A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion

Abstract: Thousands of transiting exoplanets have been discovered, but spectral analysis of their atmospheres has so far been dominated by a small number of exoplanets and data spanning relatively narrow wavelength ranges (such as 1.1 to 1.7 {\mu}m). Recent studies show that some hot-Jupiter exoplanets have much weaker water absorption features in their near-infrared spectra than predicted. The low amplitude of water signatures could be explained by very low water abundances, which may be a sign that water was depleted in the protoplanetary disk at the planet's formation location, but it is unclear whether this level of depletion can actually occur. Alternatively, these weak signals could be the result of obscuration by clouds or hazes, as found in some optical spectra. Here we report results from a comparative study of ten hot Jupiters covering the wavelength range 0.3-5 micrometres, which allows us to resolve both the optical scattering and infrared molecular absorption spectroscopically. Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres. We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the spectral strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes. This result strongly suggests that primordial water depletion during formation is unlikely and that clouds and hazes are the cause of weaker spectral signatures.

• The paper demonstrates a direct link between strong water absorption and clear atmospheres, revealing that clouds, not water depletion, obscure spectral features.
• The study employs multi-wavelength transmission spectra from HST and Spitzer to analyze optical scattering and infrared absorption in ten hot-Jupiter exoplanets.
• The findings challenge earlier theories by attributing weak water signals to cloud and haze opacity, which refines future exoplanet atmospheric surveys.

A Continuum from Clear to Cloudy Hot-Jupiter Exoplanets

The discovery of thousands of transiting exoplanets provides a fertile ground for understanding planetary atmospheres beyond our solar system. The paper "A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion" offers a meticulous exploration of the atmospheric characteristics of hot Jupiters, focusing on their spectral analysis within the wavelength range of 0.3–5 micrometers.

Methodology and Observational Strategy

The study covers a sample of ten hot Jupiters: WASP-6b, WASP-12b, WASP-17b, WASP-19b, WASP-31b, WASP-39b, HAT-P-1b, HAT-P-12b, HD 209458b, and HD 189733b. Utilizing instruments aboard the Hubble Space Telescope (HST) and the Spitzer Space Telescope, the research employs a multi-wavelength observational strategy to probe both optical scattering and infrared molecular absorption. The HST's Space Telescope Imaging Spectrograph (STIS) and Wide Field Camera 3 (WFC3) are pivotal in this analysis, allowing transmission spectra to be recorded at multiple wavelengths with high precision.

Key Findings

The work reports a diverse range of atmospheric types, with planets displaying a spectrum from clear to cloudy atmospheres. A central finding is the correlation between the planet's measured radius differences at optical and infrared wavelengths and the spectral strength of water absorption features. Notably, planets with strong water absorption lines have clear atmospheres, while those with weak or absent water signatures are associated with clouds and hazes.

This study challenges the notion of primordial water depletion as a major factor for the weak water absorption observed in some hot Jupiters. It suggests instead that cloud cover plays the dominant role. The presence of Rayleigh scattering haze and grey cloud opacity can obscure water features, complicating the derivation of molecular abundances without bias from hazes or clouds.

Implications and Theoretical Context

The findings significantly influence our understanding of exoplanet atmospheres. They suggest a pervasive role of clouds and hazes in altering the apparent composition, providing insights into the processes of cloud formation and atmospheric circulation in hot Jupiters. This work also informs hypotheses regarding cloud formation mechanisms in other celestial environments, highlighting differences with brown dwarf atmospheres, which exhibit more orderly temperature-dependent cloud formation phenomena.

Future Directions

Future research could benefit from broad atmospheric surveys that precisely measure key spectral indices—ΔZ_UB-LM, ΔZ_J-LM, and H₂O amplitude. Such measurements would further refine the classification of exoplanetary atmospheres, potentially distinguishing between clear and cloudy planets at a pre-survey stage. Additionally, probing the role of stellar activity in transmission spectra remains a critical area for exploration.

In conclusion, this study underscores the complex interplay between atmospheric dynamics and observational characteristics in hot Jupiters, enhancing predictive models and informing the design of future spectroscopic missions targeting transiting exoplanets.