HAT-P-12b: A low-density sub-saturn mass planet transiting a metal-poor K dwarf

J. D. Hartman, G. Á Bakos, G. Torres, Géza Kovcs, R. W. Noyes, A. Pál, D. W. Latham, B. Sipc'z, D. A. Fischer, J. A. Johnson, G. W. Marcy, R. P. Butler, A. W. Howard, G. A. Esquerdo, D. D. Sasselov, Gábor Kovács, R. P. Stefanik, J. M. Fernandez, J. Lázár, I. PappP. Sári

Research output: Contribution to journalArticlepeer-review

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We report on the discovery of HAT-P-12b, a transiting extrasolar planet orbiting the moderately bright V 12.8 K4 dwarf GSC 03033 - 00706, with a period P = 3.2130598±0.0000021d, transit epoch Tc = 2454419.19556± 0.00020 (BJD), and transit duration 0.0974±0.0006d. The host star has a mass of 0.73 0.02M , radius of 0.70 +0.02 -0.01R , effective temperature 4650 60K, and metallicity [Fe/H] = -0.29 0.05. We find a slight correlation between the observed spectral line bisector spans and the radial velocity, so we consider, and rule out, various blend configurations including a blend with a background eclipsing binary, and hierarchical triple systems where the eclipsing body is a star or a planet. We conclude that a model consisting of a single star with a transiting planet best fits the observations, and show that a likely explanation for the apparent correlation is contamination from scattered moonlight. Based on this model, the planetary companion has a mass of 0.211±0.012M J and radius of 0.959+0.029 -0.021R J yielding a mean density of 0.295±0.025g cm-3. Comparing these observations with recent theoretical models, we find that HAT-P-12b is consistent with a ∼1-4.5 Gyr, mildly irradiated, H/He-dominated planet with a core mass MC ≲ 10 M . HAT-P-12b is thus the least massive H/He-dominated gas giant planet found to date. This record was previously held by Saturn.

Original languageEnglish (US)
Pages (from-to)785-796
Number of pages12
JournalAstrophysical Journal
Issue number1
StatePublished - 2009
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Planetary systems
  • Stars: Individual (HAT-P-12, GSC 03033?00706)
  • Techniques: Photometric
  • Techniques: Spectroscopic


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