Exoplanet discoveries of recent years have provided a great deal of new data for studying the bulk compositions of giant planets. Here we identify 47 transiting giant planets (20M ⊕ < M < 20M J ) whose stellar insolation is low enough (F * < 2 × 10 8 erg s −1 cm −2 , or roughly T eff < 1000) that they are not affected by the hot Jupiter radius inflation mechanism(s). We compute a set of new thermal and structural evolution models and use these models in comparison with properties of the 47 transiting planets (mass, radius, age) to determine their heavy element masses. A clear correlation emerges between the planetary heavy element mass M z and the total planet mass, approximately of the form M z ∝ √ M . This finding is consistent with the core accretion model of planet formation. We also study how stellar metallicity [Fe/H] affects planetary metal-enrichment and find a weaker correlation than has been previously reported from studies with smaller sample sizes. We confirm a strong relationship between the planetary metal-enrichment relative to the parent star Z planet /Z star and the planetary mass, but see no relation in Z planet /Z star with planet orbital properties or stellar mass. The large heavy element masses of many planets (> 50 M ⊕ ) suggest significant amounts of heavy elements in H/He envelopes, rather than cores, such that metal-enriched giant planet atmospheres should be the rule. We also discuss a model of coreaccretion planet formation in a one-dimensional disk and show that it agrees well with our derived relation between mass and Z planet /Z star .

The cause of hot Jupiter radius inflation, where giant planets with T eq > 1000 K are significantly larger than expected, is an open question and the subject of many proposed explanations. Many of these hypotheses postulate an additional anomalous power which heats planets' convective interiors, leading to larger radii. Rather than examine these proposed models individually, we determine what anomalous powers are needed to explain the observed population's radii, and consider which models are most consistent with this. We examine 281 giant planets with well-determined masses and radii and apply thermal evolution and Bayesian statistical models to infer the anomalous power as a fraction of (and varying with) incident flux (F ) that best reproduces the observed radii. First, we observe that the inflation of planets below about M = 0.5 M J appears very different than their higher mass counterparts, perhaps as the result of mass loss or an inefficient heating mechanism. As such, we exclude planets below this threshold. Next, we show with strong significance that (F ) increases with T eq towards a maximum of ∼ 2.5% at T eq ≈ 1500 K, and then decreases as temperatures increase further, falling to ∼ 0.2% at T eff = 2500 K. This high-flux decrease in inflation efficiency was predicted by the Ohmic dissipation model of giant planet inflation but not other models. We also show that the thermal tides model predicts far more variance in radii than is observed. Thus, our results provide evidence for the Ohmic dissipation model and a functional form for (F ) that any future theories of hot Jupiter radii can be tested against.

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The Neptune-mass GJ 436b is one of the most studied transiting exoplanets with repeated measurements of its thermal emission and transmission spectra. We build on previous studies to answer outstanding questions about this planet, including its potentially high metallicity and tidal heating of its interior. We present new observations of GJ 436b's thermal emission at 3.6 and 4.5 μm, which reduce uncertainties in estimates of GJ 436b's flux at those wavelengths and demonstrate consistency between Spitzer observations spanning more than 7 yr. We analyze the Spitzer thermal emission photometry and Hubble WFC3 transmission spectrum. We use a dual-pronged modeling approach of both self-consistent and retrieval models. We vary the metallicity, intrinsic luminosity from tidal heating, disequilibrium chemistry, and heat redistribution. We also study clouds and photochemical hazes, but do not find strong evidence for either. The self-consistent and retrieval models combine to suggest that GJ 436b has a high atmospheric metallicity, with best fits at or above several hundred times solar metallicity, tidal heating warming its interior with best-fit intrinsic effective temperatures around 300-350 K, and disequilibrium chemistry. High metal enrichments (>600× solar) occur from the accretion of rocky, rather than icy, material. Assuming the interior temperature T int ∼300-350 K, we find a dissipation factor Q′∼2×10 5 -10 6 , larger than Neptune's Q′, implying a long tidal circularization timescale for the orbit. We suggest that Neptune-mass planets may be more diverse than imagined, with metal enhancements spanning several orders of magnitude, to perhaps over 1000× solar metallicity. High-fidelity observations with instruments like the James Webb Space Telescope will be critical for characterizing this diversity.

The Noble Element Simulation Technique (NEST) is an exhaustive collection of models explaining both the scintillation light and ionization yields of noble elements as a function of particle type (nuclear recoil, electron recoil, alphas), electric field, and incident energy or energy loss (dE/dx). It is packaged as C++ code for Geant4 that implements said models, overriding the default model which does not account for certain complexities, such as the reduction in yields for nuclear recoils (NR) compared to electron recoils (ER). We present here improvements to the existing NEST models and updates to the code which make the package even more realistic and turn it into a more full-fledged Monte Carlo simulation. All available liquid xenon data on NR and ER to date have been taken into consideration in arriving at the current models. Furthermore, NEST addresses the question of the magnitude of the light and charge yields of nuclear recoils, including their electric field dependence, thereby shedding light on the possibility of detection or exclusion of a low-mass dark matter WIMP by liquid xenon detectors.

The super-Neptune exoplanet WASP-107b is an exciting target for atmosphere characterization. It has an unusually large atmospheric scale height and a small, bright host star, raising the possibility of precise constraints on its current nature and formation history. We report the first atmospheric study of WASP-107b, a Hubble Space Telescope measurement of its near-infrared transmission spectrum. We determined the planet's composition with two techniques: atmospheric retrieval based on the transmission spectrum and interior structure modeling based on the observed mass and radius. The interior structure models set a 3 σ upper limit on the atmospheric metallicity of 30× solar. The transmission spectrum shows strong evidence for water absorption (6.5 σ confidence), and the retrieved water abundance is consistent with expectations for a solar abundance pattern. The inferred carbon-to-oxygen ratio is subsolar at 2.7 σ confidence, which we attribute to possible methane depletion in the atmosphere. The spectral features are smaller than predicted for a cloud-free composition, crossing less than one scale height. A thick condensate layer at high altitudes (0.1 -3 mbar) is needed to match the observations. We find that physically motivated cloud models with moderate sedimentation efficiency (f sed = 0.3) or hazes with a particle size of 0.3 µm reproduce the observed spectral feature amplitude. Taken together, these findings serve as an illustration of the diversity and complexity of exoplanet atmospheres. The community can look forward to more such results with the high precision and wide spectral coverage afforded by future observing facilities.

In giant planet atmosphere modelling, the intrinsic temperature T int and radiative-convective boundary (RCB) are important lower boundary conditions. Often in one-dimensional radiative-convective models and in three-dimensional general circulation models it is assumed that T int is similar to that of Jupiter itself, around 100 K, which yields a RCB around 1 kbar for hot Jupiters. In this work, we show that the inflated radii, and hence high specific entropy interiors (8-11 k b / baryon), of hot Jupiters suggest much higher T int . Assuming the effect is primarily due to current heating (rather than delayed cooling), we derive an equilibrium relation between T eq and T int , showing that the latter can take values as high as 700 K. In response, the RCB moves upward in the atmosphere. Using onedimensional radiative-convective atmosphere models, we find RCBs of only a few bars, rather than the kilobar typically supposed. This much shallower RCB has important implications for the atmospheric structure, vertical and horizontal circulation, interpretation of atmospheric spectra, and the effect of deep cold traps on cloud formation.

The atmospheric pressure–temperature profiles for transiting giant planets cross a range of chemical transitions. Here we show that the particular shapes of these irradiated profiles for warm giant planets below ∼1300 K lead to striking differences in the behavior of nonequilibrium chemistry compared to brown dwarfs of similar temperatures. Our particular focus is H2O, CO, CH4, CO2, and NH3 in Jupiter- and Neptune-class planets. We show that the cooling history of a planet, which depends most significantly on planetary mass and age, can have a dominant effect on abundances in the visible atmosphere, often swamping trends one might expect based on T eq alone. The onset of detectable CH4 in spectra can be delayed to lower T eq for some planets compared to equilibrium, or pushed to higher T eq. The detectability of NH3 is typically enhanced compared to equilibrium expectations, which is opposite to the brown dwarf case. We find that both CH4 and NH3 can become detectable at around the same T eq (at T eq values that vary with mass and metallicity), whereas these “onset” temperatures are widely spaced for brown dwarfs. We suggest observational strategies to search for atmospheric trends and stress that nonequilibrium chemistry and clouds can serve as probes of atmospheric physics. As examples of atmospheric complexity, we assess three Neptune-class planets, GJ 436b, GJ 3470b, and WASP-107, all around T eq = 700 K. Tidal heating due to eccentricity damping in all three planets heats the deep atmosphere by thousands of degrees and may explain the absence of CH4 in these cool atmospheres. Atmospheric abundances must be interpreted in the context of physical characteristics of the planet.