Exoplanet Clouds

Helling, Christiane

doi:10.1146/annurev-earth-053018-060401

Cited by 88 publications

(76 citation statements)

References 90 publications

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“…Al 2 O 3 is present in small concentrations at the high altitudes that we probe (Figure 9), but its distribution in this region is primarily controlled by transport processes rather than condensation and nucleation (Gao & Benneke 2018). We assume that Fe and Mg 2 SiO 4 nucleate heterogeneously on TiO 2 particles, similar to the treatment of Helling (2018) and related works. Although Fe can nucleate homogeneously as well, we do not consider it as this process may not be efficient (Lee et al 2018).…”

Section: Microphysical Cloud Modelmentioning

confidence: 72%

“…At high temperatures, we expect refractory species such as metal oxides, silicates, and sulphides to condense in exoplanetary atmospheres (e.g. Helling 2018;Powell et al 2018;Morley et al 2012). However, cloud formation is a complex process that depends on both microphysical processes, such as sedimentation, nucleation, and growth, and the material properties of the condensing species, many of which are highly uncertain or unknown (Helling 2018).…”

Section: Introductionmentioning

confidence: 99%

“…Helling 2018;Powell et al 2018;Morley et al 2012). However, cloud formation is a complex process that depends on both microphysical processes, such as sedimentation, nucleation, and growth, and the material properties of the condensing species, many of which are highly uncertain or unknown (Helling 2018). Consequently, the use of different underlying assumptions can lead to significantly different cloud properties, severely limiting the predictive power of these models.…”

Section: Introductionmentioning

confidence: 99%

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A Hubble PanCET Study of HAT-P-11b: A Cloudy Neptune with a Low Atmospheric Metallicity

Chachan

Knutson

Gao

et al. 2019

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We present the first comprehensive look at the 0.35−5 µm transmission spectrum of the warm (∼ 800 K) Neptune HAT-P-11b derived from thirteen individual transits observed using the Hubble and Spitzer Space Telescopes. Along with the previously published molecular absorption feature in the 1.1−1.7 µm bandpass, we detect a distinct absorption feature at 1.15 µm and a weak feature at 0.95 µm, indicating the presence of water and/or methane with a combined significance of 4.4 σ. We find that this planet's nearly flat optical transmission spectrum and attenuated near-infrared molecular absorption features are best-matched by models incorporating a high-altitude cloud layer. Atmospheric retrievals using the combined 0.35−1.7 µm HST transmission spectrum yield strong constraints on atmospheric cloudtop pressure and metallicity, but we are unable to match the relatively shallow Spitzer transit depths without under-predicting the strength of the near-infrared molecular absorption bands. HAT-P-11b's HST transmission spectrum is well-matched by predictions from our microphysical cloud models. Both forward models and retrievals indicate that HAT-P-11b most likely has a relatively low atmospheric metallicity (< 4.6 Z and < 86 Z at the 2σ and 3σ levels respectively), in contrast to the expected trend based on the solar system planets. Our work also demonstrates that the wide wavelength coverage provided by the addition of the HST STIS data is critical for making these inferences.

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Section: Microphysical Cloud Modelmentioning

confidence: 72%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A Hubble PanCET Study of HAT-P-11b: A Cloudy Neptune with a Low Atmospheric Metallicity

Chachan

Knutson

Gao

et al. 2019

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“…The spherical surface associated with R0 has previously been interpreted by Bétrémieux & Swain (2017, 2018 to represent the boundary associated with an opaque (optically thick) cloud deck. Furthermore, Bétrémieux & Swain (2017, 2018 claim that variations of equation (1) When computing the transmission spectrum, one needs to specify the cross section or opacity (cross section per unit mass) as a function of wavelength, temperature and pressure. Physically, the opacity function includes contributions from the extinction (absorption and scattering) of radiation by atoms, ions, molecules and aerosols/hazes/clouds, whether in the form of spectral lines or a continuum.…”

Section: Introductionmentioning

confidence: 99%

On physical interpretations of the reference transit radius of gas-giant exoplanets

Heng

2019

Monthly Notices of the Royal Astronomical Society

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Two theoretical quandaries involving transmission spectra of gas-giant exoplanets are elucidated. When computing the transit radius as a function of wavelength, one needs to specify a reference transit radius corresponding to a reference pressure. Mathematically, the reference transit radius is a constant of integration that originates from evaluating an integral for the transit depth. Physically, its interpretation has been debated in the literature. Jordán & Espinoza (2018) suggested that the optical depth is discontinuous across, and infinite below, the reference transit radius. Bétrémieux & Swain (2017, 2018 interpreted the spherical surface located at the reference transit radius to represent the boundary associated with an opaque cloud deck. It is demonstrated that continuous functions for the optical depth may be found. The optical depth below and at the reference transit radius need not take on special or divergent values. In the limit of a spatially uniform grey cloud with constant opacity, the transit chord with optical depth on the order of unity mimics the presence of a "cloud top". While the surface located at the reference pressure may mimic the presence of grey clouds, it is more natural to include the effects of these clouds as part of the opacity function because the cloud opacity may be computed from first principles. It is unclear how this mimicry extends to non-grey clouds comprising small particles.

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“…Several of the known exoplanets orbit their host star very closely such that a rocky surface turns into magma or Jupiter-like exoplanets expand their atmosphere considerably. Inside the atmospheres of exoplanets, clouds form and the cloud particles (also called aerosols) are made of a mix of minerals, for example silicates and iron oxide [1]. Spectroscopic studies of exoplanets are repeatedly frustrated by clouds blocking the view into the underlying atmosphere to reveal the existence of potential biosignatures.…”

Section: Introductionmentioning

confidence: 99%

Lightning in other planets

Helling

2019

J. Phys.: Conf. Ser.

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More than 4000 planet are known that orbit stars other than our Sun. Many harbor a dynamic atmosphere that is cold enough that cloud particles can form in abundance. The diversity of exoplanets leads to differences in cloud coverage depending on global system parameters. Some planets will be fully covered in clouds, some have clouds on the nightside but are largely cloud-free on the dayside. These cloud particles can easily be charged and lightning discharges will occur in cloudy, dynamic exoplanet atmosphere. Lightning supports a Global Electric Circuit (GCE) on Earth and we argue that exoplanet may develop a GCE in particular if parts of the exoplanet atmospheres can remain cloud free.

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Exoplanet Clouds

Cited by 88 publications

References 90 publications

A Hubble PanCET Study of HAT-P-11b: A Cloudy Neptune with a Low Atmospheric Metallicity

A Hubble PanCET Study of HAT-P-11b: A Cloudy Neptune with a Low Atmospheric Metallicity

On physical interpretations of the reference transit radius of gas-giant exoplanets

Lightning in other planets

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