Persistence of flare-driven atmospheric chemistry on rocky habitable zone worlds


Low-mass stars show evidence of vigorous magnetic activity in the form of large flares and coronal mass ejections. Such space weather events may have important ramifications for the habitability and observational fingerprints of exoplanetary atmospheres. Here, using a suite of three-dimensional coupled chemistry–climate model simulations, we explore effects of time-dependent stellar activity on rocky planet atmospheres orbiting G, K and M dwarf stars. We employ observed data from the MUSCLES campaign and the Transiting Exoplanet Survey Satellite and test a range of rotation period, magnetic field strength and flare frequency assumptions. We find that recurring flares drive the atmospheres of planets around K and M dwarfs into chemical equilibria that substantially deviate from their pre-flare regimes, whereas the atmospheres of G dwarf planets quickly return to their baseline states. Interestingly, simulated O2-poor and O2-rich atmospheres experiencing flares produce similar mesospheric nitric oxide abundances, suggesting that stellar flares can highlight otherwise undetectable chemical species. Applying a radiative transfer model to our chemistry–climate model results, we find that flare-driven transmission features of bio-indicating chemical species, such as nitrogen dioxide, nitrous oxide and nitric acid, show particular promise for detection by future instruments.

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Fig. 1: Observed flare light curves and spectra used as inputs for CCM simulations.
Fig. 2: Spatial and temporal atmospheric effects of repeated stellar flares on a G-star planet.
Fig. 3: Spatial and temporal atmospheric effects of repeated stellar flaring on a K-star planet.
Fig. 4: Global-mean vertical profiles of atmospheric species.
Fig. 5: Global-mean vertical profiles of atmospheric species for non-modern-Earth climate archetypes.
Fig. 6: Simulated transmission spectra for two endmember planetary scenarios.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon request. The raw data are publicly available at (MAST) and (MUSCLES) and the solar ion pair production rates are available at

Code availability

The unmodified climate model used in this study is available for public download at Components of the modified version of the climate can be obtained via ExoCam at and by request from E.T.W. ( The stella package can be downloaded at The remaining codes that support the results within this paper and other findings of this study are available from the corresponding author on request.


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H.C. and D.E.H. acknowledge support from the Future Investigators in NASA Earth and Space Science and Technology (FINESST) Graduate Research Award Number 80NSSC19K1523. H.C. thanks P. Loyd for assistance in the use of his MUSCLES flare code and for sharing it with the public. Z.Z. acknowledges support from the MIT BOSE Fellow programme, the Change Happens Foundation and the Heising-Simons Foundation. E.T.W. acknowledges support from NASA Habitable Worlds grant number 80NSSC17K0257. A.D.F. acknowledges support from NSF Graduate Research Fellowship Program grant number DGE-1746045. We thank A. Gu for ozone variability analysis inspiration and the QUEST high-performance computing facility at Northwestern University for computational and staff resources. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the funding agencies. This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. TESS data were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract number NAS5-26555. Support for MAST is provided by the NASA Office of Space Science via grant number NNX13AC07G and by other grants and contracts.

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H.C., E.T.W. and D.E.H. conceived and designed the study. H.C. conducted the numerical model simulations and data analysis. Z.Z. performed the radiative transfer model simulations. A.Y. provided stellar input data from the MUSCLES and Mega-MUSCLES surveys. A.D.F. performed the machine learning TESS data reductions. H.C. wrote the manuscript with input from all co-authors.

Corresponding author

Correspondence to Howard Chen.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Antigona Segura and Eric Hébrard for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Input broadband (a) and UV (b) spectral energy distributions for the Sun, HD85512, and TRAPPIST-1.

The Sun represents the G-star archetype, HD85512 a K-star, and TRAPPIST-1 a late M-star. We refer to the stellar spectral types these stars represent (G-star, K-star, and M-star) instead of the specific star in the main text and throughout the paper.

Extended Data Fig. 2 Timeseries of TESS lightcurves used in this study.

The stellar data used are those of TIC 671393 (a) and TIC 1636399 (b), showing identified flares by orange ‘ × ’s. Flares are identified by a convolutional neural network algorithm described in Feinstein et al. (2020).

Extended Data Fig. 3 Three scenarios of input vertical-mean ionization rates to explore the effects of flare frequency.

Three different assumptions are investigated: α = 0.7, 0.82, 0.54. Supplementary Table 1 lists the specific experiments and their assumed flare frequency.

Extended Data Fig. 4 Spatial and temporal atmospheric effects of repeated stellar flaring on an M-star planet.

Simulated global time slice distributions of upper atmospheric NO (a-d), OH (e-h), and O3 (i-l) concentrations and their global average time-series (m) that result from exposure to flares with time-evolving proton fluences (n). The simulated planet rotates around M-star TRAPPIST-1 synchronously and has a weak magnetic field. and OH mixing ratios are reported at 0.1 hPa, whereas O3 mixing ratios are reported at 1.0 hPa. Spherical projections are centered on 40 N latitude and 225 longitude. Red cross denotes the substellar point.

Extended Data Fig. 5 Temporal evolution of global-mean mixing ratios of NO, OH, and O3 experiencing TESS flares.

Result demonstrate that small flares over a short timespan do not substantially affect exoplanetary atmospheres. NO and OH mixing ratios are reported at 0.1 hPa, whereas O3 mixing ratios are reported at 1.0 hPa.

Extended Data Fig. 6 Global-mean vertical profiles of ozone number density at three different stellar flare frequencies.

These results show the cumulative effect (300 Earth days) of repeated stellar flares. Results that assume α approaching those of observed MUSCLES stars (α = 0.7) established a new chemical equilibrium, whereas those using a values close to very active stars (α = 0.54) have their ozone layers rapidly depleted.

Extended Data Fig. 7 Zonal mean of zonal wind, O3 mixing ratios (10−8), and meridional circulation stream functions for hypothetical O2-rich planets around a G-star, K-star, and M-star as denoted.

Results demonstrate the convolved effects of dynamics and atmospheric chemistry.

Extended Data Fig. 8 NO concentration averaged over the poles (\(\left|{\rm{latitude}}\right|>6{5}^{\circ }\)) as a function of time and pressure for hypothetical O2-rich planets.

The rotation periods of these simulations are 24 hours, 92 Earth days, and 4.32 Earth days around a G-dwarf (a), K-dwarf (b), and M-dwarf (c) star. Note the log10-scale.

Supplementary information

Supplementary Information

Supplementary Tables 1 and 2.

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Chen, H., Zhan, Z., Youngblood, A. et al. Persistence of flare-driven atmospheric chemistry on rocky habitable zone worlds. Nat Astron (2020).

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