The extreme-UV (EUV) emission of cool stars has become very fashionable in recent years, since it is thought to be the most important driver for exoplanet evaporation. Since no observatories have been active at this wavelength range for a long time, scaling laws are typically used to infer the stellar EUV fluxes. However, stellar coronae and transition regions are exciting places with interesting physics that still host some surprises. I will present results from our new catalog of exoplanet X-ray irradiation, showing that high-energy environments can be much more extreme than what is found for the well-studied Hot Jupiters. I will also show that some EUV assumptions made in exoplanet atmosphere studies are off by an order of magnitude, and that certain nondetections of atmospheres are actually unsurprising when one takes into account the physics of emission lines in the stellar corona and transition region.

{"references": ["Poppenhaeger K., 2022, MNRAS, 512, 1751. doi:10.1093/mnras/stac507 arXiv:2202.08838", "Ketzer L., Poppenhaeger K., 2023, MNRAS, 518, 1683. doi:10.1093/mnras/stac2643 arXiv:2209.05860", "Foster G., Poppenhaeger K., Ilic N., Schwope A., 2022, A\\&A, 661, A23. doi:10.1051/0004-6361/202141097 arXiv:2106.14550", "France K., Loyd R.~O.~P., Youngblood A., Brown A., Schneider P.~C., Hawley S.~L., Froning C.~S., et al., 2016, ApJ, 820, 89. doi:10.3847/0004-637X/820/2/89 arXiv:1602.09142"]}

Plenary talk given at the Cool Stars 21 conference. Research project received funding from the German Leibniz Community under grant number 67/2018.