Helium in Exoplanet Exospheres: Orbital and Stellar Influences

Searches for helium in the exospheres of exoplanets via the metastablenear-infrared triplet have yielded 17 detections and 40 non-detections. Weperformed a comprehensive re-analysis of published studies to investigate theinfluence of stellar XUV flux and orbital parameters on the detectability ofhelium in exoplanetary atmospheres. We identified a distinct 'orbital sweetspot' for helium detection, 0.03 to 0.08 AU from the host star, where themajority of detections occurred. This sweet spot is influenced by the stellarXUV flux and planet size. Notably, a lower ratio of XUV flux to mid-UV flux ispreferred for planets compared to non-detections. We also found that heliumdetections occur for planets around stars with effective temperatures of4400-6500 K, with a sharp gap at 5400 to 6000 K, where no detections occur.Additionally, our analysis of the cumulative XUV flux versus escape velocityshows planets with helium detections are found above the 'cosmic shoreline',suggesting the shoreline needs revision. The trends we found in our analysiscontribute to a deeper understanding of exosphere evolution.