The Earth Similiarity Index

New indexing and surface temperature analysis of exoplanets

Authors:

Kashyap et al

Abstract:

Study of exoplanets is the holy grail of present research in planetary sciences and astrobiology. Analysis of huge planetary data from space missions such as CoRoT and Kepler is directed ultimately at finding a planet similar to Earth\-the Earth's twin, and answering the question of potential exo-habitability. The Earth Similarity Index (ESI) is a first step in this quest, ranging from 1 (Earth) to 0 (totally dissimilar to Earth). It was defined for the four physical parameters of a planet: radius, density, escape velocity and surface temperature. The ESI is further sub-divided into interior ESI (geometrical mean of radius and density) and surface ESI (geometrical mean of escape velocity and surface temperature). The challenge here is to determine which exoplanet parameter(s) is important in finding this similarity; how exactly the individual parameters entering the interior ESI and surface ESI are contributing to the global ESI. Since the surface temperature entering surface ESI is a non-observable quantity, it is difficult to determine its value. Using the known data for the Solar System objects, we established the calibration relation between surface and equilibrium temperatures to devise an effective way to estimate the value of the surface temperature of exoplanets for further analysis with our graphical methodology. ESI is a first step in determining potential exo-habitability that may not be very similar to a known life. A new approach, called Mars Similarity Index (MSI), is introduced to identify planets that may be habitable to the extreme forms of life.

The ExoMol database: molecular line lists for exoplanet and other hot atmospheres

The ExoMol database: molecular line lists for exoplanet and other hot atmospheres

Authors:

Tennyson et al

Abstract:

The ExoMol database (www.exomol.com) provides extensive line lists of molecular transitions which are valid over extended temperatures ranges. The status of the current release of the database is reviewed and a new data structure is specified. This structure augments the provision of energy levels (and hence transition frequencies) and Einstein A coefficients with other key properties, including lifetimes of individual states, temperature-dependent cooling functions, Land\'e g-factors, partition functions, cross sections, k-coefficients and transition dipoles with phase relations. Particular attention is paid to the treatment of pressure broadening parameters. The new data structure includes a definition file which provides the necessary information for utilities accessing ExoMol through its application programming interface (API). Prospects for the inclusion of new species into the database are discussed.

Analytical Models of hot Exoplanetary Atmospheres

Analytical Models of Exoplanetary Atmospheres. III. Gaseous C-H-O-N Chemistry with 9 Molecules

Authors:

Heng et al

Abstract:

We present novel, analytical, equilibrium-chemistry formulae for the abundances of molecules in hot exoplanetary atmospheres that include the carbon, oxygen and nitrogen networks. Our hydrogen-dominated solutions involve acetylene (C2H2), ammonia (NH3), carbon dioxide (CO2), carbon monoxide (CO), ethylene (C2H4), hydrogen cyanide (HCN), methane (CH4), molecular nitrogen (N2) and water (H2O). By considering only the gaseous phase, we prove that the mixing ratio of carbon monoxide is governed by a decic equation (polynomial equation of degree 10). We validate our solutions against numerical calculations of equilibrium chemistry that perform Gibbs free energy minimization and demonstrate that they are accurate for temperatures from 500--3000 K. In hydrogen-dominated atmospheres, the ratio of abundances of HCN to CH4 is nearly constant across a wide range of carbon-to-oxygen ratios, which makes it a robust diagnostic of the metallicity in the gas phase. Our validated formulae allow for the convenient benchmarking of chemical kinetics codes and provide an efficient way of enforcing chemical equilibrium in atmospheric retrieval calculations.

Simulating the Thermal Light Curve of Earth-like Exoplanets

Thermal light curves of Earth-like planets: 1. Varying surface and rotation on planets in a terrestrial orbit

Authors:

Gómez-Leal et al

Abstract:

The integrated thermal emission of an exoplanet and its variations along the orbital motion can carry information about the climatic conditions and the rotation of the planet. In this study, we use the LMDZ 3D Global Climate Model (GCM) to simulate the climate of a synthetic Earth and three quasi-Earth configurations: a slowly rotating Earth, an ocean-covered Earth and its snowball counterpart. We also generate the time-dependent broadband thermal emission of the planet from these simulations. In a first step, we validate the model by comparing the synthetic Earth emission with the actual emission of our planet as constrained by observations. Then, we determine the main properties of the climate and emission of the three Earth-like planets and compare them to those of the Earth. We show that planets with an uneven distribution of continents exhibit a maximum of emission during the summer of the hemisphere with larger continental masses, and they may exhibit a maximum of emission at apastron. Large convective clouds might form over the continents of slow rotating planets, having an important effect over their climate and their emission. We also show that, in all the modeled cases, the equilibrium temperature, the Bond albedo and the rotation period can in theory be retrieved from the light curve by a distant observer. The values obtained at transiting geometries have a low deviation from the global values for cases with an axis tilt similar to that of the Earth, and we are able to distinguish between the four planets presented here by the data obtained from their light curves. However, this might not be the case under different conditions.

Temperature has Huge Impacts on the Structure and Volume of Water Rich SuperEarths

In hot water: effects of temperature-dependent interiors on the radii of water-rich super-Earths

Authors:

Thomas et al

Abstract:

Observational advancements are leading to increasingly precise measurements of super-Earth masses and radii. Such measurements are used in internal structure models to constrain interior compositions of super-Earths. It is now critically important to quantify the effect of various model assumptions on the predicted radii. In particular, models often neglect thermal effects, a choice justified by noting that the thermal expansion of a solid Earth-like planet is small. However, the thermal effects for water-rich interiors may be significant. We have systematically explored the extent to which thermal effects can influence the radii of water-rich super-Earths over a wide range of masses, surface temperatures, surface pressures and water mass fractions. We developed temperature-dependent internal structure models of water-rich super-Earths that include a comprehensive temperature-dependent water equation of state. We found that thermal effects induce significant changes in their radii. For example, for super-Earths with 10 per cent water by mass, the radius increases by up to 0.5R⊕ when the surface temperature is increased from 300 to 1000K, assuming a surface pressure of 100bar and an adiabatic temperature gradient in the water layer. The increase is even larger at lower surface pressures and/or higher surface temperatures, while changing the water fraction makes only a marginal difference. These effects are comparable to current super-Earth radial measurement errors, which can be better than 0.1R⊕. It is therefore important to ensure that the thermal behaviour of water is taken into account when interpreting super-Earth radii using internal structure models.

The Differences Between Dayside-Nightside Temperature of Hot Jupiters

Atmospheric Circulation of Hot Jupiters: Dayside-Nightside Temperature Differences

Authors:

Komacek et al

Abstract:

The full-phase infrared light curves of low-eccentricity hot Jupiters show a trend of increasing dayside-to-nightside brightness temperature difference with increasing equilibrium temperature. Here we present a three-dimensional model that explains this relationship, in order to shed insight on the processes that control heat redistribution in tidally-locked planetary atmospheres. This three-dimensional model combines predictive analytic theory for the atmospheric circulation and dayside-nightside temperature differences over a range of equilibrium temperature, atmospheric composition, and potential frictional drag strengths with numerical solutions of the circulation that verify this analytic theory. This analytic theory shows that the longitudinal propagation of waves mediates dayside-nightside temperature differences in hot Jupiter atmospheres, analogous to the wave adjustment mechanism that regulates the thermal structure in Earth's tropics. These waves can be damped in hot Jupiter atmospheres by either radiative cooling or potential frictional drag. This frictional drag would likely be caused by Lorentz forces in a partially ionized atmosphere threaded by a background magnetic field, and would increase in strength with increasing temperature. Additionally, the amplitude of radiative heating and cooling increases with increasing temperature, and hence both radiative heating/cooling and frictional drag damp waves more efficiently with increasing equilibrium temperature. Radiative heating and cooling play the largest role in controlling day-night temperature temperature differences in both our analytic theory and numerical simulations, with frictional drag only being important if it is stronger than the Coriolis force. As a result, dayside-nightside temperature differences in hot Jupiter atmospheres increase with increasing stellar irradiation and decrease with increasing pressure.

Hot Jupiter WASP-33b's Atmosphere Does NOT Exhibit Temperature Inversion

A temperature inversion in WASP-33b? Large Binocular Telescope occultation data confirm significant thermal flux at short wavelengths

Authors:

von Essen

Abstract:

We observed a secondary eclipse of WASP-33b quasi-simultaneously in the optical (~0.55 {\mu}m) and the near-infrared (~1.05 {\mu}m) using the 2x8.4 m Large Binocular Telescope. WASP-33 is a {\delta} Scuti star pulsating with periods comparable to the eclipse duration, making the determination of the eclipse depth challenging. We use previously determined oscillation frequencies to model and remove the pulsation signal from the light curves, isolating the secondary eclipse. The determined eclipse depth is dF = 1.03 +/- 0.34 parts per thousand, corresponding to a brightness temperature of Tb = 3398 +/- 302 K. Combining previously published data with our new measurement we find the equilibrium temperature of WASP-33b to be Tb = 3358 +/- 165 K. We compare all existing eclipse data to a blackbody spectrum, to a carbon-rich non-inverted model and to a solar composition model with an inverted temperature structure. We find that current available data on WASP-33b's atmosphere can be best represented by a simple blackbody emission, without the need for more sophisticated atmospheric models with temperature inversions. Although our data cannot rule out models with or without a temperature inversion, they do confirm a high brightness temperature for the planet at short wavelengths. WASP-33b is one of the hottest exoplanets known till date, and its equilibrium temperature is consistent with rapid reradiation of the absorbed stellar light and a low albedo.

ESTM: a new Model for Simulating Terrestrial Exoplanet Surface Temperatures

Modeling the surface temperature of Earth-like planets

Authors:

Vladilo et al

Abstract:

We introduce a novel Earth-like planet surface temperature model (ESTM) for habitability studies based on the spatial-temporal distribution of planetary surface temperatures. The ESTM adopts a surface Energy Balance Model complemented by: radiative-convective atmospheric column calculations, a set of physically-based parameterizations of meridional transport, and descriptions of surface and cloud properties more refined than in standard EBMs. The parameterization is valid for rotating terrestrial planets with shallow atmospheres and moderate values of axis obliquity (epsilon greater than or equal to 45^o). Comparison with a 3D model of atmospheric dynamics from the literature shows that the equator-to-pole temperature differences predicted by the two models agree within ~5K when the rotation rate, insolation, surface pressure and planet radius are varied in the intervals 0.5 less than or equal to Omega/Omega_o less than or equal 2, 0.75 less than or equal S/S_o less than or equal 1.25, 0.3 less than or equal p/(1 bar) less than or equal 10, and 0.5 less than or equal R/R_o less than or equal 2, respectively. The ESTM has an extremely low computational cost and can be used when the planetary parameters are scarcely known (as for most exoplanets) and/or whenever many runs for different parameter configurations are needed. Model simulations of a test-case exoplanet (Kepler-62e) indicate that an uncertainty in surface pressure within the range expected for terrestrial planets may impact the mean temperature by ~60 K. Within the limits of validity of the ESTM, the impact of surface pressure is larger than that predicted by uncertainties in rotation rate, axis obliquity, and ocean fractions. We discuss the possibility of performing a statistical ranking of planetary habitability taking advantage of the flexibility of the ESTM.

Characterization Hot Jupiter HAT-P-32Ab's Atmosphere

Characterization of the atmosphere of the hot Jupiter HAT-P-32Ab and the M-dwarf companion HAT-P-32B

Authors:

Zhao et al

Abstract:

We report secondary eclipse photometry of the hot Jupiter HAT-P-32Ab, taken with Hale/WIRC in H and Ks bands and with Spitzer/IRAC at 3.6 and 4.5 micron. We carried out adaptive optics imaging of the planet host star HAT-P-32A and its companion HAT-P-32B in the near-IR and the visible. We clearly resolve the two stars from each other and find a separation of 2.923" +/- 0. 004" and a position angle 110.64 deg +/- 0.12 deg. We measure the flux ratios of the binary in g' r' i' z' and H & Ks bands, and determine Teff = 3565 +/- 82 K for the companion star, corresponding to an M1.5 dwarf. We use PHOENIX stellar atmosphere models to correct the dilution of the secondary eclipse depths of the hot Jupiter due to the presence of the M1.5 companion. We also improve the secondary eclipse photometry by accounting for the non-classical, flux-dependent nonlinearity of the WIRC IR detector in the H band. We measure planet-to-star flux ratios of 0.090 +/- 0.033%, 0.178 +/- 0.057%, 0.364 +/- 0.016%, and 0.438 +/- 0.020% in the H, Ks, 3.6 and 4.5 micron bands, respectively. We compare these with planetary atmospheric models, and find they prefer an atmosphere with a temperature inversion and inefficient heat redistribution. However, we also find that the data are equally well-described by a blackbody model for the planet with Tp = 2042 +/- 50 K. Finally, we measure a secondary eclipse timing offset of 0.3 +/- 1.3 min from the predicted mid-eclipse time, which constrains e = 0.0072 +0.0700/-0.0064 when combined with RV data and is more consistent with a circular orbit.

WISE 0855−0714 is the Coldest Brown Dwarf

Temperature constraints on the coldest brown dwarf known WISE 0855-0714

Authors:

Beamín et al

Abstract:

Context.

Nearby isolated planetary mass objects are beginning to be discovered, but their individual properties are poorly constrained because their low surface temperatures and strong molecular self-absorption make them extremely faint.

Aims.

We aimed to detect the near infrared emission of the coldest brown dwarf (BD) found so far WISE0855−0714, located ∼2.2 pc away, and to improve its temperature estimate (Teff= 225-260 K) from a comparison with state of the art models of BD atmospheres.
Methods. We observed the field containing WISE0855-0714 with HAWK-I at the VLT in the Y band. For BDs with Teff less than 500 K theoretical models predict strong signal (or rather less molecular absorption) in this band.

Results.

WISE0855-0714 was not detected in our Y-band images, thus placing an upper limit on its brightness to Y greater than 24.4 mag at 3-σ level, leading to Y-[4.5] greater than 10.5. Combining this limit with previous detections and upper limits at other wavelengths, WISE0855-0714 is confirmed as the reddest BD detected. We applied spectral energy distribution fitting with collections of models from two independent groups for extremely cool BD atmospheres leading to an effective temperature of Teff less than 250 K, making WISE0855-0714 the coldest BD discovered to date.

Hot Jupiter HD 209458b Observed to Have Atmospheric Superrotation

The 4.5 μm full-orbit phase curve of the hot Jupiter HD 209458b

Authors:

Zellem et al

Abstract:

he hot Jupiter HD 209458b is particularly amenable to detailed study as it is among the brightest transiting exoplanet systems currently known (V-mag = 7.65; K-mag = 6.308) and has a large planet-to-star contrast ratio. HD 209458b is predicted to be in synchronous rotation about its host star with a hot spot that is shifted eastward of the substellar point by superrotating equatorial winds. Here we present the first full-orbit observations of HD 209458b, in which its 4.5 $\mu$m emission was recorded with $Spitzer$/IRAC. Our study revises the previous 4.5 $\mu$m measurement of HD 209458b's secondary eclipse emission downward by $\sim$35 % to $0.1391\%^{+0.0072\%}_{-0.0069\%}$,changing our interpretation of the properties of its dayside atmosphere. We find that the hot spot on the planet's dayside is shifted eastward of the substellar point by $40.9^{\circ}\pm{6.0^{\circ}}$, in agreement with circulation models predicting equatorial superrotation. HD 209458b's dayside (T$_{bright}$ = 1499 $\pm$ 15 K) and nightside (T$_{bright}$ = 972 $\pm$ 44 K) emission indicates a day-to-night brightness temperature contrast smaller than that observed for more highly irradiated exoplanets, suggesting that the day-to-night temperature contrast may be partially a function of the incident stellar radiation. The observed phase curve shape deviates modestly from global circulation model predictions potentially due to disequilibrium chemistry or deficiencies in the current hot CH$_{4}$ line lists used in these models. Observations of the phase curve at additional wavelengths are needed in order to determine the possible presence and spatial extent of a dayside temperature inversion, as well as to improve our overall understanding of this planet's atmospheric circulation.

Temperatures & Albedos of 20 Confirmed Kepler Gas Giants

A comprehensive study of Kepler phase curves and secondary eclipses -- temperatures and albedos of confirmed Kepler giant planets

Authors:

Angerhausen

Abstract:

We present a comprehensive study of phase curves and secondary eclipses in the Kepler data set using all available data from 15 quarters. Our original sample consists of 489 Kepler Objects of Interest (KOI) with R_p > 4 R_e , P < 10d, V_mag < 15 from the latest data release. Here we focus on 20 confirmed planets from that sample and derive their temperatures and albedos. Our results confirm and in most cases improve parameters derived by previous studies. We present new results for Kepler 1b-8b, 12b-15b, 17b, 40b, 41b, 43b, 44b, 76b, 77b, and 412b derived in a consistent manner. Furthermore we present a lightcurve analysis of Kepler 91b and Kepler 74b. Both show extra dimmings at times other than of the expected primary and secondary eclipses. Corrected for thermal emission we find the 20 planets we analyzed separate into two groups of high (>0.1) and low (<0 .1="" albedo.="" albedos="" all="" any="" are="" comment-0--="" correlation="" from="" however="" in="" low="" massive="" most="" no="" or="" our="" parameters.="" planetary="" planets="" sample="" significant="" stellar="" the="" to="" with="">