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A JWST transmission spectrum of the nearby Earth-sized exoplanet LHS 475 b

Nature Astronomy volume 7, pages 1317–1328 (2023)Cite this article

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Abstract

The critical first step in the search for life on exoplanets over the next decade is to determine whether rocky planets transiting small M-dwarf stars possess atmospheres and, if so, what processes sculpt them over time. Because of its broad wavelength coverage and improved resolution compared with previous instruments, spectroscopy with the James Webb Space Telescope (JWST) offers a new capability to detect and characterize the atmospheres of Earth-sized, M-dwarf planets. Here we use the JWST to independently validate the discovery of LHS 475 b, a warm (586 K), 0.99 Earth-radius exoplanet, interior to the habitable zone, and report a precise 2.9–5.3 μm transmission spectrum using the Near Infrared Spectrograph G395H instrument. With two transit observations, we rule out primordial hydrogen-dominated and cloudless pure methane atmospheres. Thus far, the featureless transmission spectrum remains consistent with a planet that has a high-altitude cloud deck (similar to Venus), a tenuous atmosphere (similar to Mars) or no appreciable atmosphere at all (akin to Mercury). There are no signs of stellar contamination due to spots or faculae. Our observations show that the JWST has the requisite sensitivity to constrain the secondary atmospheres of terrestrial exoplanets with absorption features <50 ppm, and that our current atmospheric constraints speak to the nature of the planet itself, rather than instrumental limits.

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Fig. 1: White-light curves from both LHS 475 b visits using the FIREFLy reduction.
Fig. 2: 2D light curves of LHS 475 b as a function of time and wavelength for the first visit, measured with NIRSpec G395H.
Fig. 3: Final, binned spectrum compared with models.
Fig. 4: Retrieval results showing preferred atmospheric properties for models containing H2O, CO2, CH4 and CO, plus a variable bulk gas composition for LHS 475 b given the transmission spectrum measurements.

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Data availability

The data used in this paper are from the JWST Cycle 1 General Observer Program 1981 and are publicly available on the Mikulski Archive for Space Telescopes (https://mast.stsci.edu). Fully reduced data products from this paper are available in the following Zenodo long-term public archive: https://doi.org/10.5281/zenodo.7925111. All additional data, such as intermediate data products and model outputs, are available upon request.

Code availability

The codes used throughout this work for data analysis, atmospheric modelling and paper preparation are as follows: Astropy88,89, batman56, CHIMERA68,69, dynesty63, emcee57, Eureka!33, ExoCTK90, Forecaster38, IPython91, jwst47, Matplotlib92, NumPy93,94, PICASO67, POSEIDON80, PyMC395, SciPy96 and smarter76.

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Acknowledgements

This work is based in part on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with programme 1981. Support for programme 1981 was provided by NASA through a grant from the STScI. This paper includes data collected with the TESS mission, obtained from the MAST data archive at the STScI. Funding for the TESS mission is provided by the NASA Explorer Program. The material is based on work supported by NASA under award number 80GSFC21M0002. This material is based upon work performed as part of NASA’s CHAMPs team, supported by NASA under Grant No. 80NSSC21K0905 issued through the Interdisciplinary Consortia for Astrobiology Research (ICAR) programme. R.J.M. is an NHFP Sagan Fellow.

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Authors and Affiliations

  1. Johns Hopkins APL, Laurel, MD, USA

    Jacob Lustig-Yaeger, E. M. May, Kevin B. Stevenson, L. C. Mayorga & Kristin S. Sotzen

  2. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA

    Guangwei Fu & David K. Sing

  3. Center for Astrophysics, Harvard & Smithsonian, Cambridge, MA, USA

    Kevin N. Ortiz Ceballos, Mercedes López-Morales & Jéa I. Adams Redai

  4. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

    Sarah E. Moran

  5. University of Maryland, Baltimore County, MD, USA

    Sarah Peacock

  6. NASA Goddard Space Flight Center, Greenbelt, MD, USA

    Sarah Peacock & Ethan Kruse

  7. Department of Physics, Imperial College London, London, UK

    James Kirk

  8. Department of Astronomy, University of Michigan, Ann Arbor, MI, USA

    Ryan J. MacDonald

  9. Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA

    David K. Sing, Katherine A. Bennett, Junellie Gonzalez-Quiles & Zafar Rustamkulov

  10. Space Telescope Science Institute, Baltimore, MD, USA

    Jeff A. Valenti

  11. Carnegie Earth and Planets Laboratory, Washington DC, USA

    Munazza K. Alam

  12. NASA Ames Research Center, Moffett Field, CA, USA

    Natasha E. Batalha

  13. Department of Astronomy, University of Maryland, College Park, MD, USA

    Ethan Kruse

  14. Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD, USA

    Ethan Kruse

  15. Department of Physics, Utah Valley University, Orem, UT, USA

    Joshua D. Lothringer

  16. School of Physics, HH Wills Physics Laboratory, University of Bristol, Bristol, UK

    Hannah R. Wakeford

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  1. Jacob Lustig-Yaeger
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All authors played a substantial role in one or more of the following: development of the original proposal, management of the project, observation planning, analysis of the data, theoretical modelling and preparation of this paper. J.L.-Y., G.F., K.B.S., E.M.M., K.N.O.C., S.E.M., S.P., M.L.-M., R.J.M. and J.K. made notable contributions to the figures and text of the paper. K.B.S., J.L.-Y., M.L.-M. and D.K.S. provided general programme leadership and management. K.B.S., J.L.-Y., E.M.M., S.E.M., D.K.S., K.S.S., N.E.B., J.D.L. and H.R.W. made important contributions to the design of the programme. K.B.S. created the observing plan with input from the team. L.C.M., J.K., D.K.S., K.S.S., J.A.V., J.I.A.R., M.K.A., N.E.B., K.A.B., J.G.-Q., E.K., J.D.L., Z.R. and H.R.W. contributed to the preparation of the paper.

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Correspondence to Jacob Lustig-Yaeger or Guangwei Fu.

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Nature Astronomy thanks Hannah Diamond-Lowe and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Fit to the stellar spectrum.

Comparison of our 30 closest matching PHOENIX models (\({{\chi }^{2}}_{\nu } <\) 50) to all available archival photometry of LHS 475 from the VizieR Photometry Viewer. These models have Teff = 3300+80−30 K, log(g) = 5.2 ± 0.5 g/cm2, M = 0.262 M⊙. The error bars are 1-σ standard deviations.

Extended Data Fig. 2 Validation of the LHS 475 b.

Left: A 3.36’ × 3.36’ DSS image centered on LHS 475 taken 1999 June 20. The red circle depicts the star’s J2000 position per Simbad, whereas the blue circle indicates the star’s position for the JWST observations in September of 2022. We see no indication of a background star at the 2022 position that could be the source of the observed transit signal. For reference, NIRSpec’s field of view is 1.6 × 1.6 pixels on this image. Right: Sensitivity curve of LHS 475 from a NIRSpec target acquisition image (upper right) spanning 1.6" × 1.6", using the SUB32 subarray and CLEAR filter. The sensitivity curve and visual inspection of the image show no contamination sources (down to Δmag=3) to within 0.1 arcsec, which corresponds to a distance of 1.25 AU.

Extended Data Fig. 3 Relative transit depths from our three independent reductions.

Blue circles: FIREFly; red squares: Eureka!; yellow triangles: Tiberius. The error bars are 1-σ standard deviations. For plotting purposes, we bin the transmission spectra to a resolution of ~ 40 nm. We see excellent agreement between reductions from 2.8 - 4.5 μm. At > 4.5 μm, the binned spectra begin to diverge, though the unbinned data (not shown) are all consistent within 1σ.

Extended Data Fig. 4 Corner plot on a subset of the fitting parameters in the TLS contamination retrieval.

Prior PDFs are in orange and posterior PDFs are in dark blue. The flat spectrum reveals a consistent spot (and faculae) coverage along the transit chord compared to the full stellar disk.

Extended Data Fig. 5 Comparison of the observations to atmospheric models using compositions of the Solar System terrestrial bodies.

LHS 475 b’s binned spectrum is represented by black circles. The error bars are 1-σ standard deviations showing the uncertainties in the transit depth and width of each wavelength bin. Our data weakly rule out Earth composition (blue solid), clear Titan composition (orange solid), and clear Venus composition atmospheres (yellow solid). However, the data are all consistent within error to that of a hazy Titan composition with a haze-top at 0.01 mbar (dotted orange), a cloudy Venus composition with a cloud-top at 1 mbar (dotted yellow), and a Mars composition atmosphere (red solid), as well as that of an airless body, like Mercury (grey dotted line).

Extended Data Fig. 6 Corner plot showing the 1D and 2D marginalized posterior probability distribution for a subset of the smarter model parameters.

The upper right axis shows the transmission spectrum data with 1-? standard deviation error bars along with 1σ and 3σ envelopes around the median retrieved spectrum, which corresponds to the multidimensional posterior PDF projected onto the observed spectrum. Disfavored atmospheres are thick (large P0), hot (large T0), and composed primarily of light molecules (low μ).

Extended Data Fig. 7 Retrieved volume mixing ratios from the POSEIDON retrievals of LHS 475 b’s transmission spectrum.

Two retrievals with different prior treatments for the atmospheric composition are overplotted: centered log-ratio (CLR) transformed abundances with a priori unknown composition (green); and log-uniform abundances assuming an N2-dominated atmosphere (orange). Statistical 2σ upper and lower limits are annotated (or ‘N/A’ if unconstrained). Both retrievals rule out H2-dominated atmospheres. The log- uniform retrieval finds upper limits on H2O, CH4, CO2, and CO due to the assumption that N2 dominates the atmosphere, while the agnostic CLR treatment does not find upper limits for their abundances. For clarity in viewing upper limits, we switch from a logarithmic to linear x-axis at a mixing ratio of 10%. The probability densities for the linear histogram bins are renormalized to match the probability density of the nearest logarithmic bin left of the 10% boundary.

Extended Data Fig. 8 Corner plot showing the 1D and 2D marginalized posterior probability distributions from the POSEIDON retrieval using CLR mixing ratio parameters.

The units are: Rp, ref (R⊕), g (cm s−2), Psurf (bar), and T (K). The inset shows the corresponding retrieved transmission spectrum model (1σ and 2σ confidence regions) compared to the NIRSpec G395H observations. The error bars are 1-σ standard deviations. The solution rules out H2-dominated atmospheres (to > 5σ) and thick atmospheres (Psurf ≳ 10 mbar) dominated by CH4 (to 3σ).

Extended Data Fig. 9 Precision achieved as a function of wavelength.

Top: Eureka! spectrophotometric precision at the native pixel resolution, compared to expected noise levels for both events. The expected noise level, as well as 1.25 × and 2 × the expected noise are shown as grey lines, these have been smoothed to the resolution of the final transit spectrum for visualization purposes. Squares denote columns which are greater than 1.5 × the expected noise level in both transits, dark blue circles denote columns which are greater than 1.5 × the expected noise level in only one transit. These columns are flagged and not used to generate the final transmission spectrum. Bottom: Comparison of uncertainty on planet radius derived from light curves fit at the native pixel resolution and fitting of pre- binned light curves. The y-axis is in parts per thousand. We find little to no difference in the uncertainty, suggesting that our 1/f correction is sufficient to address the column-column variances.

Extended Data Fig. 10 Allan variance plots from the white and spectroscopic light curve fits.

Panel (a) illustrates that the white light curve residuals from two of the analyses exhibit some correlated noise at timescales of < 5 minutes ( < 35 integrations). This is likely due to uncorrected 1/f noise from the thermal cycling of on-board heaters [61, Section 4.5.3]. At longer timescales ( > 120 integrations, > 18 minutes), the Eureka! pipeline returns to the expected standard error with RMS values below 10 ppm. The Tiberius reduction did not sum the flux across both detectors and was not used for this noise floor analysis. The spectroscopic RMS values in panels (b)–(d) are more consistent with the standard error, thus confirming that the spectroscopic light curves are dominated by white noise.

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Lustig-Yaeger, J., Fu, G., May, E.M. et al. A JWST transmission spectrum of the nearby Earth-sized exoplanet LHS 475 b. Nat Astron 7, 1317–1328 (2023). https://doi.org/10.1038/s41550-023-02064-z

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