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A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c

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Abstract

Three Earth-sized exoplanets were recently discovered close to the habitable zone1,2 of the nearby ultracool dwarf star TRAPPIST-1 (ref. 3). The nature of these planets has yet to be determined, as their masses remain unmeasured and no observational constraint is available for the planetary population surrounding ultracool dwarfs, of which the TRAPPIST-1 planets are the first transiting example. Theoretical predictions span the entire atmospheric range, from depleted to extended hydrogen-dominated atmospheres4,5,6,7,8. Here we report observations of the combined transmission spectrum of the two inner planets during their simultaneous transits on 4 May 2016. The lack of features in the combined spectrum rules out cloud-free hydrogen-dominated atmospheres for each planet at ≥10σ levels; TRAPPIST-1 b and c are therefore unlikely to have an extended gas envelope as they occupy a region of parameter space in which high-altitude cloud/haze formation is not expected to be significant for hydrogen-dominated atmospheres9. Many denser atmospheres remain consistent with the featureless transmission spectrum—from a cloud-free water-vapour atmosphere to a Venus-like one.

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Figure 1: Hubble/WFC3 white-light curve for the TRAPPIST-1b and TRAPPIST-1c double transit of 4 May 2016.
Figure 2: Hubble/WFC3 spectrophotometry of the TRAPPIST-1b and TRAPPIST-1c double transit of 4 May 2016.
Figure 3: Transmission spectra of TRAPPIST-1b and TRAPPIST-1c compared with models.

References

  1. Kopparapu, R. K. et al. Habitable zones around main-sequence stars: new estimates. Astrophys. J. 765, 131 (2013)

    ADS  Article  Google Scholar 

  2. Zsom, A., Seager, S., de Wit, J. & Stamenkovic, V. Towards the minimum inner edge distance of the habitable zone. Astrophys. J. 778, 109 (2013)

    ADS  Article  Google Scholar 

  3. Gillon, M. et al. Temperate Earth-sized planets transiting a nearby ultracool dwarf star. Nature 533, 221–224 (2016)

    CAS  ADS  Article  Google Scholar 

  4. Owen, J. E. & Wu, Y. Kepler planets: a tale of evaporation. Astrophys. J. 775, 105 (2013)

    ADS  Article  Google Scholar 

  5. Jin, S. et al. Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation. Astrophys. J. 795, 65 (2014)

    ADS  Article  Google Scholar 

  6. Johnstone, C. P. et al. The evolution of stellar rotation and the hydrogen atmospheres of habitable-zone terrestrial planets. Astrophys. J. 815, L12 (2015)

    ADS  Article  Google Scholar 

  7. Luger, R. & Barnes, R. Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs. Astrobiology 15, 119–143 (2015)

    CAS  ADS  Article  Google Scholar 

  8. Owen, J. E. & Mohanty, S. Habitability of terrestrial-mass planets in the HZ of M dwarfs. I. H/He-dominated atmospheres. Mon. Not. R. Astron. Soc. 459, 4088–4108 (2016)

    CAS  ADS  Article  Google Scholar 

  9. Morley, C. V. et al. Thermal emission and reflected light spectra of super Earths with flat transmission spectra. Astrophys. J. 815, 110 (2015)

    ADS  Article  Google Scholar 

  10. McCullough, P. & MacKenty, J. Considerations for using spatial scans with WFC3. Instr. Sci. Report WFC3 2012-08 (Space Telescope Science Institute, 2012)

  11. Deming, D. et al. Infrared transmission Spectroscopy of the exoplanets HD 209458b and XO-1b using the wide field camera-3 on the Hubble Space Telescope. Astrophys. J. 774, 95 (2013)

    ADS  Article  Google Scholar 

  12. Wakeford, H. R., Sing, D. K., Evans, T., Deming, D. & Mandell, A. Marginalizing instrument systematics in HST WFC3 transit light curves. Astrophys. J. 819, 10 (2016)

    ADS  Article  Google Scholar 

  13. Sing, D. K. et al. A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion. Nature 529, 59–62 (2016)

    CAS  ADS  Article  Google Scholar 

  14. Kreidberg, L. et al. Clouds in the atmosphere of the super-Earth exoplanet GJ1214b. Nature 505, 69–72 (2014)

    ADS  Article  Google Scholar 

  15. Husser, T.-O. et al. A new extensive library of PHOENIX stellar atmospheres and synthetic spectra. Astron. Astrophys. 553, A6 (2013)

    Article  Google Scholar 

  16. Hirano, T. et al. Planet-planet eclipse and the Rossiter-McLaughlin effect of a multiple transiting system: joint analysis of the Subaru spectroscopy and the Kepler photometry. Astrophys. J. 759, L36 (2012)

    ADS  Article  Google Scholar 

  17. Mandel, K. & Agol, E. Analytic light curves for planetary transit searches. Astrophys. J. 580, L171–L175 (2002)

    ADS  Article  Google Scholar 

  18. Sing, D. K. Stellar limb-darkening coefficients for CoRot and Kepler. Astron. Astrophys. 510, A21 (2010)

    ADS  Article  Google Scholar 

  19. de Wit, J. & Seager, S. Constraining exoplanet mass from transmission spectroscopy. Science 342, 1473–1477 (2013)

    CAS  ADS  Article  Google Scholar 

  20. Howe, A. R., Burrows, A. & Verne, W. Mass-radius relations and core-envelope decompositions of super-Earths and sub-Neptunes. Astrophys. J. 787, 173 (2014)

    ADS  Article  Google Scholar 

  21. Bean, J. L., Miller-Ricci Kempton, E. & Homeier, D. A ground-based transmission spectrum of the super-Earth exoplanet GJ 1214b. Nature 468, 669–672 (2010)

    CAS  ADS  Article  Google Scholar 

  22. Berta, Z. K. et al. The flat transmission spectrum of the super-Earth GJ1214b from wide field camera 3 on the Hubble Space Telescope. Astrophys. J. 747, 35 (2012)

    ADS  Article  Google Scholar 

  23. Leconte, J., Forget, F. & Lammer, H. On the (anticipated) diversity of terrestrial planet atmospheres. Exp. Astron. 40, 449–467 (2015)

    ADS  Article  Google Scholar 

  24. Tellmann, S., Pätzold, M., Häusler, B., Bird, M. K. & Tyler, G. L. Structure of the Venus neutral atmosphere as observed by the Radio Science experiment VeRa on Venus Express. J. Geophys. Res. Planets 114, E00B36 (2009)

    ADS  Article  Google Scholar 

  25. Wilquet, V. et al. Preliminary characterization of the upper haze by SPICAV/SOIR solar occultation in UV to mid-IR onboard Venus Express. J. Geophys. Res. Planets 114, E00B42 (2009)

    Article  Google Scholar 

  26. Ehrenreich, D. et al. Transmission spectrum of Venus as a transiting exoplanet. Astron. Astrophys. 537, L2 (2012)

    ADS  Article  Google Scholar 

  27. Evans, T. M. et al. Detection of H2O and evidence for TiO/VO in an ultra-hot exoplanet atmosphere. Astrophys. J. 822, L4 (2016)

    ADS  Article  Google Scholar 

  28. Huitson, C. M. et al. An HST optical-to-near-IR transmission spectrum of the hot Jupiter WASP-19b: detection of atmospheric water and likely absence of TiO. Mon. Not. R. Astron. Soc. 434, 3252–3274 (2013)

    CAS  ADS  Article  Google Scholar 

  29. Eastman, J., Siverd, R. & Gaudi, B. S. Achieving better than 1 minute accuracy in the heliocentric and barycentric Julian dates. Publ. Astron. Soc. Pacif. 122, 935–946 (2010)

    ADS  Article  Google Scholar 

  30. Knutson, H. A. et al. Hubble Space Telescope near-IR transmission spectroscopy of the super-Earth HD 97658b. Astrophys. J. 794, 155 (2014)

    ADS  Article  Google Scholar 

  31. Gibson, N. P. Reliable inference of exoplanet light-curve parameters using deterministic and stochastic systematics models. Mon. Not. R. Astron. Soc. 445, 3401–3414 (2014)

    ADS  Article  Google Scholar 

  32. Gillon, M. et al. The TRAPPIST survey of southern transiting planets. I. Thirty eclipses of the ultra-short period planet WASP-43 b. Astron. Astrophys. 542, A4 (2012)

    ADS  Article  Google Scholar 

  33. de Wit, J. et al. Direct measure of radiative and dynamical properties of an exoplanet atmosphere. Astrophys. J. 820, L33 (2016)

    ADS  Article  Google Scholar 

  34. Espinoza, N. & Jordán, A. Limb darkening and exoplanets: testing stellar model atmospheres and identifying biases in transit parameters. Mon. Not. R. Astron. Soc. 450, 1879–1899 (2015)

    ADS  Article  Google Scholar 

  35. Benneke, B. & Seager, S. Atmospheric retrieval for super-Earths: uniquely constraining the atmospheric composition with transmission spectroscopy. Astrophys. J. 753, 100 (2012)

    ADS  Article  Google Scholar 

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Acknowledgements

This work is based on observations made with the NASA/ESA Hubble Space Telescope that were obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. These observations are associated with program HST-GO-14500 (principal investigator J.d.W.), support for which was provided by NASA through a grant from the Space Telescope Science Institute. The research leading to our results was funded in part by the European Research Council (ERC) under the FP/2007-2013 ERC grant 336480, and through an Action de Recherche Concertée (ARC) grant financed by the Wallonia-Brussels Federation. H.R.W. acknowledges support through an appointment to the NASA Postdoctoral Program at Goddard Space Flight Center, administered by the Universities Space Research Association through a contract with NASA. M.G. is Research Associate at the Belgian Fonds (National) de la Recherche Scientifique (FRS–FNRS). L.D. acknowledges support of the Fund for Research Training in Industry and Agriculture of the FRS–FNRS. We thank D. Taylor, S. Deustua, P. McCullough, and N. Reid for their assistance in planning and executing our observations. We are also grateful for discussions with Z. Berta-Thompson and Pierre Magain about this study and manuscript. We thank the ATLAS and PHOENIX teams for providing stellar models.

Author information

Authors and Affiliations

Authors

Contributions

J.d.W. and H.R.W. led the data reduction and analysis, with the support of M.G., N.K.L. and B.-O.D. J.d.W., H.R.W., and N.K.L. led the data interpretation, with the support of M.G. and J.A.V. J.A.V. provided the limb-darkening coefficients and further insights into TRAPPIST-1’s properties and emission together with A.J.B. Every author contributed to writing both the manuscript and the HST proposal behind these observations.

Corresponding author

Correspondence to Julien de Wit.

Additional information

Reviewer Information Nature thanks D. Ehrenreich and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Marginal effects of instrumental systematics on transit depth estimates.

a, Evidence-based weight, Wq, for each systematic model12 applied to the white-light curve. b, Combined transit depth estimate (ΔFb+c) obtained by correcting the data, using each systematic model. c, d, Individual transit depth estimates for TRAPPIST-1b and TRAPPIST-1c, ΔFb and ΔFc. The horizontal lines indicate the final marginalized measurements and associated uncertainties. The scale of the values here indicates that all of the systematic models fit equally well to the data.

Extended Data Figure 2 TRAPPIST-1’s limb darkening.

Stellar limb-darkening relationships for TRAPPIST-1 (black curves) and four stellar models (coloured curves) that bracket the effective temperature and surface gravity of TRAPPIST-1 (shown in coloured and black numbers in a; temperature is in K and surface gravity is expressed in log(g). The circles are theoretical15 specific intensities (I) relative to disc centre (Ic) as a function of μ′ (the cosine of the angle between an outward radial vector and the direction towards the observer). We fitted I/Ic averaged over the indicated wavelength intervals to determine the quadratic (dashed curves) and four-parameter (solid curves) limb-darkening coefficients. a, Stellar limb-darkening relationship integrated over WFC3’s spectral band. bl, Stellar limb-darkening relationship over the 11 spectral channels used here.

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de Wit, J., Wakeford, H., Gillon, M. et al. A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c. Nature 537, 69–72 (2016). https://doi.org/10.1038/nature18641

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