[Paper Review] Transit spectrophotometry of the exoplanet HD189733b. I. Searching for water but finding haze with HST NICMOS

David K. Sing, Jean-Michel Désert+6 more|arXiv (Cornell University)|Jul 29, 2009

Stellar, planetary, and galactic studies44 references112 citations

TL;DR

This study uses Hubble Space Telescope NICMOS narrowband photometry of HD189733b's transits to search for atmospheric water vapor in the near-infrared. Despite targeting H2O absorption, the data reveal no significant H2O feature at 5σ confidence, instead indicating that Rayleigh scattering by sub-micron haze particles dominates the transmission spectrum from 0.5 to 2 μm, obscuring molecular signatures at these wavelengths.

ABSTRACT

We present Hubble Space Telescope near-infrared transit photometry of the nearby hot-Jupiter HD189733b. The observations were taken with the NICMOS instrument during five transits, with three transits executed with a narrowband filter at 1.87 microns and two performed with a narrowband filter at 1.66 microns. Our observing strategy using narrowband filters is insensitive to the usual HST intra-orbit and orbit-to-orbit measurement of systematic errors, allowing us to accurately and robustly measure the near-IR wavelength dependance of the planetary radius. Our measurements fail to reproduce the Swain et al. absorption signature of atmospheric water below 2 microns at a 5-sigma confidence level. We measure a planet-to-star radius contrast of 0.15498+/-0.00035 at 1.66 microns and a contrast of 0.15517+/-0.00019 at 1.87 microns. Both of our near-IR planetary radii values are in excellent agreement with the levels expected from Rayleigh scattering by sub-micron haze particles, observed at optical wavelengths, indicating that upper-atmospheric haze still dominates the near-IR transmission spectra over the absorption from gaseous molecular species at least below 2 microns.

Motivation & Objective

• To precisely measure the near-infrared planetary radius of HD189733b using Hubble Space Telescope NICMOS narrowband photometry.
• To test the presence of atmospheric water vapor absorption features below 2 μm, as previously reported by Swain et al. (2008).
• To determine whether upper-atmospheric haze or gaseous molecular species dominate the near-infrared transmission spectrum.
• To assess the robustness of near-infrared transit photometry in the presence of systematic errors by using narrowband filters.
• To provide a critical benchmark for future James Webb Space Telescope observations of exoplanet atmospheres.

Proposed method

• Conducted five Hubble Space Telescope NICMOS transits using two narrowband filters at 1.66 μm and 1.87 μm to minimize systematic errors.
• Employed a photometric strategy insensitive to intra-orbit and orbit-to-orbit systematic errors common in spectroscopic modes.
• Measured the planet-to-star radius contrast at each wavelength with high precision, achieving uncertainties of ±0.00035 and ±0.00019.
• Compared the measured radii to predictions from Rayleigh scattering by sub-micron haze particles and to models including H2O absorption.
• Used the wavelength dependence of the radius contrast to infer particle size distribution and opacity sources.
• Evaluated consistency with prior Swain et al. (2008) spectroscopic data, excluding extreme spectral edges to test for compatibility with haze models.

Experimental results

Research questions

• RQ1 Does the near-infrared transmission spectrum of HD189733b show detectable absorption features from atmospheric water vapor below 2 μm?
• RQ2 To what extent do sub-micron haze particles dominate the near-infrared transmission spectrum of HD189733b?
• RQ3 Are the previously reported H2O absorption features in the Swain et al. (2008) spectrum robust to systematic error contamination?
• RQ4 What is the inferred particle size distribution of the upper-atmospheric haze based on the wavelength dependence of the planetary radius?
• RQ5 How do the results from narrowband photometry compare with broadband spectroscopic measurements from HST and Spitzer?

Key findings

• The planet-to-star radius contrast at 1.66 μm is 0.15498 ± 0.00035, and at 1.87 μm is 0.15517 ± 0.00019, showing no significant wavelength dependence.
• The observed radii are in excellent agreement with predictions from Rayleigh scattering by sub-micron haze particles, not H2O absorption.
• The study rules out the presence of H2O absorption features below 2 μm at a 5σ confidence level, challenging the findings of Swain et al. (2008).
• The data suggest that the observed spectral features in Swain et al. (2008) are likely artifacts of residual systematic errors, particularly at spectral edges.
• The maximum particle size in the haze distribution is constrained to 0.009–0.086 μm, consistent with MgSiO3 condensates.
• The transition from haze-dominated to gas-dominated transmission occurs between 2 and 3 μm, as Spitzer IRAC photometry shows larger radii beyond 3 μm.