Discovery of a massive giant planet with extreme density around the sub-giant star TOI-4603

We present the discovery of a transiting massive giant planet around TOI-4603, a sub-giant F-type star from NASA's Transiting Exoplanet Survey Satellite (TESS). The newly discovered planet has a radius of 1.042_−0.035^+0.038 R_J and an orbital period of 7.24599_−0.00021^+0.00022 days. Using radial velocity measurements with the PARAS and TRES spectrographs, we determined the planet's mass to be 12.89_−0.57^+0.58 M_J, resulting in a bulk density of 14.1_−1.6^+1.7 g cm⁻³. This makes it one of the few known massive giant planets with an extreme density. It lies in the transition mass region of massive giant planets and low-mass brown dwarfs, an important addition to the population of fewer than five known objects in this mass range. The eccentricity of 0.325 ± 0.020 and an orbital separation of 0.0888 ± 0.0010 AU from its host star suggest that the planet is likely undergoing high-eccentricity tidal migration. We find a fraction of heavy elements of 0.13_−0.06^+0.05 and metal enrichment of the planet (Z_P/Z_star) of 4.2_−2.0^+1.6. Detection of such systems will enable us to gain valuable insights into the governing mechanisms of massive planets and improve our understanding of their dominant formation and migration mechanisms.