Hubble's Multi-Year Search for Exospheres in the TRAPPIST-1 System Reveals Frequent Microflares
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| Main Authors: | , , , , , , , , , , , , , , , , |
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| Format: | Preprint |
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2025
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| author | Berardo, David de Wit, Julien Gillon, Michael Howard, Ward S. Bourrier, Vincent Cotton, Matthew W. Quatresooz, Florian Hoerner, Léonie Bolmont, Emeline Burdanov, Artem Burgasser, Adam J. Demory, Brice-Olivier Enhrenreich, David Lederer, Susan M. Rackham, Benjamin V. Seager, Sara Triaud, Amaury |
| author_facet | Berardo, David de Wit, Julien Gillon, Michael Howard, Ward S. Bourrier, Vincent Cotton, Matthew W. Quatresooz, Florian Hoerner, Léonie Bolmont, Emeline Burdanov, Artem Burgasser, Adam J. Demory, Brice-Olivier Enhrenreich, David Lederer, Susan M. Rackham, Benjamin V. Seager, Sara Triaud, Amaury |
| contents | 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 $\gtrsim20\%$, which translates into an upper limit on the escape rate of $1064~EO_H$/Gyr for planet b ($1~EO_H$ is the Earth-ocean-equivalent hydrogen content), in agreement with recent claims that planet b should be airless. These upper limits are $\sim$3 times larger than expected from the photon noise due to a large baseline scatter, which we ultimately link to TRAPPIST-1's intrinsic Ly-$α$ variability from frequent ``microflares.'' While JWST observations of TRAPPIST-1 in the near infrared have shown that $\sim10^{30}$-erg flares occur every $\sim$6 hours, we report here $\sim10^{29}$-erg flares on sub-hour timescales in the HST/STIS and also Very Large Telescope (VLT) $g^{'}$ observations. The FUV and optical amplitudes ($\sim$400$\%$ vs $\sim$3$\%$, respectively) for flares with similar waiting-times indicate flare temperatures of 11000$^{+4200}_{-3100}$~K over 0.011$^{+0.03}_{-0.01}$\% of the stellar disk. Finally, our multi-year baseline reveals a variability with $P = 3.27 \pm 0.04$ 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2506_12140 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Hubble's Multi-Year Search for Exospheres in the TRAPPIST-1 System Reveals Frequent Microflares Berardo, David de Wit, Julien Gillon, Michael Howard, Ward S. Bourrier, Vincent Cotton, Matthew W. Quatresooz, Florian Hoerner, Léonie Bolmont, Emeline Burdanov, Artem Burgasser, Adam J. Demory, Brice-Olivier Enhrenreich, David Lederer, Susan M. Rackham, Benjamin V. Seager, Sara Triaud, Amaury Earth and Planetary Astrophysics Solar and Stellar Astrophysics 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 $\gtrsim20\%$, which translates into an upper limit on the escape rate of $1064~EO_H$/Gyr for planet b ($1~EO_H$ is the Earth-ocean-equivalent hydrogen content), in agreement with recent claims that planet b should be airless. These upper limits are $\sim$3 times larger than expected from the photon noise due to a large baseline scatter, which we ultimately link to TRAPPIST-1's intrinsic Ly-$α$ variability from frequent ``microflares.'' While JWST observations of TRAPPIST-1 in the near infrared have shown that $\sim10^{30}$-erg flares occur every $\sim$6 hours, we report here $\sim10^{29}$-erg flares on sub-hour timescales in the HST/STIS and also Very Large Telescope (VLT) $g^{'}$ observations. The FUV and optical amplitudes ($\sim$400$\%$ vs $\sim$3$\%$, respectively) for flares with similar waiting-times indicate flare temperatures of 11000$^{+4200}_{-3100}$~K over 0.011$^{+0.03}_{-0.01}$\% of the stellar disk. Finally, our multi-year baseline reveals a variability with $P = 3.27 \pm 0.04$ 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. |
| title | Hubble's Multi-Year Search for Exospheres in the TRAPPIST-1 System Reveals Frequent Microflares |
| topic | Earth and Planetary Astrophysics Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2506.12140 |