We derive improved system parameters for the HDÂ 209458 system using a model that simultaneously fits both photometric transit and radial velocity observations .
The photometry consists of previous Hubble Space Telescope STIS and FGS observations , twelve I –band transits observed between 2001–2003 with the Mt. Laguna Observatory 1m telescope , and six Strömgren b + y transits observed between 2001–2004 with two of the Automatic Photometric Telescopes at Fairborn Observatory .
The radial velocities were derived from Keck/HIRES observations .
The model properly treats the orbital dynamics of the system , and thus yields robust and physically self-consistent solutions .
Our set of system parameters agrees with previously published results though with improved accuracy .
For example , applying robust limits on the stellar mass of 0.93–1.20Â M _ { \odot } , we find 1.26 < R _ { planet } < 1.42 Â R _ { Jup } Â and 0.59 < M _ { planet } < 0.70 Â M _ { Jup } .
We can reduce the uncertainty on these estimates by including a stellar mass–radius relation constraint , yielding R _ { planet } =1.35 \pm 0.07Â R _ { Jup } Â and M _ { planet } =0.66 \pm 0.04Â M _ { Jup } .
Our results verify that the planetary radius is 10–20 % larger than predicted by irradiated planet evolution models , confirming the need for an additional mechanism to slow the evolutionary contraction of the planet .
A revised ephemeris is derived , T _ { 0 } = 2452854.82545 + 3.52474554 E ( HJD ) , which now contains an uncertainty in the period of 0.016Â s and should facilitate future searches for planetary satellites and other bodies in the HDÂ 209458 system .