Mineral snowflakes on exoplanets and brown dwarfs

Samra, D.; Helling, Ch.; Birnstiel, T.

doi:10.1051/0004-6361/202142651

Cited by 9 publications

(2 citation statements)

References 111 publications

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“…However, there is some tentative evidence of porous cloud particles (generated through collisions) as an explanation for the flat spectrum of the mini-Neptune GJ-1214b (Ohno et al 2020). Although, as shown in Samra et al (2022), collisions in exoplanet atmospheres when including turbulence are expected to be destructive rather than producing larger aggregate particles. However, hazes of fractal aggregates have been inferred for a number of bodies, both Solar system and exoplanets (e.g.…”

Section: Where Clouds On Wasp-96b Get Optically Thickmentioning

confidence: 93%

Clouds form on the hot Saturn JWST ERO target WASP-96b

Samra¹,

Helling²,

Chubb³

et al. 2022

Preprint

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Context. WASP-96b is a hot Saturn exoplanet, with an equilibrium temperature of ≈ 1300 K, well within the regime of thermodynamically expected extensive cloud formation. Prior observations with Hubble/WFC3, Spitzer/IRAC, and VLT/FORS2 have been combined into a single spectra for which retrievals suggest a cold but cloud-free atmosphere. Recently, the planet was observed with the James Webb Space Telescope (JWST) as part of the Early Release Observations (ERO). Aims. The formation of clouds in the atmosphere of the exoplanet WASP-96b is explored. Methods. 1D profiles are extracted from the 3D GCM expeRT/MITgcm results and used as input for a kinetic, non-equilibrium model to study the formation of mineral cloud particles of mixed composition. The ARCiS retrieval framework is applied to the pre-JWST WASP-96b transit spectra to investigate the apparent contradiction between cloudy models and assumed cloud-free transit spectra. Results. Clouds are predicted to be ubiquitous throughout the atmosphere of WASP-96b. Silicate materials contribute between 40% and 90%, hence, also metal oxides do contribute with up to 40% in the low-pressure regimes that effect the spectra. We explore how to match these cloudy models with currently available atmospheric transit spectra. A reduced vertical mixing acts to settle clouds to deeper in the atmosphere, and an increased cloud particles porosity reduces the opacity of clouds in the near-IR and optical region. Both these effects allow for clearer molecular features to be observed, while still allowing clouds to be in the atmosphere. Conclusions. The atmosphere of WASP-96b is unlikely to be cloud free. Also retrievals of HST, Spitzer and VLT spectra show that multiple cloudy solutions reproduce the data. JWST observations will be affected by clouds, where within even the NIRISS wavelength range the cloud top pressure varies by an order of magnitude. The long wavelength end of NIRSpec and short end of MIRI may probe atmospheric asymmetries between the limbs of the terminator on WASP-96b. Key words. planets and satellites: individual: WASP-96b -planets and satellites: atmospheres -planets and satellites: gaseous planets -planets and satellites: fundamental parameters was assumed because of broad Na I line wings by (Nikolov et al. 2018). Nikolov et al. (2022 more recently used space-based instruments to observe further primary transits of WASP-96b, including the Infrared Array Camera (IRAC) on the Spitzer Space Telescope (Spitzer), and the Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope (HST). These were combined with ground-based observations from the VLT to do a more complete analysis of the planetary atmosphere. These are the set of observations that we use for some comparisons to our atmospheric models in this work. Nikolov et al. (2022) argue that their retrieval approach of these combined data suggests cloud/haze free terminators at the pressures probed by the observations. The retrieval procedure treats cloud particles rain out and the element abundances for Na...

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Section: Where Clouds On Wasp-96b Get Optically Thickmentioning

confidence: 93%

Clouds form on the hot Saturn JWST ERO target WASP-96b

Samra¹,

Helling²,

Chubb³

et al. 2022

Preprint

View full text Add to dashboard Cite

show abstract

“…However, there is some tentative evidence of porous cloud particles (generated through collisions) as an explanation for the flat spectrum of the mini-Neptune GJ-1214b (Ohno et al 2020). As shown in Samra et al (2022), however, collisions in exoplanet atmospheres when turbulence is included are expected to be destructive and not to produce larger aggregate particles. However, hazes of fractal aggregates have been inferred for a number of bodies in the Solar System and for exoplanets (e.g.…”

Section: Where Clouds On Wasp-96b Become Optically Thickmentioning

confidence: 96%

Clouds form on the hot Saturn JWST ERO target WASP-96b

Samra

Helling

Chubb

et al. 2023

A&A

View full text Add to dashboard Cite

Context. WASP-96b is a hot Saturn exoplanet, with an equilibrium temperature of ≈ 1300 K. This is well within the regime of thermodynamically expected extensive cloud formation. Prior observations with Hubble/WFC3, Spitzer/IRAC, and VLT/FORS2 have been combined into a single spectrum for which retrievals suggest a cold but cloud-free atmosphere. Recently, the planet was observed with the James Webb Space Telescope (JWST) as part of the Early Release Observations (ERO). Aims. The formation of clouds in the atmosphere of exoplanet WASP-96b is explored. Methods. One-dimensional profiles were extracted from the 3D GCM expeRT/MITgcm results and used as input for a kinetic, nonequilibrium model to study the formation of mineral cloud particles of mixed composition. The ARCiS retrieval framework was applied to the pre-JWST WASP-96b transit spectrum to investigate the apparent contradiction between cloudy models and assumed cloud-free transit spectrum. Results. Clouds are predicted to be ubiquitous throughout the atmosphere of WASP-96b. Silicate materials contribute between 40% and 90% cloud particle volume, which means that metal oxides also contribute with up to 40% cloud particle volume in the lowpressure regimes that affect spectra. We explore how these cloudy models match currently available transit spectra. Reduced vertical mixing acts to settle clouds to deeper in the atmosphere, and an increased cloud particle porosity reduces the opacity of clouds in the near-IR and optical region. These two effects allow for clearer molecular features to be observed while still allowing clouds to be in the atmosphere. Conclusions. The atmosphere of WASP-96b is unlikely to be cloud free. Retrievals of HST, Spitzer, and VLT spectra also show that multiple cloudy solutions reproduce the data. JWST observations will be affected by clouds, where the cloud top pressure varies by an order of magnitude within even the NIRISS wavelength range. The long-wavelength end of NIRSpec and the short-wavelength end of MIRI may probe atmospheric asymmetries between the limbs of the terminator on WASP-96b.

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Exoplanet weather and climate regimes with clouds and thermal ionospheres

Helling

Samra

Lewis

et al. 2023

A&A

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Context. Gaseous exoplanets are the targets that enable us to explore fundamentally our understanding of planetary physics and chemistry. With observational efforts moving from the discovery into the characterisation mode, systematic campaigns that cover large ranges of global stellar and planetary parameters will be needed to disentangle the diversity of exoplanets and their atmospheres that all are affected by their formation and evolutionary paths. Ideally, the spectral range includes the high-energy (ionisation) and the low-energy (phase-transitions) processes as they carry complementary information of the same object. Aims. We aim to uncover cloud formation trends and globally changing chemical regimes into which gas-giant exoplanets may fall due to the host star’s effect on the thermodynamic structure of their atmospheres. We aim to examine the emergence of an ionosphere as indicator for potentially asymmetric magnetic field effects on these atmospheres. We aim to provide input for exoplanet missions such as JWST, PLATO, and Ariel, as well as potential UV missions ARAGO, PolStar, or POLLUX on LUVOIR. Methods. Pre-calculated 3D GCMs for M, K, G, F host stars are the input for our kinetic cloud model for the formation of nucleation seeds, the growth to macroscopic cloud particles and their evaporation, gravitational settling, element conservation and gas chemistry. Results. Gaseous exoplanets fall broadly into three classes: i) cool planets with homogeneous cloud coverage, ii) intermediate temperature planets with asymmetric dayside cloud coverage, and iii) ultra-hot planets without clouds on the dayside. In class ii), the dayside cloud patterns are shaped by the wind flow and irradiation. Surface gravity and planetary rotation have little effect. For a given effective temperature, planets around K dwarfs are rotating faster compared to G dwarfs leading to larger cloud inhomogeneities in the fast rotating case. Extended atmosphere profiles suggest the formation of mineral haze in form of metal-oxide clusters (e.g. (TiO2)N). Conclusions. The dayside cloud coverage is the tell-tale sign for the different planetary regimes and their resulting weather and climate appearance. Class (i) is representative of planets with a very homogeneous cloud particle size and material compositions across the globe (e.g., HATS-6b, NGTS-1b), classes (ii, e.g., WASP-43b, HD 209458b) and (iii, e.g., WASP-121b, WP 0137b) have a large day-night divergence of the cloud properties. The C/O ratio is, hence, homogeneously affected in class (i), but asymmetrically in class (ii) and (iii). The atmospheres of class (i) and (ii) planets are little affected by thermal ionisation, but class (iii) planets exhibit a deep ionosphere on the dayside. Magnetic coupling will therefore affect different planets differently and will be more efficient on the more extended, cloud-free dayside. How the ionosphere connects atmospheric mass loss at the top of the atmosphere with deep atmospheric layers need to be investigated to coherently interpret high resolution observations of ultra-hot planets.

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Mineral snowflakes on exoplanets and brown dwarfs

Cited by 9 publications

References 111 publications

Clouds form on the hot Saturn JWST ERO target WASP-96b

Clouds form on the hot Saturn JWST ERO target WASP-96b

Clouds form on the hot Saturn JWST ERO target WASP-96b

Exoplanet weather and climate regimes with clouds and thermal ionospheres

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