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~Å (continuum), 1700~Å (broadband), and three extreme-UV (EUV) narrowbands at 304~Å, 171~Å, and 94~Å. We find that the disk-integrated transit signal is clearly detectable in the 6173~Å band, whereas strong solar activity-induced fluctuations obscure the transit in the EUV channels. Notably, the 1700~Å 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.

Figures (12)

• Figure 1: The transit of Venus across the sun on 5 June 2012 observed in 5 SDO channels. The black dots represent the path of Venus and do not depict the actual planet size or the cadence at which it was observed. Top row shows the images obtained in 6173Å and 1700Å channels; bottom row shows the images obtained in 304Å, 171Å, and 94Å channels respectively.
• Figure 2: Disk-integrated light curves in five wavelengths bands from SDO's time series images taken between 4-7 June, 2012. A vertical cyan box marks the Venus transit, starting at 22:10 UTC on 5 June and ending at 04:50 UTC on 6 June 2012. The yellow dotted lines indicate the time when solar flares occurred during this 96 hours observation window. The strength of each flare is labeled to the right of the respective dotted lines in the top panel. Panels from top-to-bottom corresponds to 94 Å, 171 Å, 304 Å, 1700 Å, 6173 Å channel, respectively.
• Figure 3: HMI 6173 Å disk-integrated light curve and detrending steps. Top left: Raw disk-integrated light curve showing strong instrumental artifacts consisting of a 24-h periodic modulation and a slow downward drift. The gray curve is a low-order polynomial fit to peaks (red crosses mark) which models the slow drift. Transit start and end times are indicated by vertical gray dashed lines. Bottom left: The drift-corrected light curve. A 24-h, transit-free reference segment (3-4 June, blue band) was chosen to model the periodic artifacts. A yellow band (5-6 June) denotes a 24 hours transit segment. Top right: Transit segment folded onto transit-free reference segment. A red curve is polynomial fit to transit-free reference segment. Bottom right: Extracted transit light curve from the polynomial fit and the transit segment. The black line indicates the best fit transit model obtained from python batman-package.
• Figure 4: The 4 contact points during the ToV and their timings between June 5 and June 6, 2012. Note that the figure represents the definitions of the contact points and not the path of the transit.
• Figure 5: Transit depth of Venus (in ppm) as a function of observer distance. The horizontal red-dashed line represents a 78 ppm mark.