Using small automated telescopes in Arizona and Hawaii , the HATNet project has detected an object transiting one member of the double star system ADS 16402 .
This system is a pair of G0 main-sequence stars with age about 3 Gyr at a distance of \sim 139 pc and projected separation of \sim 1550 AU .
The transit signal has a period of 4.46529 days and depth of 0.015 mag .
From follow-up photometry and spectroscopy , we find that the object is a “ hot Jupiter ” planet with mass about 0.53 M _ { J } and radius \sim 1.36 R _ { J } traveling in an orbit with semimajor axis 0.055 AU and inclination about 85 \fdg 9 , thus transiting the star at impact parameter 0.74 of the stellar radius .
Based on a data set spanning three years , ephemerides for the transit center are : T _ { C } = 2453984.397 + N _ { tr } * 4.46529 .
The planet , designated HAT-P-1b , appears to be at least as large in radius , and smaller in mean density , than any previously-known planet .