Hubble’s Multi-Year Search for Exospheres in the TRAPPIST-1 System Reveals Frequent Microflares
Ly-α 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 ≳20%, which translates into an upper limit on the escape rate of 1064 EOH/Gyr for planet b (1 EOH is the Earth-ocean-equivalent hydrogen content), in agreement with recent claims that planet b should be airless.
These upper limits are ∼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-α variability from frequent “microflares.” While JWST observations of TRAPPIST-1 in the near infrared have shown that ∼1030-erg flares occur every ∼6 hours, we report here ∼1029-erg flares on sub-hour timescales in the HST/STIS and also Very Large Telescope (VLT) g′ observations.
The FUV and optical amplitudes (∼400% vs ∼3%, respectively) for flares with similar waiting-times indicate flare temperatures of 11000+4200−3100~K over 0.011+0.03−0.01\% of the stellar disk.
Finally, our multi-year baseline reveals a variability with P=3.27±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.
David Berardo, Julien de Wit, Michael Gillon, Ward S. Howard, Vincent Bourrier, Matthew W. Cotton, Florian Quatresooz, Léonie Hoerner, Emeline Bolmont, Artem Burdanov, Adam J. Burgasser, Brice-Olivier Demory, David Enhrenreich, Susan M. Lederer, Benjamin V. Rackham, Sara Seager, Amaury Triaud
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2506.12140 [astro-ph.EP] (or arXiv:2506.12140v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2506.12140
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From: David Berardo
[v1] Fri, 13 Jun 2025 18:02:12 UTC (3,216 KB)
https://arxiv.org/abs/2506.12140
Astrobiology,
