Venus as an exoplanet analog: extended UV transit signatures and coronal occultations

Abstract

Stellar activity manifests differently across wavelengths, causing flux variability that can obscure planetary transits. While transit observations are typically performed in the visible and infrared bands, where stellar flux is relatively stable, short-wavelength regimes exhibit high variability, complicating reliable detections. Here, we analyze the 2012 transit of Venus as an exoplanet analog using multiwavelength observations taken by the Solar Dynamics Observatory (SDO) in five channels: 6173~\AA\ (continuum), 1700~\AA\ (broadband), and three extreme-UV (EUV) narrowbands at 304~\AA, 171~\AA, and 94~\AA. We find that the disk-integrated transit signal is clearly detectable in the 6173~\AA\ band, whereas strong solar activity-induced fluctuations obscure the transit in the EUV channels. Notably, the 1700~\AA\ UV transit is noisier but significantly longer ($\approx 9.2$~hrs) than the visible-band transit ($\approx 6.7$~hrs), because Venus began occulting the extended coronal features before ingress onto the visible disk. This observation highlights the potential of UV transits to probe the spatial extent of stellar coronae in exoplanetary systems. Numerical simulations further suggest that limb-brightened stars in quiescence phase may exhibit distinctive UV/EUV transit signatures, opening new possibilities for exoplanet detection and characterization in these spectral regimes.