Phase Curve Amplitude Research Articles

The tidal deformation and atmosphere of WASP-12 b from its phase curve

Context. Ultra-hot Jupiters present a unique opportunity to understand the physics and chemistry of planets, their atmospheres, and interiors at extreme conditions. WASP-12 b stands out as an archetype of this class of exoplanets, with a close-in orbit around its star that results in intense stellar irradiation and tidal effects. Aims. The goals are to measure the planet’s tidal deformation, atmospheric properties, and also to refine its orbital decay rate. Methods. We performed comprehensive analyses of the transits, occultations, and phase curves of WASP-12b by combining new CHEOPS observations with previous TESS and Spitzer data. The planet was modeled as a triaxial ellipsoid parameterized by the second-order fluid Love number of the planet, h2, which quantifies its radial deformation and provides insight into the interior structure. Results. We measured the tidal deformation of WASP-12b and estimated a Love number of h2 = 1.55−0.49+0.45 (at 3.2σ) from its phase curve. We measured occultation depths of 333 ± 24 ppm and 493 ± 29 ppm in the CHEOPS and TESS bands, respectively, while the nightside fluxes are consistent with zero, and also marginal eastward phase offsets. Our modeling of the dayside emission spectrum indicates that CHEOPS and TESS probe similar pressure levels in the atmosphere at a temperature of ~2900 K. We also estimated low geometric albedos of Ag = 0.086 ± 0.017 and Ag = 0.01 ± 0.023 in the CHEOPS and TESS passbands, respectively, suggesting the absence of reflective clouds in the high-temperature dayside of the planet. The CHEOPS occultations do not show strong evidence for variability in the dayside atmosphere of the planet at the median occultation depth precision of 120 ppm attained. Finally, combining the new CHEOPS timings with previous measurements refines the precision of the orbital decay rate by 12% to a value of −30.23 ± 0.82 ms yr−1, resulting in a modified stellar tidal quality factor of Q′★ = 1.70 ± 0.14 × 105. Conclusions. WASP-12 b becomes the second exoplanet, after WASP-103b, for which the Love number has been measured from the effect of tidal deformation in the light curve. However, constraining the core mass fraction of the planet requires measuring h2 with a higher precision. This can be achieved with high signal-to-noise observations with JWST since the phase curve amplitude, and consequently the induced tidal deformation effect, is higher in the infrared.

Hot Jupiter diversity and the onset of TiO/VO revealed by a large grid of non-grey global circulation models

ABSTRACT The population of hot Jupiters is extremely diverse, with large variations in their irradiation, period, gravity, and chemical composition. To understand the intrinsic planet diversity through the observed population level trends, we explore the a priori scatter in the population created by the different responses of atmospheric circulation to planetary parameters. We use the SPARC/MITgcm three-dimensional global circulation model to simulate 345 planets spanning a wide range of instellation, metallicity, gravity, and rotation periods typical for hot Jupiters, while differentiating between models with and without titanium oxide (TiO)/vanadium oxide (VO) in their atmosphere. We show that the combined effect of the planetary parameters leads to a large diversity in the ability of atmospheres to transport heat from day-side to night-side at a given equilibrium temperature. We further show that the hotspot offset is a non-monotonic function of planetary rotation period and explain our findings by a competition between the rotational and divergent parts of the circulation. As a consequence, hotspot offset and phase curve amplitude are not necessarily correlated. Finally, we compare the observables from our grid to the population of Spitzer and Hubble observations of hot Jupiters. We find that the sudden jump in brightness temperature observed in the Spitzer secondary eclipse measurements can be naturally explained by the cold-trapping of TiO/VO at approximately 1800 K. Spectra, phase curves and thermal structures from the model grid used in this study are available to the community, for more details see the data availability section.

Effects of tidal deformation on planetary phase curves

With the continuous improvement in the precision of exoplanet observations, it has become feasible to probe for subtle effects that can enable a more comprehensive characterization of exoplanets. A notable example is the tidal deformation of ultra-hot Jupiters by their host stars, whose detection can provide valuable insights into the planetary interior structure. In this work we extend previous research on modeling deformation in transit light curves by proposing a straightforward approach to account for tidal deformation in phase curve observations. The planetary shape is modeled as a function of the second fluid Love number for radial deformation h2f. For a planet in hydrostatic equilibrium, h2f provides constraints on the interior structure of the planet. We show that the effect of tidal deformation manifests across the full orbit of the planet as its projected area varies with phase, thereby allowing us to better probe the planet’s shape in phase curves than in transits. Comparing the effects and detectability of deformation by different space-based instruments (JWST, HST, PLATO, CHEOPS, and TESS), we find that the effect of deformation is more prominent in infrared observations where the phase curve amplitude is the largest. A single JWST phase curve observation of a deformed planet, such as WASP-12 b, can allow up to a 17σ measurement of h2f compared to 4σ from transit-only observation. This high-precision h2f measurement can constrain the core mass of the planet to within 19% of the total mass, thus providing unprecedented constraints on the interior structure. Due to the lower phase curve amplitudes in the optical, the other instruments provide ≤ 4σ precision on h2f depending on the number of phase curves observed. We also find that detecting deformation from infrared phase curves is less affected by uncertainty in limb darkening, unlike detection in transits. Finally, the assumption of sphericity when analyzing the phase curve of deformed planets can lead to biases in several system parameters (radius, dayside and nightside temperatures, and hotspot offset, among others), thereby significantly limiting their accurate characterization.

Investigating the visible phase-curve variability of 55 Cnc e

Context. 55 Cnc e is an ultra-short period super-Earth transiting a Sun-like star. Previous observations in the optical range detected a time-variable flux modulation that is phased with the planetary orbital period, whose amplitude is too large to be explained by reflected light and thermal emission alone. Aims. The goal of the study is to investigate the origin of the variability and timescale of the phase-curve modulation in 55 Cnc e. To this end, we used the CHaracterising ExOPlanet Satellite (CHEOPS), whose exquisite photometric precision provides an opportunity to characterise minute changes in the phase curve from one orbit to the next. Methods. CHEOPS observed 29 individual visits of 55 Cnc e between March 2020 and February 2022. Based on these observations, we investigated the different processes that could be at the origin of the observed modulation. In particular, we built a toy model to assess whether a circumstellar torus of dust driven by radiation pressure and gravity might match the observed flux variability timescale. Results. We find that the phase-curve amplitude and peak offset of 55 Cnc e do vary between visits. The sublimation timescales of selected dust species reveal that silicates expected in an Earth-like mantle would not survive long enough to explain the observed phase-curve modulation. We find that silicon carbide, quartz, and graphite are plausible candidates for the circumstellar torus composition because their sublimation timescales are long. Conclusions. The extensive CHEOPS observations confirm that the phase-curve amplitude and offset vary in time. We find that dust could provide the grey opacity source required to match the observations. However, the data at hand do not provide evidence that circumstellar material with a variable grain mass per unit area causes the observed variability. Future observations with the James Webb Space Telescope (JWST) promise exciting insights into this iconic super-Earth.

Combining photometry and astrometry to improve orbit retrieval of directly imaged exoplanets

ABSTRACT Future missions like Roman, HabEx, and LUVOIR will directly image exoplanets in reflected light. While current near-infrared direct imaging searches are only sensitive to young, self-luminous planets whose brightness is independent of their orbital phase, reflected light direct imaging will reveal changes in planet brightness throughout an orbit due to phase variations. One of the first objectives will be determining the planet’s orbit via astrometry, the projected position of the planet with respect to its host star in the sky plane. We show that phase variations can significantly improve the accuracy and precision of orbital retrieval with two or three direct images. This would speed up the classification of exoplanets and improve the efficiency of subsequent spectroscopic characterization. We develop a forward model to generate synthetic observations of the 2D astrometry and the planet/star flux ratio. Synthetic data are fitted with Keplerian orbits and Henyey–Greenstein phase variations to retrieve orbital and phase parameters. For astrometric uncertainties of 0.01 au in projected separation and flux ratio uncertainties of 10−12, using photometry in orbit retrieval improves the accuracy of semimajor axis by 47 per cent for two epochs and 61 per cent for three epochs if the phase curves have a known shape, but unknown amplitude. In a realistic scenario where phase curve shape and amplitude are a priori unknown, photometry improves accuracy by 16 per cent for two epochs and 50 per cent for three epochs. In general, we find that if the planetary flux is measured to better than 10σ at multiple epochs, it usefully contributes to orbit retrieval.

Mapping the Pressure-dependent Day–Night Temperature Contrast of a Strongly Irradiated Atmosphere with HST Spectroscopic Phase Curve

Abstract Many brown dwarfs are on ultrashort-period and tidally locked orbits around white dwarf hosts. Because of these small orbital separations, the brown dwarfs are irradiated at levels similar to hot Jupiters. Yet, they are easier to observe than hot Jupiters because white dwarfs are fainter than main-sequence stars at near-infrared wavelengths. Irradiated brown dwarfs are, therefore, ideal hot Jupiter analogs for studying the atmospheric response under strong irradiation and fast rotation. We present the 1.1–1.67 μm spectroscopic phase curve of the irradiated brown dwarf (SDSS1411-B) in the SDSS J141126.20 + 200911.1 brown dwarf–white dwarf binary with the near-infrared G141 grism of the Hubble Space Telescope Wide Field Camera 3. SDSS1411-B is a 50M Jup brown dwarf with an irradiation temperature of 1300 K and has an orbital period of 2.02864 hr. Our best-fit model suggests a phase-curve amplitude of 1.4% and places an upper limit of 11° for the phase offset from the secondary eclipse. After fitting the white dwarf spectrum, we extract the phase-resolved brown dwarf emission spectra. We report a highly wavelength-dependent day–night spectral variation, with a water-band flux variation of about 360% ± 70% and a comparatively small J-band flux variation of 37% ± 2%. By combining the atmospheric modeling results and the day–night brightness temperature variations, we derive a pressure-dependent temperature contrast. We discuss the difference in the spectral features of SDSS1411-B and hot Jupiter WASP-43b, as well as the lower-than-predicted day–night temperature contrast of J4111-BD. Our study provides the high-precision observational constraints on the atmospheric structures of an irradiated brown dwarf at different orbital phases.

Spitzer Phase-curve Observations and Circulation Models of the Inflated Ultrahot Jupiter WASP-76b

Abstract The large radii of many hot Jupiters can only be matched by models that have hot interior adiabats, and recent theoretical work has shown that the interior evolution of hot Jupiters has a significant impact on their atmospheric structure. Due to its inflated radius, low gravity, and ultrahot equilibrium temperature, WASP-76b is an ideal case study for the impact of internal evolution on observable properties. Hot interiors should most strongly affect the nonirradiated side of the planet, and thus full phase-curve observations are critical to ascertain the effect of the interior on the atmospheres of hot Jupiters. In this work, we present the first Spitzer phase-curve observations of WASP-76b. We find that WASP-76b has an ultrahot dayside and relatively cold nightside with brightness temperatures of 2471 ± 27 K/1518 ± 61 K at 3.6 μm and 2699 ± 32 K/1259 ± 44 K at 4.5 μm, respectively. These results provide evidence for a dayside thermal inversion. Both channels exhibit small phase offsets of 0.68 ± 0.°48 at 3.6 μm and 0.67 ± 0.°2 at 4.5 μm. We compare our observations to a suite of general circulation models (GCMs) that consider two endmembers of interior temperature along with a broad range of frictional drag strengths. Strong frictional drag is necessary to match the small phase offsets and cold nightside temperatures observed. From our suite of cloud-free GCMs, we find that only cases with a cold interior can reproduce the cold nightsides and large phase-curve amplitude at 4.5 μm, hinting that the hot interior adiabat of WASP-76b does not significantly impact its atmospheric dynamics or that clouds blanket its nightside.

CHEOPS precision phase curve of the Super-Earth 55 Cancri e

Context. 55 Cnc e is a transiting super-Earth (radius 1.88 R⊕ and mass 8 M⊕) orbiting a G8V host star on a 17-h orbit. Spitzer observations of the planet’s phase curve at 4.5 μm revealed a time-varying occultation depth, and MOST optical observations are consistent with a time-varying phase curve amplitude and phase offset of maximum light. Both broadband and high-resolution spectroscopic analyses are consistent with either a high mean molecular weight atmosphere or no atmosphere for planet e. A long-term photometric monitoring campaign on an independent optical telescope is needed to probe the variability in this system. Aims. We seek to measure the phase variations of 55 Cnc e with a broadband optical filter with the 30 cm effective aperture space telescope CHEOPS and explore how the precision photometry narrows down the range of possible scenarios. Methods. We observed 55 Cnc for 1.6 orbital phases in March of 2020. We designed a phase curve detrending toolkit for CHEOPS photometry which allowed us to study the underlying flux variations in the 55 Cnc system. Results. We detected a phase variation with a full-amplitude of 72 ± 7 ppm, but did not detect a significant secondary eclipse of the planet. The shape of the phase variation resembles that of a piecewise-Lambertian; however, the non-detection of the planetary secondary eclipse, and the large amplitude of the variations exclude reflection from the planetary surface as a possible origin of the observed phase variations. They are also likely incompatible with magnetospheric interactions between the star and planet, but may imply that circumplanetary or circumstellar material modulate the flux of the system. Conclusions. This year, further precision photometry of 55 Cnc from CHEOPS will measure variations in the phase curve amplitude and shape over time.

A comprehensive reanalysis of Spitzer’s 4.5 μm phase curves, and the phase variations of the ultra-hot Jupiters MASCARA-1b and KELT-16b

ABSTRACT We have developed an open-source pipeline for the analysis of Spitzer/IRAC channel 1 and 2 time-series photometry, incorporating some of the most popular decorrelation methods. We applied this pipeline to new phase curve observations of ultra-hot Jupiters MASCARA-1b and KELT-16b, and we performed the first comprehensive reanalysis of 15 phase curves. We find that MASCARA-1b and KELT-16b have phase offsets of $6^{+11}_{-11}~^{\circ }$W and $38^{+16}_{-15}~^{\circ }$W, dayside temperatures of $2952^{+100}_{-97}$ and $3070^{+160}_{-150}$ K, and nightside temperatures of $1300^{+340}_{-340}$ and $1900^{+430}_{-440}$ K, respectively. We confirm a strong correlation between dayside and irradiation temperatures with a shallower dependence for nightside temperature. We also find evidence that the normalized phase curve amplitude (peak-to-trough divided by eclipse depth) is correlated with stellar effective temperature. In addition, while our different models often retrieve similar parameters, significant differences occasionally arise between them, as well as between our preferred model and the literature values. Nevertheless, our preferred models are consistent with published phase offsets to within −8 ± 21 degrees (−1.6 ± 3.2 sigma), and normalized phase curve amplitudes are on average reproduced to within −0.01 ± 0.24 (−0.1 ± 1.6 sigma). Finally, we find that BLISS performs best in most cases, but not all; we therefore recommend future analyses consider numerous detector models to ensure an optimal fit and to assess model dependencies.

The cloudy shape of hot Jupiter thermal phase curves

ABSTRACT Hot Jupiters have been predicted to have a strong day/night temperature contrast and a hotspot shifted eastward of the substellar point. This was confirmed by numerous phase curve observations probing the longitudinal brightness variation of the atmosphere. Global circulation models, however, systematically underestimate the phase curve amplitude and overestimate the shift of its maximum. We use a global circulation model including non-grey radiative transfer and realistic gas and cloud opacities to systematically investigate how the atmospheric circulation of hot Jupiters varies with equilibrium temperature from 1000 to 2200 K. We show that the heat transport is very efficient for cloudless planets cooler than 1600 K and becomes less efficient at higher temperatures. When nightside clouds are present, the day-to-night heat transport becomes extremely inefficient, leading to a good match to the observed low nightside temperatures. The constancy of this low temperature is, however, due to the strong dependence of the radiative time-scale with temperature. We further show that nightside clouds increase the phase curve amplitude and decrease the phase curve offset at the same time. This change is very sensitive to the cloud chemical composition and particle size, meaning that the diversity of observed phase curves can be explained by a diversity of nightside cloud properties. Finally, we show that phase curve parameters do not necessarily track the day/night contrast nor the shift of the hotspot on isobars, and propose solutions to to recover the true hotspot shift and day/night contrast.

Evidence for H2 Dissociation and Recombination Heat Transport in the Atmosphere of KELT-9b

Abstract Phase curve observations provide an opportunity to study the energy budgets of exoplanets by quantifying the amount of heat redistributed from their daysides to their nightsides. Theories of phase curves for hot Jupiters have focused on the balance between radiation and dynamics as the primary parameter controlling heat redistribution. However, recent phase curves have shown deviations from the trends that emerge from this theory, which has led to work on additional processes that may affect hot Jupiter energy budgets. One such process, molecular hydrogen dissociation and recombination, can enhance energy redistribution on ultra-hot Jupiters with temperatures above ∼2000 K. In order to study the impact of H2 dissociation on ultra-hot Jupiters, we present a phase curve of KELT-9b observed with the Spitzer Space Telescope at 4.5 μm. KELT-9b is the hottest known transiting planet, with a 4.5 μm dayside brightness temperature of and a nightside temperature of . We observe a phase curve amplitude of 0.609 ± 0.020 and an offset of . The observed amplitude is too small to be explained by a simple balance between radiation and advection. General circulation models (GCMs) and an energy balance model that include the effects of H2 dissociation and recombination provide a better match to the data. The GCMs, however, predict a maximum phase offset of 5°, which disagrees with our observations at >5σ confidence. This discrepancy may be due to magnetic effects in the planet’s highly ionized atmosphere.

Temporal Variability in Hot Jupiter Atmospheres

Abstract Hot Jupiters receive intense incident stellar light on their daysides, which drives vigorous atmospheric circulation that attempts to erase their large dayside-to-nightside flux contrasts. Propagating waves and instabilities in hot Jupiter atmospheres can cause emergent properties of the atmosphere to be time-variable. In this work, we study such weather in hot Jupiter atmospheres using idealized cloud-free general circulation models with double-gray radiative transfer. We find that hot Jupiter atmospheres can be time-variable at the ∼0.1%–1% level in globally averaged temperature and at the ∼1%–10% level in globally averaged wind speeds. As a result, we find that observable quantities are also time-variable: the secondary eclipse depth can be variable at the ≲2% level, the phase-curve amplitude can change by ≲1%, the phase-curve offset can shift by ≲5°, and terminator-averaged wind speeds can vary by ≲2 km s−1. Additionally, we calculate how the eastern and western limb-averaged wind speeds vary with incident stellar flux and the strength of an imposed drag that parameterizes Lorentz forces in partially ionized atmospheres. We find that the eastern limb is blueshifted in models over a wide range of equilibrium temperature and drag strength, while the western limb is only redshifted if equilibrium temperatures are ≲1500 K and drag is weak. Lastly, we show that temporal variability may be observationally detectable in the infrared through secondary eclipse observations with the James Webb Space Telescope, phase-curve observations with future space telescopes (e.g., ARIEL), and/or Doppler wind speed measurements with high-resolution spectrographs.

The Atmospheric Circulation of Ultra-hot Jupiters

Abstract Recent observations of ultra-hot Jupiters with dayside temperatures in excess of 2500 K have found evidence for new physical processes at play in their atmospheres. In this work, we investigate the effects of the dissociation of molecular hydrogen and recombination of atomic hydrogen on the atmospheric circulation of ultra-hot Jupiters. To do so, we incorporate these effects into a general circulation model (GCM) for hot Jupiter atmospheres and run a large suite of models varying the incident stellar flux, rotation period, and strength of frictional drag. We find that including hydrogen dissociation and recombination reduces the fractional day-to-night temperature contrast of ultra-hot Jupiter atmospheres and causes the speed of the equatorial jet to decrease in simulations with fixed rotation. This is because the large energy input required for hydrogen dissociation cools the dayside of the planet, and the energy released due to hydrogen recombination warms the nightside. The resulting decrease in the day-to-night temperature contrast reduces the day-to-night pressure gradient that drives the circulation, resulting in weaker wind speeds. The results from our GCM experiments qualitatively agree with previous theory that found that the fractional day–night temperature contrast of ultra-hot Jupiters should decrease with increasing equilibrium temperature owing to hydrogen dissociation and recombination. Lastly, we compute full-phase light curves from our suite of GCM experiments, finding that the reduced day-to-night temperature contrast in ultra-hot Jupiter atmospheres causes a smaller phase curve amplitude. The reduction in phase curve amplitude due to hydrogen dissociation and recombination could explain the relatively small phase curve amplitudes of observed ultra-hot Jupiters.

Variability in the Atmosphere of the Hot Jupiter Kepler-76b

Abstract 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 data set well suited for exploring phase curve variability, which provides additional insights into atmospheric dynamics. Observations of the hot Jupiter Kepler-76b span more than 1000 days, providing an ideal data set to search for atmospheric variability. In this study, we find that Kepler-76b’s secondary eclipse, with a depth of 87 ± 6 ppm, corresponds to an effective temperature of 2830 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 eastward of the substellar 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 feedback loop. Looking forward, the Transiting Exoplanet Survey Satellite (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.

Linking the Climate and Thermal Phase Curve of 55 Cancri e

Abstract The thermal phase curve of 55 Cancri e is the first measurement of the temperature distribution of a tidally locked super-Earth, but raises a number of puzzling questions about the planet’s climate. The phase curve has a high amplitude and peak offset, suggesting that it has a significant eastward hot-spot shift as well as a large day–night temperature contrast. We use a general circulation model to model potential climates, and investigate the relation between bulk atmospheric composition and the magnitude of these seemingly contradictory features. We confirm theoretical models of tidally locked circulation are consistent with our numerical model of 55 Cnc e, and rule out certain atmospheric compositions based on their thermodynamic properties. Our best-fitting atmosphere has a significant hot-spot shift and day–night contrast, although these are not as large as the observed phase curve. We discuss possible physical processes that could explain the observations, and show that night-side cloud formation from species such as SiO from a day-side magma ocean could potentially increase the phase curve amplitude and explain the observations. We conclude that the observations could be explained by an optically thick atmosphere with a low mean molecular weight, a surface pressure of several bars, and a strong eastward circulation, with night-side cloud formation a possible explanation for the difference between our model and the observations.

3.6 AND 4.5 μm PHASE CURVES AND EVIDENCE FOR NON-EQUILIBRIUM CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HD 189733b

We present new, full-orbit observations of the infrared phase variations of the canonical hot Jupiter HD 189733b obtained in the 3.6 and 4.5 micron bands using the Spitzer Space Telescope. When combined with previous phase curve observations at 8.0 and 24 micron, these data allow us to characterize the exoplanet's emission spectrum as a function of planetary longitude. We utilize improved methods for removing the effects of intrapixel sensitivity variations and accounting for the presence of time-correlated noise in our data. We measure a phase curve amplitude of 0.1242% +/- 0.0061% in the 3.6 micron band and 0.0982% +/- 0.0089% in the 4.5 micron band. We find that the times of minimum and maximum flux occur several hours earlier than predicted for an atmosphere in radiative equilibrium, consistent with the eastward advection of gas by an equatorial super-rotating jet. The locations of the flux minima in our new data differ from our previous observations at 8 micron, and we present new evidence indicating that the flux minimum observed in the 8 micron is likely caused by an over-shooting effect in the 8 micron array. We obtain improved estimates for HD 189733b's dayside planet-star flux ratio of 0.1466% +/- 0.0040% at 3.6 micron and 0.1787% +/- 0.0038% at 4.5 micron; these are the most accurate secondary eclipse depths obtained to date for an extrasolar planet. We compare our new dayside and nightside spectra for HD 189733b to the predictions of models from Burrows et al. (2008) and Showman et al. (2009). We find that HD 189733b's 4.5 micron nightside flux is 3.3 sigma smaller than predicted by the Showman et al. models, which assume that the chemistry is in local thermal equilibrium. We conclude that this discrepancy is best-explained by vertical mixing, which should lead to an excess of CO and correspondingly enhanced 4.5 micron absorption in this region. [abridged]

A NEW 24 μm PHASE CURVE FOR υ ANDROMEDAE b

We report the detection of 24 micron variations from the planet-hosting upsilon Andromedae system consistent with the orbital periodicity of the system's innermost planet, upsilon And b. We find a peak-to-valley phase curve amplitude of 0.00130 times the mean system flux. Using a simple model with two hemispheres of constant surface brightness and assuming a planetary radius of 1.3 Jupiter radii gives a planetary temperature contrast of >900 K and an orbital inclination of >28 degrees. We further report the largest phase offset yet observed for an extrasolar planet: the flux maximum occurs ~80 degrees before phase 0.5. Such a large phase offset is difficult to reconcile with most current atmospheric circulation models. We improve on earlier observations of this system in several important ways: (1) observations of a flux calibrator star demonstrate the MIPS detector is stable to 10^-4 on long timescales, (2) we note that the background light varies systematically due to spacecraft operations, precluding use of this background as a flux calibrator (stellar flux measured above the background is not similarly affected), and (3) we calibrate for flux variability correlated with motion of the star on the MIPS detector. A reanalysis of our earlier observations of this system is consistent with our new result.

ON THE METHOD TO INFER AN ATMOSPHERE ON A TIDALLY LOCKED SUPER EARTH EXOPLANET AND UPPER LIMITS TO GJ 876d

We develop a method to infer or rule out the presence of an atmosphere on a tidally-locked hot super Earth. The question of atmosphere retention is a fundamental one, especially for planets orbiting M stars due to the star's long-duration active phase and corresponding potential for stellar-induced planetary atmospheric escape and erosion. Tidally-locked planets with no atmosphere are expected to show a Lambertian-like thermal phase curve, causing the combined light of the planet-star system to vary with planet orbital phase. We report Spitzer 8 micron IRAC observations of GJ 876 taken over 32 continuous hours and reaching a relative photometric precision of 3.9e-04 per point for 25.6 s time sampling. This translates to a 3 sigma limit of 5.13e-05 on a planet thermal phase curve amplitude. Despite the almost photon-noise limited data, we are unable to conclusively infer the presence of an atmosphere or rule one out on the non-transiting short-period super Earth GJ 876d. The limiting factor in our observations was the miniscule, monotonic photometric variation of the slightly active host M star, because the partial sine wave due to the planet has a component in common with the stellar linear trend. The proposed method is nevertheless very promising for transiting hot super Earths with the James Webb Space Telescope and is critical for establishing observational constraints for atmospheric escape.