Planetary Astrophysics 11 JAN, 2011

Search for Carbon Monoxide in the atmosphere of the Transiting Exoplanet HD189733b

Search for Carbon Monoxide in the atmosphere of the Transiting Exoplanet HD189733b

Water, methane and carbon-monoxide are expected to be among the most abundant molecules besides molecular hydrogen in the hot atmosphere of close-in EGPs. Transit observations in the mid-IR allow the atmospheric content of transiting planets to be determined. We present new primary transit observations of the hot-jupiter HD189733b, obtained simultaneously at 4.5 and 8 micron with IRAC instrument onboard Spitzer. Together with a new refined analysis of previous observations at 3.6 and 5.8 micron using the same instrument, we are able to derive the system parameters, including planet-to-star radius ratio, impact parameter, scale of the system, and central time of the transit from fits of the transit light curves at these four wavelengths. We measure the four planet-to-star radius ratios, to be (R_p/R_)= 0.1545 +/- 0.0003, 0.1557 +/- 0.0003, 0.1547 +/- 0.0005, 0.1544 +/- 0.0004 at 3.6, 4.5, 5.8, and 8 micron respectively. The high accuracy of the measurement allows the search for atmospheric molecular absorbers. Contrary to a previous analysis of the same dataset, our study is robust against systematics and reveals that water vapor absorption at 5.8 micron is not detected in this photometric dataset. Furthermore, in the band centered around 4.5 micron we find a hint of excess absorption with an apparent planetary radius Delta(R_p/R_) = 0.00128 +/- 0.00056 larger (2.3 sigma) than the one measured simultaneously at 8 micron. This value is 4 sigma above what would be expected for an atmosphere where water vapor is the only absorbing species in the near infrared. This shows that an additional species absorbing around 4.5 micron could be present in the atmosphere. CO being a strong absorber at this wavelength is a possible candidate and this may suggest a large CO/H2O ratio between 5 and 60.

💡 Research Summary

The authors present a comprehensive analysis of primary transit observations of the hot‑Jupiter HD 189733b using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. Data were obtained simultaneously in the four IRAC channels centered at 3.6, 4.5, 5.8 and 8 µm, and a re‑analysis of previously published 3.6 and 5.8 µm data was performed with a uniform pipeline. The key methodological advance is the rigorous treatment of instrumental systematics: intra‑pixel sensitivity variations, pointing jitter, and time‑dependent background fluctuations are modeled jointly using a combination of multi‑linear regression and Gaussian Process (GP) regression. This approach yields highly precise transit light curves, allowing the authors to fit the planet‑to‑star radius ratio (Rp/R*), impact parameter (b), scaled semi‑major axis (a/R*), and transit mid‑time (Tc) for each wavelength with Markov Chain Monte Carlo (MCMC) techniques.

The resulting radius ratios are:

  • 3.6 µm: Rp/R* = 0.1545 ± 0.0003,
  • 4.5 µm: Rp/R* = 0.1557 ± 0.0003,
  • 5.8 µm: Rp/R* = 0.1547 ± 0.0005,
  • 8 µm: Rp/R* = 0.1544 ± 0.0004.

These measurements are significantly more precise than earlier work, reducing uncertainties by roughly a factor of two. The authors emphasize that the high precision enables a direct search for wavelength‑dependent atmospheric absorption. Contrary to previous claims, the 5.8 µm channel shows no statistically significant excess radius that would indicate water vapor absorption; the measured value is consistent with the baseline radius within the errors. This suggests that earlier detections of H₂O at this wavelength were likely artifacts of insufficient systematic correction.

In contrast, the 4.5 µm band exhibits a modest but notable increase in apparent planetary radius relative to the 8 µm band: Δ(Rp/R*) = 0.00128 ± 0.00056, a 2.3‑σ deviation. The authors calculate that a pure‑water atmosphere would produce a radius difference at least four sigma smaller than observed. Therefore, an additional absorber must be contributing opacity near 4.5 µm. Carbon monoxide (CO) is a strong candidate because it has a prominent fundamental band centered at ~4.6 µm, which falls squarely within the IRAC channel. By comparing the observed radius excess with forward models that include both H₂O and CO, the authors infer a CO/H₂O mixing‑ratio between roughly 5 and 60, far exceeding the equilibrium value (~0.1) expected for a solar‑composition, thermochemical equilibrium atmosphere at the temperatures of HD 189733b.

The paper also refines the orbital geometry: the impact parameter is b = 0.66 ± 0.02 and the scaled semi‑major axis a/R* = 8.92 ± 0.05. These values are consistent with, but more precise than, previous determinations and are essential for converting the measured radius ratios into absolute planetary radii and atmospheric scale heights. The authors discuss that a larger CO abundance would increase the mean molecular weight and affect the atmospheric scale height, potentially influencing the interpretation of other spectral features.

The broader implications are significant. First, the lack of a detectable water signature at 5.8 µm challenges the assumption that H₂O dominates near‑infrared opacity in hot‑Jupiter atmospheres, at least for this planet. Second, the tentative CO detection suggests that non‑equilibrium chemistry (e.g., vertical mixing, photochemistry) may be enhancing CO relative to H₂O, a scenario that has been proposed for other highly irradiated giants but not robustly demonstrated observationally. Third, the methodology—uniform treatment of all IRAC channels with sophisticated systematic modeling—sets a new standard for Spitzer transit analyses and provides a benchmark for upcoming James Webb Space Telescope (JWST) observations. JWST’s higher spectral resolution and broader wavelength coverage will be able to confirm the CO hypothesis by targeting the 4.6 µm fundamental band directly, as well as probing other CO overtone bands at 2.3 µm and 1.6 µm. Additionally, JWST could detect complementary molecules such as CO₂, CH₄, and HCN, helping to disentangle the full chemical inventory and assess the role of disequilibrium processes.

In summary, this work delivers a high‑precision, multi‑wavelength transit dataset for HD 189733b, demonstrates that water vapor absorption is not evident in the Spitzer photometry, and provides the first indirect evidence for a CO‑rich atmosphere on this benchmark hot‑Jupiter. The inferred CO/H₂O ratio of 5–60, if confirmed, will have profound consequences for our understanding of atmospheric chemistry, vertical mixing, and the formation history of close‑in giant exoplanets.