A Framework for Characterizing the Atmospheres of Low-Mass Low-Density Transiting Planets

Fortney, Jonathan J.; Mordasini, Christoph; Nettelmann, Nadine; Kempton, Eliza M.-R.; Greene, Thomas P.; Zahnle, Kevin

doi:10.1088/0004-637x/775/1/80

Cited by 276 publications

(288 citation statements)

References 144 publications

(208 reference statements)

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“…A systematic noise floor value of 50 ppm was adopted for the simulations following the framework outlined in Fortney et al (2013) with additional photon noise estimated using the MRS throughput (Glasse et al 2010). As discussed in Sect.…”

Section: James Webb Space Telescopementioning

confidence: 99%

Transmission spectral properties of clouds for hot Jupiter exoplanets

Wakeford

Sing

2015

A&A

189

205

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Clouds play an important role in the atmospheres of planetary bodies. It is expected that, like all the planetary bodies in our solar system, exoplanet atmospheres will also have substantial cloud coverage, and evidence is mounting for clouds in a number of hot Jupiters. To better characterise planetary atmospheres, we need to consider the effects these clouds will have on the observed broadband transmission spectra. Here we examine the expected cloud condensate species for hot Jupiter exoplanets and the effects of various grain sizes and distributions on the resulting transmission spectra from the optical to infrared, which can be used as a broad framework when interpreting exoplanet spectra. We note that significant infrared absorption features appear in the computed transmission spectrum, the result of vibrational modes between the key species in each condensate, which can potentially be very constraining. While it may be hard to differentiate between individual condensates in the broad transmission spectra, it may be possible to discern different vibrational bonds, which can distinguish between cloud formation scenarios, such as condensate clouds or photochemically generated species. Vibrational mode features are shown to be prominent when the clouds are composed of small sub-micron sized particles and can be associated with an accompanying optical scattering slope. These infrared features have potential implications for future exoplanetary atmosphere studies conducted with JWST, where such vibrational modes distinguishing condensate species can be probed at longer wavelengths.

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Section: James Webb Space Telescopementioning

confidence: 99%

Transmission spectral properties of clouds for hot Jupiter exoplanets

Wakeford

Sing

2015

A&A

189

205

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“…Motivated by previous observational work on GJ 436b, there have been recent suggestions along multiple theoretical lines that Neptune-class exoplanet atmospheres may commonly have extremely high metallicities, perhaps several hundred times solar, or higher. Fortney et al (2013) have suggested, based on atmospheric accretion of planetesimals in population synthesis Fig. 24.…”

Section: On the Atmospheric Modelmentioning

confidence: 99%

A global analysis ofSpitzerand new HARPS data confirms the loneliness and metal-richness of GJ 436 b

Lanotte

Gillon

Demory

et al. 2014

A&A

130

135

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Context. GJ 436b is one of the few transiting warm Neptunes for which a detailed characterisation of the atmosphere is possible, whereas its non-negligible orbital eccentricity calls for further investigation. Independent analyses of several individual datasets obtained with Spitzer have led to contradicting results attributed to the different techniques used to treat the instrumental effects. Aims. We aim at investigating these previous controversial results and developing our knowledge of the system based on the full Spitzer photometry dataset combined with new Doppler measurements obtained with the HARPS spectrograph. We also want to search for additional planets. Methods. We optimise aperture photometry techniques and the photometric deconvolution algorithm DECPHOT to improve the data reduction of the Spitzer photometry spanning wavelengths from 3-24 µm. Adding the high-precision HARPS radial velocity data, we undertake a Bayesian global analysis of the system considering both instrumental and stellar effects on the flux variation.Results. We present a refined radius estimate of R P = 4.10 ± 0.16 R ⊕ , mass M P = 25.4 ± 2.1 M ⊕ , and eccentricity e = 0.162 ± 0.004 for GJ 436b. Our measured transit depths remain constant in time and wavelength, in disagreement with the results of previous studies. In addition, we find that the post-occultation flare-like structure at 3.6 µm that led to divergent results on the occultation depth measurement is spurious. We obtain occultation depths at 3.6, 5.8, and 8.0 µm that are shallower than in previous works, in particular at 3.6 µm. However, these depths still appear consistent with a metal-rich atmosphere depleted in methane and enhanced in CO/CO 2 , although perhaps less than previously thought. We could not detect a significant orbital modulation in the 8 µm phase curve. We find no evidence of a potential planetary companion, stellar activity, or a stellar spin-orbit misalignment. Conclusions. Recent theoretical models invoking high-metallicity atmospheres for warm Neptunes are a reasonable match to our results, but we encourage new modelling efforts based on our revised data. Future observations covering a wide wavelength range of GJ 436b and other Neptune-class exoplanets will further illuminate their atmosphere properties, whilst future accurate radial velocity measurements might explain the eccentricity.

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“…We found that the opacity again remains virtually identical. This means that the small bodies would not substantially increase the opacity, but they would increase the mean molecular weight of the envelope gas in the layers that are sufficiently hot for grain evaporation (see Fortney et al 2013 for estimates of the mean molecular weight in protoplanetary atmosphere). A high mean molecular weight is known to reduce the critical core mass (Stevenson 1982;Hori & Ikoma 2011).…”

Section: Implications For the Metallicity Effect And Pebble Accretionmentioning

confidence: 99%

“…Instead, all mass is directly added to the core using the sinking approximation for the calculation of the core luminosity. The enrichment of the gaseous envelope is thus not considered, while it could be very important for a reduction of the critical core mass (Hori & Ikoma 2011), because high enrichments of Z ≈ 0.8 are expected for planets with masses between 1 and 10 M ⊕ (Fortney et al 2013).…”

Section: Core Accretion Model and Initial Conditionsmentioning

confidence: 99%

“…We will also take into account effects like the concurrent formation of several embryos , the effect of envelope enrichment by heavy elements (Fortney et al 2013), or the evaporation of the primordial H/He envelope of close-in planets (Jin et al 2014). This will help to better understand the role of the grain opacity in shaping the observed mass-radius relationship and thus bulk composition of the extrasolar planets.…”

Section: Introductionmentioning

confidence: 99%

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Grain opacity and the bulk composition of extrasolar planets

Mordasini

2014

A&A

100

114

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Context. We investigate the grain opacity κ gr in the atmosphere (outer radiative zone) of forming planets. This is important for the observed planetary mass-radius relationship since κ gr affects the primordial H/He envelope mass of low-mass planets and the critical core mass of giant planets. Aims. The goal of this study is to derive a simple analytical model for κ gr and to explore its implications for the atmospheric structure and resulting gas accretion rate. Methods. Our model is based on the comparison of the timescales of the most important microphysical processes. We consider grain settling in the Stokes and Epstein drag regime, growth by Brownian motion coagulation and differential settling, grain evaporation in hot layers, and grain advection due to the contraction of the envelope. With these timescales and the assumption of a radially constant grain flux, we derive the typical grain size, abundance, and opacity. Results. We find that the dominating growth process is differential settling. In this regime, κ gr has a simple functional form; it is given as 27Q/8Hρ in the Epstein regime in the outer atmosphere and as 2Q/Hρ for Stokes drag in the deeper layers. Grain growth leads to a typical radial structure of κ gr with high ISM-like values in the outer layers but a strong decrease towards the deeper parts where κ gr becomes so low that the grain-free molecular opacities take over. Conclusions. In agreement with earlier results, we find that κ gr is typically much lower than in the ISM. In retrospect, this suggests that classical giant planet formation models should have considered the grain-free case to be as equally meaningful as the full ISM opacity case. The equations also show that a higher dust input in the top layers does not strongly increase κ gr . This has two important implications. First, for the formation of giant planet cores via pebbles, there could be the adverse effect that pebbles tend to increase the grain input high in the atmosphere because of ablation. This could in principle increase the opacity, making giant planet formation difficult. Our study indicates that this potentially adverse effect is not important. Second, it means that a higher stellar [Fe/H] which presumably leads to a higher surface density of planetesimals only favors giant planet formation without being detrimental to it because of an increased κ gr . This corroborates the result that core accretion can explain the observed increase of the giant planet frequency with stellar [Fe/H].

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A Framework for Characterizing the Atmospheres of Low-Mass Low-Density Transiting Planets

Cited by 276 publications

References 144 publications

Transmission spectral properties of clouds for hot Jupiter exoplanets

Transmission spectral properties of clouds for hot Jupiter exoplanets

A global analysis ofSpitzerand new HARPS data confirms the loneliness and metal-richness of GJ 436 b

Grain opacity and the bulk composition of extrasolar planets

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