HD 209458 b
Hubble team announces the first direct detection of an exoplanet atmosphere—sodium around HD 209458 b
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Original prompt & settings (JSON) ↗The atmospheric signal was an extra dimming of about 0.023 percent, pulled out of 684 Hubble spectra — and the element was sodium, not hydrogen.
27 November 2001: sodium, not hydrogen
NASA announced on 27 November 2001 that David Charbonneau, Timothy Brown, Robert Noyes and Ronald Gilliland had used the Hubble Space Telescope to make the first direct detection and chemical measurement of an atmosphere around an extrasolar planet. The element detected was neutral sodium.
Hydrogen belongs to a later result. An extended, escaping hydrogen atmosphere around the same planet was reported by Alfred Vidal-Madjar's team in Nature on 13 March 2003.
684 spectra, four transits
Hubble's Space Telescope Imaging Spectrograph measured starlight filtered through the atmosphere of HD 209458 b during four transits; the planet circles its star in about 3.5 Earth days. The team collected 684 spectra.
They found extra absorption around sodium's 589.3-nanometre resonance doublet: the sodium-band transit ran deeper than adjacent bands by (2.32 ± 0.57) × 10⁻⁴. Nothing was photographed — the chemical fingerprint was read from starlight passing through the planet's limb.
Less sodium than the model predicted
The measured absorption fell short of what the team's cloudless, solar-abundance model predicted. The explanations they offered included high clouds, sodium chemistry, photoionisation and a lower sodium abundance.
The accepted paper was posted as a preprint on 28 November and appeared in the Astrophysical Journal on 20 March 2002.
Sources
Researched 23 Aug 2026 5 sources 2 audit passes
How this was checked
Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →
What the sources leave uncertain
- November 27 is securely supported as NASA’s announcement date. The peer-reviewed article was accepted November 19, posted publicly on November 28, and published in March 2002.
- The event description supplied with the slug is materially wrong about the substance detected: hydrogen escape was reported separately in March 2003.
Checked against
- NASA: Hubble Makes First Direct Measurements of Atmosphere on World Around Another Star
- Charbonneau et al., Detection of an Extrasolar Planet Atmosphere
- Vidal-Madjar et al., An Extended Upper Atmosphere Around the Extrasolar Planet HD 209458 b
- Vidal-Madjar et al., Detection of Oxygen and Carbon in the Hydrodynamically Escaping Atmosphere
- NASA: Super-Hot Planet with a Unique Comet-Like Tail