Hubble's Multi-Year Search for Exospheres in the TRAPPIST-1 System Reveals Frequent Microflares

Abstract

Ly-$\alpha$ observations provide a powerful probe of stellar activity and atmospheric escape in exoplanetary systems. We present here an analysis of 104 HST/STIS orbits monitoring the TRAPPIST-1 system between 2017 and 2022, covering 3--5 transits for each of its seven planets. We rule out transit depths $\gtrsim 20\%$, which translates into an upper limit on the escape rate of $1064E{O}_H$/Gyr for planet b ($1E{O}_H$ is the Earth-ocean-equivalent hydrogen content), in agreement with recent claims that planet b should be airless. These upper limits are $\sim$3 times larger than expected from the photon noise due to a large baseline scatter, which we ultimately link to TRAPPIST-1's intrinsic Ly-$\alpha$ variability from frequent ``microflares.'' While JWST observations of TRAPPIST-1 in the near infrared have shown that $\sim 10^{30}$-erg flares occur every $\sim$6 hours, we report here $\sim 10^{29}$-erg flares on sub-hour timescales in the HST/STIS and also Very Large Telescope (VLT) $g^{{}^{\prime }}$ observations. The FUV and optical amplitudes ($\sim$400$\%$ vs $\sim$3$\%$, respectively) for flares with similar waiting-times indicate flare temperatures of 11000${}_{-3100}^{+4200}$~K over 0.011${}_{-0.01}^{+0.03}$\% of the stellar disk. Finally, our multi-year baseline reveals a variability with $P=3.27\pm 0.04$ days, providing further validation of the previously reported 3.295-day rotation period for TRAPPIST-1. These results highlight the importance of accounting for stellar microvariability when searching for exospheres around active M dwarfs.