Hubble’s Multiyear Search for Exospheres in the TRAPPIST-1 System Reveals Frequent Microflares

Abstract Ly α 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 O ( 10 ) × larger than previously reported (e.g., 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 Ly α variability from frequent “microflares.” While JWST’s near-infrared observations have shown that ∼10 30 erg flares occur every ∼6 hr, we identify ∼10 29 erg flares on subhour timescales in both HST/STIS and Very Large Telescope g ′ 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.