2026ApJ...998L..31B

Lyα observations provide a powerful probe of stellar activity and atmospheric escape in exoplanetary systems. We present here an analysis of 109 Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) orbits monitoring the TRAPPIST-1 system between 2017 and 2022, covering three to five transits for each of its seven planets. We rule out transit depths ≳30%, prompting a revision of atmospheric escape rates, with upper limits ${ \mathcal O }(10)\times $ larger than previously reported (e.g., ${ \mathcal O }(70)$ Earth-ocean-equivalent hydrogen content per Gyr for planet b, consistent with claims that it is airless). The transit depth upper limits are ∼3 times larger than expected from the photon noise owing to a large baseline scatter, which we link to TRAPPIST-1's intrinsic Lyα variability from frequent “microflares.” While JWST's near-infrared observations have shown that ∼10³⁰ erg flares occur every ∼6 hr, we identify ∼10²⁹ erg flares on subhour timescales in both HST/STIS and Very Large Telescope ${g}^{{\prime} }$observations. The far-UV and optical amplitudes (∼400% vs. ∼3%, respectively) for flares with similar waiting times indicate flare temperatures of 9700${}_{-2400}^{+1500}$ K over 0.017${}_{-0.007}^{+0.03}$% of the stellar disk. Finally, our multiyear baseline reveals a variability with P = 3.27 ± 0.04 days-the first independent validation of TRAPPIST-1's 3.3-day rotation period.