The discovery of planetary transits across the star HD 209458 confirms that its radial velocity variations are due to an orbiting planet. High precision photometric measurements show two transit events consistent with radial velocity data. The planet has a radius of 1.27 ± 0.02 R_Jup and an orbital inclination of 87.1 ± 0.2 degrees. The planet is a gas giant with a surface gravity of ~970 cm/s² and an average density of ~0.38 g/cm³. The transit data allow for the calculation of the planet's mass, which is 0.63 M_Jup. The planet's temperature is estimated to be ~1400 K (1 - A)^1/4, where A is the albedo. The thermal velocity of hydrogen is ~6 km/s, much less than the escape velocity of ~42 km/s, indicating the planet is not losing significant mass. The transit observations suggest that other planets in the system may also transit, especially if their orbits are coplanar. The study highlights the importance of high cadence photometry for future transit observations and the potential for detecting planetary atmospheres and surface features. The results provide critical insights into the structure and evolution of exoplanets, and the study underscores the value of combining radial velocity and photometric data to understand planetary systems.The discovery of planetary transits across the star HD 209458 confirms that its radial velocity variations are due to an orbiting planet. High precision photometric measurements show two transit events consistent with radial velocity data. The planet has a radius of 1.27 ± 0.02 R_Jup and an orbital inclination of 87.1 ± 0.2 degrees. The planet is a gas giant with a surface gravity of ~970 cm/s² and an average density of ~0.38 g/cm³. The transit data allow for the calculation of the planet's mass, which is 0.63 M_Jup. The planet's temperature is estimated to be ~1400 K (1 - A)^1/4, where A is the albedo. The thermal velocity of hydrogen is ~6 km/s, much less than the escape velocity of ~42 km/s, indicating the planet is not losing significant mass. The transit observations suggest that other planets in the system may also transit, especially if their orbits are coplanar. The study highlights the importance of high cadence photometry for future transit observations and the potential for detecting planetary atmospheres and surface features. The results provide critical insights into the structure and evolution of exoplanets, and the study underscores the value of combining radial velocity and photometric data to understand planetary systems.