Exoplanet Atmosphere Report · JWST MIRI LRS

HD 209458 b

Nicknamed "Osiris" — the first known exoplanet observed to transit its star, and the first exoplanet whose atmosphere was directly detected. More than two decades later, JWST is still finding new structure in its atmosphere: a 2026 MIRI spectrum testing for magnesium silicate clouds.

Discovered 1999, RV + transit NASA Exoplanet Archive parameters 2026 JWST MIRI LRS spectrum
Artist's concept of HD 209458 b, the first exoplanet with a directly detected atmosphere

AI-generated artist's concept of HD 209458 b — not a real photograph. All data and figures in this report come from actual JWST MIRI observations (see below).

The planet, in numbers

Queried live from the NASA Exoplanet Archive TAP service (pscomppars).

Radius15.58 Earth radii (~1.39 Jupiter radii)
Mass232.0 Earth masses (~0.73 Jupiter masses)
Orbital period3.52 days
Semi-major axis0.047 AU
Equilibrium temperature1459 K
Host starHD 209458, G-type dwarf, Teff = 6091 K, 1.19 Rsun, 1.23 Msun
Distance48.3 parsecs (~157 light-years)
Discovery1999, radial velocity, transit confirmed the same year

A quarter-century of firsts

1999 — Discovered by radial velocity (Mazeh et al., Latham et al.); its transit was detected within months, making it the first exoplanet confirmed by both methods.
2000 — Henry et al. and Charbonneau et al. independently report the transit photometry, giving the first directly measured exoplanet radius.
2002 — Charbonneau et al. detect sodium absorption in transit with HST STIS: the first exoplanet atmosphere ever directly measured.
2007-2010s — Extensive HST and Spitzer follow-up establishes escaping hydrogen, water vapor, and haze/scattering in its atmosphere.
2026 — Chubb, Grant, and collaborators publish a JWST MIRI LRS transmission spectrum testing for magnesium silicate cloud absorption, using a rule-based tree-structured data reduction to quantify how much pipeline choices affect the result (the data behind this report).
Close-up 3D-rendered concept of HD 209458 b's cloudy atmosphere

AI-generated 3D-render-style concept of the magnesium silicate cloud layer this report's data tests for — not an actual image of the planet.

JWST MIRI spectrum, and how much the reduction pipeline matters

The figure audits the reduced data behind Figure 4 of the 2026 paper and two publication-supplied PICASO/Virga cloud-model products. The four “leaves” share the same exposures and most processing, so they are correlated sensitivity branches—not four independent experiments. Leaf 3 is the version used in most source-paper retrievals.

HD 209458 b JWST MIRI LRS transmission spectrum from four correlated reduction-tree leaves
Observed leaves, vertically aligned cloud-model shapes, standardized residuals, and wavelength-resolved reduction sensitivity. Generated from DOI-pinned products by scripts/analyze_spectrum.py.
Wavelength bins28
Mean transit depth14458 ppm
Spread > mean reported 1σ18 / 28 bins
Mean leaf spread122 ppm

A predeclared 8.0–10.2 minus 5.2–7.8 μm contrast is 267–294 ppm across the four leaves. After fitting one vertical offset, the supplied Mg2SiO4 and mixed Mg2SiO4/MgSiO3 median curves have descriptive χ² values of 69.90 and 84.07 across 28 bins.

These are reproducibility diagnostics, not new detection significances. The leaves are correlated, spectral covariance is unavailable in the compact table, and the cloud curves are posterior products fitted by the source team to a larger combined dataset. The source retrieval—not this page—supports the magnesium-silicate identification.

Data and method notes

System parameters are preserved as a dated NASA Exoplanet Archive TAP CSV. The spectrum, two model files, and publisher READMEs come from Zenodo v1 record 10.5281/zenodo.20089901. The integrity manifest records archive MD5 values, committed SHA-256 digests, retrieval dates, and inference boundaries. Reproduce the outputs with python scripts/analyze_spectrum.py.

AI-generated illustration of the James Webb Space Telescope

AI-generated illustration of the James Webb Space Telescope, whose MIRI instrument took the real spectrum used in this report. Not an official mission photograph — see NASA/JWST for real imagery.

References

  1. Charbonneau, D. et al., 2002. Detection of an Extrasolar Planet Atmosphere. The Astrophysical Journal, 568(1), pp.377-384.
  2. Chubb, K.L., Grant, D. et al., 2026. Magnesium Silicate Clouds in the Atmosphere of HD 209458b from a Rule-Based Tree-Structured Data Reduction. Zenodo record 10.5281/zenodo.20089901.
  3. Henry, G.W. et al., 2000. A Transiting "51 Peg-like" Planet. The Astrophysical Journal Letters, 529(1), pp.L41-L44.
  4. Vidal-Madjar, A. et al., 2003. An extended upper atmosphere around the extrasolar planet HD209458b. Nature, 422, pp.143-146 (escaping hydrogen).
  5. NASA Exoplanet Archive, exoplanetarchive.ipac.caltech.edu — system parameters, queried live via TAP.