The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b

Santos, Leonardo A. dos; Ehrenreich, D.; Bourrier, V.; Astudillo-Defru, N.; Bonfils, X.; Forget, F.; Lovis, C.; Pepe, F.; Udry, S.

doi:10.1051/0004-6361/201937327

Cited by 34 publications

(27 citation statements)

References 34 publications

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“…A recent analysis of HST/WFC3 transit observations revealed an absorption at 1.4 µm, which is interpreted as water vapour absorption (Tsiaras et al 2019;Benneke et al 2019b) or methane absorption (Bézard et al 2020) in a low-molecular-weight (H 2 /Hedominated) atmosphere. Recent observations by HST/STIS also confirm the interpretation that K2-18b has a H2-dominated atmosphere (dos Santos et al 2020). Benneke et al (2019b) also suggested that water clouds (most likely icy, but possibly liquid for a Bond albedo around 0.3) may form in the atmosphere of K2-18b.…”

Section: Introductionmentioning

confidence: 59%

Formation and dynamics of water clouds on temperate sub-Neptunes: the example of K2-18b

Charnay

Blain

Bézard

et al. 2021

A&A

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Context. Hubble Space Telescope (HST) spectroscopic transit observations of the temperate sub-Neptune K2-18b were interpreted as the presence of water vapour with potential water clouds. 1D modelling studies also predict the formation of water clouds in K2-18b’s atmosphere in some conditions. However, such models cannot predict the cloud cover, which is driven by atmospheric dynamics and thermal contrasts, and thus neither can they predict the real impact of clouds on spectra. Aims. The main goal of this study is to understand the formation, distribution, and observational consequences of water clouds on K2-18b and other temperate sub-Neptunes. Methods. We simulated the atmospheric dynamics, water cloud formation, and spectra of K2-18b for a H2-dominated atmosphere using a 3D general circulation model. We analysed the impact of atmospheric composition (with metallicity from 1× solar to 1000× solar), concentration of cloud condensation nuclei, and planetary rotation rate. Results. Assuming that K2-18b has a synchronous rotation, we show that the atmospheric circulation in the upper atmosphere essentially corresponds to a symmetric day-to-night circulation with very efficient heat redistribution. This regime preferentially leads to cloud formation at the sub-stellar point or at the terminator. Clouds form at metallicity ≥100× solar with relatively large particles (radius = 30–450 μm). At 100–300× solar metallicity, the cloud fraction at the terminators is small with a limited impact on transit spectra. At 1000× solar metallicity, very thick clouds form at the terminator, greatly flattening the transit spectrum. The cloud distribution appears very sensitive to the concentration of cloud condensation nuclei and to the planetary rotation rate, although the impact on transit spectra is modest in the near-infrared. Fitting HST transit data with our simulated spectra suggests a metallicity of ~100–300× solar, which is consistent with the mass-metallicity trend of giant planets in the Solar System. In addition, we found that the cloud fraction at the terminator can be highly variable in some conditions, leading to a potential variability in transit spectra that is correlated with spectral windows. This effect could be common on cloudy exoplanets and could be detectable with multiple transit observations. Finally, the complex cloud dynamics revealed in this study highlight the inherent 3D nature of clouds shaped by couplings between microphysics, radiation, and atmospheric circulation.

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Section: Introductionmentioning

confidence: 59%

Formation and dynamics of water clouds on temperate sub-Neptunes: the example of K2-18b

Charnay

Blain

Bézard

et al. 2021

A&A

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“…Unfortunately, no clear detections of the upper atmospheres of super‐Earths have been made so far. The most promising result is the tentative detection of neutral hydrogen for K2‐18b based on a partial transit observed with Hubble (dos Santos et al., 2020). This is consistent with the observation of the bulk atmosphere described above, but more data are needed to confirm the result.…”

Section: A Decade Of Super‐earth Atmosphere Studiesmentioning

confidence: 99%

“…Beyond the attempts to directly observe the bulk atmospheres of sub-Neptune size planets, there have also been clever attempts to deduce the composition of these planets' atmospheres through observations of their thermospheres, exospheres, and winds. These observations have been obtained for neutral hydrogen at Lyman α (Bourrier et al, 2017;dos Santos et al, 2020;Ehrenreich et al, 2012;García Muñoz et al, 2020;Waalkes et al, 2019) and in the helium IR triplet (Kasper et al, 2020). The detection of neutral hydrogen in an escaping wind would constrain the mass-loss rate of the atmosphere but would have an ambiguous interpretation with regards to the composition because neutral hydrogen could be produced from the photodissociation of H 2 and H 2 O.…”

Section: A Decade Of Super-earth Atmosphere Studiesmentioning

confidence: 99%

The Nature and Origins of Sub‐Neptune Size Planets

2021

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Planets intermediate in size between the Earth and Neptune, and orbiting closer to their host stars than Mercury does the Sun, are the most common type of planet revealed by exoplanet surveys over the last quarter century. Results from NASA's Kepler mission have revealed a bimodality in the radius distribution of these objects, with a relative underabundance of planets between 1.5 and 2.0 R⊕. This bimodality suggests that sub‐Neptunes are mostly rocky planets that were born with primary atmospheres a few percent by mass accreted from the protoplanetary nebula. Planets above the radius gap were able to retain their atmospheres (“gas‐rich super‐Earths”), while planets below the radius gap lost their atmospheres and are stripped cores (“true super‐Earths”). The mechanism that drives atmospheric loss for these planets remains an outstanding question, with photoevaporation and core‐powered mass loss being the prime candidates. As with the mass‐loss mechanism, there are two contenders for the origins of the solids in sub‐Neptune planets: the migration model involves the growth and migration of embryos from beyond the ice line, while the drift model involves inward‐drifting pebbles that coagulate to form planets close‐in. Atmospheric studies have the potential to break degeneracies in interior structure models and place additional constraints on the origins of these planets. However, most atmospheric characterization efforts have been confounded by aerosols. Observations with upcoming facilities are expected to finally reveal the atmospheric compositions of these worlds, which are arguably the first fundamentally new type of planetary object identified from the study of exoplanets.

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“…Mass loss can in turn leave substantial imprints on observed planetary statistics, such as the dearth of planets between 1.5 and 2 Earth radii (the "radius gap" or "evaporation valley") and the so-called "Neptune desert" in the radius-period plane (Lopez & Fortney 2013;Owen & Wu 2013Fulton et al 2017;Fulton & Petigura 2018;van Eylen et al 2018;Cloutier & Menou 2020;Hardegree-Ullman et al 2020). Over the past two decades, most measurements of mass-loss rates for close-in planets have been conducted at ultraviolet wavelengths, with Lyα detections for HD 209458b (Vidal-Madjar et al 2003), HD 189733b (Lecavelier Des Etangs et al 2010;Lecavelier des Etangs et al 2012), GJ 436b (Kulow et al 2014;Ehrenreich et al 2015;Lavie et al 2017), and GJ 3470b (Bourrier et al 2018); tentative/marginal signals for TRAPPIST-1b and c (Bourrier et al 2017a), Kepler-444e and f (Bourrier et al 2017b), and K2-18b (dos Santos et al 2020); and nondetections for 55 Cnc e (Ehrenreich et al 2012), HD 97658b (Bourrier et al 2017c), GJ 1132 b (Waalkes et al 2019), and π Men c (García Muñoz et al 2020). While in theory the large cross section of this line should result in strong absorption during exoplanet transits, in practice geocoronal emission and interstellar absorption effectively mask out the line core for most stars, requiring these studies to study the absorption in the line's extended wings.…”

Section: Introductionmentioning

confidence: 99%

Constraints on Metastable Helium in the Atmospheres of WASP-69b and WASP-52b with Ultranarrowband Photometry

Vissapragada

Knutson

Jovanović

et al. 2020

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Infrared observations of metastable 2 3 S helium absorption with ground-and space-based spectroscopy are rapidly maturing, as this species is a unique probe of exoplanet atmospheres. Specifically, the transit depth in the triplet feature (with vacuum wavelengths near 1083.3 nm) can be used to constrain the temperature and mass-loss rate of an exoplanet's upper atmosphere. Here, we present a new photometric technique to measure metastable 2 3 S helium absorption using an ultranarrowband filter (FWHM 0.635 nm) coupled to a beam-shaping diffuser installed in the Wide-field Infrared Camera on the 200 inch Hale Telescope at Palomar Observatory. We use telluric OH lines and a helium arc lamp to characterize refractive effects through the filter and to confirm our understanding of the filter transmission profile. We benchmark our new technique by observing a transit of WASP-69b and detect an excess absorption of 0.498%±0.045% (11.1σ), consistent with previous measurements after considering our bandpass. We then use this method to study the inflated gas giant WASP-52b and place a 95th percentile upper limit on excess absorption in our helium bandpass of 0.47%. Using an atmospheric escape model, we constrain the massloss rate for WASP-69b to be-+-M 5.25 10 Gyr 0.46

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The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b

Cited by 34 publications

References 34 publications

Formation and dynamics of water clouds on temperate sub-Neptunes: the example of K2-18b

Formation and dynamics of water clouds on temperate sub-Neptunes: the example of K2-18b

The Nature and Origins of Sub‐Neptune Size Planets

Constraints on Metastable Helium in the Atmospheres of WASP-69b and WASP-52b with Ultranarrowband Photometry

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