The 8 μm phase variation of the hot saturn HD 149026b

Heather A. Knutson, David Charbonneau, Nicolas B. Cowan, Jonathan J. Fortney, Adam P. Showman, Eric Agol, Gregory W. Henry

Research output: Contribution to journalArticlepeer-review

101 Scopus citations


We monitor the star HD 149026 and its Saturn-mass planet at 8.0 μm over slightly more than half an orbit using the Infrared Array Camera on the Spitzer Space Telescope. We find an increase of 0.0227% 0.0066% (3.4σ significance) in the combined planet-star flux during this interval. The minimum flux from the planet is 45% 19% of the maximum planet flux, corresponding to a difference in brightness temperature of 480 140 K between the two hemispheres. We derive a new secondary eclipse depth of 0.0411% 0.0076% in this band, corresponding to a dayside brightness temperature of 1440 150 K. Our new secondary eclipse depth is half that of a previous measurement (3.0σ difference) in this same bandpass by Harrrington et al. We re-fit the Harrrington et al. data and obtain a comparably good fit with a smaller eclipse depth that is consistent with our new value. In contrast to earlier claims, our new eclipse depth suggests that this planet's dayside emission spectrum is relatively cool, with an 8 μm brightness temperature that is less than the maximum planet-wide equilibrium temperature. We measure the interval between the transit and secondary eclipse and find that that the secondary eclipse occurs 20.9+7.2 -6.5 minutes earlier (2.9σ) than predicted for a circular orbit, a marginally significant result. This corresponds to ecos(ω) = -0.0079+0.0027 -0.0025, where e is the planet's orbital eccentricity and ω is the argument of pericenter.

Original languageEnglish (US)
Pages (from-to)769-784
Number of pages16
JournalAstrophysical Journal
Issue number1
StatePublished - 2009
Externally publishedYes


  • Eclipses
  • Planetary systems
  • Stars: individual (HD 149026b)
  • Techniques: photometric

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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