The formation of hazes at microbar pressures has been explored by theoretical models of exoplanet atmospheres to explain Rayleigh scattering and/or featureless transmission spectra; however observational evidence of aerosols in the low-pressure formation environments has proved elusive. Here, we show direct evidence of aerosols existing at ∼1 microbar pressures in the atmosphere of the warm sub-Saturn WASP-69b using observations taken with the Space Telescope Imaging Spectrograph and Wide Field Camera 3 instruments on the Hubble Space Telescope. The transmission spectrum shows a wavelength-dependent slope induced by aerosol scattering that covers 11 scale heights of spectral modulation. Drawing on the extensive studies of haze in our solar system, we model the transmission spectrum based on a scaled version of Jupiter’s haze-density profile to show that the WASP-69b transmission spectrum can be produced by scattering from an approximately constant density of particles extending throughout the atmospheric column from 40 millibar to microbar pressures. These results are consistent with theoretical expectations based on microphysics of the aerosol particles that have suggested haze can exist at microbar pressures in exoplanet atmospheres.

系外行星大气的理论模型已经探索了微巴压力下雾霾的形成，以解释瑞利散射和/或无特征的透射光谱；然而，在低压地层环境中气溶胶的观测证据已证明难以捉摸。在这里，我们使用太空望远镜成像光谱仪和哈勃太空望远镜上的宽视场相机 3 仪器进行的观测，展示了在土星下温暖的 WASP-69b 大气中存在约 1 微巴压力的气溶胶的直接证据。透射光谱显示了由气溶胶散射引起的波长相关斜率，覆盖光谱调制的 11 个标度高度。借鉴对太阳系雾霾的广泛研究，我们基于木星雾度密度剖面的缩放版本对透射光谱进行建模，以表明 WASP-69b 透射光谱可以通过从 40 毫巴到微巴压力延伸到整个大气柱的近似恒定密度的粒子散射来产生。这些结果与基于气溶胶粒子微观物理学的理论预期一致，这些理论表明在系外行星大气中的微巴压力下可能存在雾霾。