospheric temperature, which in turn may cause changes in the chemistry of the atmosphere from higher CO abundances at periapse to higher CH4 abundances at apoapse. Here we examine chemical timescales for CO<->CH4 interconversion compared to orbital timescales and vertical mixing timescales for the highly eccentric exoplanets HAT-P-2b and CoRoT-10b. As exoplanet atmospheres cool, the chemical timescales for CO<->CH4 tend to exceed orbital and/or vertical mixing timescales, leading to quenching. The relative roles of orbit-induced thermal quenching and vertical quenching depend upon mixing timescales relative to orbital timescales. For both HAT-P-2b and CoRoT-10b, vertical quenching will determine disequilibrium CO<->CH4 chemistry at faster vertical mixing rates (Kzz > 10^7 cm^2 s^-1), whereas orbit-induced thermal quenching may play a significant role at slower mixing rates (Kzz < 10^7 cm^2 s^-1). The general abundance and chemical timescale results - calculated as a function of pressure, temperature, and metallicity - can be applied for different atmospheric profiles in order to estimate the quench level and disequilibrium abundances of CO and CH4 on hydrogen-dominated exoplanets. Observations of CO and CH4 on highly eccentric exoplanets may yield important clues to the chemical and dynamical properties of their atmospheres.
Clouds have been shown to be present in many exoplanetary atmospheres. Cloud formation modeling predicts considerable inhomogeneities of cloud cover, consistent with optical phase curve observations. However, optical phase curves cannot resolve some existing degeneracies between cloud location and cloud optical properties. We present a conceptually simple technique to detect inhomogeneous cloud cover on exoplanets. Such an inhomogeneous cloud cover produces an asymmetric primary transit of the planet in front of the host star. Asymmetric transits produce characteristic residuals compared to a standard symmetric model. Furthermore, bisector spans can be used to determine asymmetries in the transit light curve. We apply a model of asymmetric transits to the light curves of HAT-P-7b, Kepler-7b and HD209458b and search for possible cloud signatures. The nearly uninterrupted Kepler photometry is particularly well-suited for this method since it allows for a very high time resolution. We do not find any statistically sound cloud signature in the data of the considered planets. For HAT-P-7b, a tentative detection of an asymmetric cloud cover is found, consistent with analysis of the optical phase curve. Based on Bayesian probability arguments, a symmetric model with an offset in the transit ephemeris remains, however, the most viable model. Still, this work demonstrates that for suitable targets, namely low-gravity planets around bright stars, the method can be used to constrain cloud cover characteristics and is thus a helpful additional tool to study exoplanetary atmospheres.
Exoplanets on eccentric orbits experience an incident stellar flux that can be markedly larger at periastron versus apoastron. This variation in instellation can lead to dramatic changes in atmospheric structure in regions of the atmosphere where the radiative and advective heating/cooling timescales are shorter than the orbital timescale. To explore this phenomenon, we develop a sophisticated one-dimensional (vertical) time-stepping atmospheric structure code, EGP+, capable of simulating the dynamic response of atmospheric thermal and chemical structure to time-dependent perturbations. Critically, EGP+ can efficiently simulate multiple orbits of a planet, thereby providing new opportunities for exoplanet modeling without the need for more computationally-expensive models. We make the simplifying assumption of cloud-free atmospheres, and apply our model to HAT-P-2b, HD~17156b, and HD~80606b, which are known to be on higher-eccentricity orbits. We find that for those planets which have Spitzer observations, our planet-to-star ratio predictions are roughly consistent with observations. However, we are unable to reproduce the observed peak offsets from periastron passage. Finally, we discuss promising pathways forward for adding new model complexity that would enable more detailed studies of clear and cloudy eccentric planets as well as worlds orbiting active host stars.
The increasing number of transiting exoplanets sparked a significant interest in discovering their moons. Most of the methods in the literature utilize timing analysis of the raw light curves. Here we propose a new approach for the direct detection of a moon in the transit light curves via the so called Scatter Peak. The essence of the method is the valuation of the local scatter in the folded light curves of many transits. We test the ability of this method with different simulations: Kepler short cadence, Kepler long cadence, ground-based millimagnitude photometry with 3-min cadence, and the expected data quality of the planned ESA mission of PLATO. The method requires ~100 transit observations, therefore applicable for moons of 10-20 day period planets, assuming 3-4-5 year long observing campaigns with space observatories. The success rate for finding a 1 R_Earth moon around a 1 R_Jupiter exoplanet turned out to be quite promising even for the simulated ground-based observations, while the detection limit of the expected PLATO data is around 0.4 R_Earth. We give practical suggestions for observations and data reduction to improve the chance of such a detection: (i) transit observations must include out-of-transit phases before and after a transit, spanning at least the same duration as the transit itself; (ii) any trend filtering must be done in such a way that the preceding and following out-of-transit phases remain unaffected.
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