Discovery of a massive giant planet with extreme density around a 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+0.038-0.035 RJ, and an orbital period of 7.24599+0.00022-0.00021 days. Using radial velocity measurements with the PARAS {and TRES} spectrographs, we determined the planet's mass to be 12.89+0.58-0.57 MJ, resulting in a bulk density of 14.1+1.7-1.6 g cm-3. This makes it one of the few massive giant planets with extreme density and lies in the transition mass region of massive giant planets and low-mass brown dwarfs, an important addition to the population of less than five 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 (HET) migration. We find a fraction of heavy elements of 0.13+0.05-0.06 and metal enrichment of the planet (ZP/Zstar) of 4.2+1.6-2.0. Detection of such systems will offer us to gain valuable insights into the governing mechanisms of massive planets and improve our understanding of their dominant formation and migration mechanisms.