The planet was discovered by TESS using the transit method, which involves measuring light curves during a planet’s eclipse. The paper states that it’s inflated due to heating from its host star, which has a high luminosity.[2] Its discovery was announced in October 2021.
Properties
Orbit and mass
TOI-4138 b has an orbital period of 3.6 days, typical for a hot Jupiter. This corresponds to a separation from its host close to one eighth of the distance of Mercury from the Sun. Since the inclination is known, doppler spectroscopy measurements give the planet a mass only 67% that of Jupiter.[1] Its separation is comparable with HD 209458 b, but is much larger due to the evolved state of the host star.[1]
Radius and density
TOI-4138 b’s transit gives it a radius 1.49 times that of Jupiter; this combined with its low mass of 0.67MJ gives it a density only 25% that of water.[1]
Host star
TOI-4138 b orbits TOI-4138, a subgiant star[1] located in the constellation Ursa Minor. The star has an enlarged radius of 1.82 R☉, a luminosity of 4.37 L☉ and an effective temperature of 6,128 K (5,855 °C).[1] It has 1.32 times the Sun's mass, and it has an intermediate age of around 3.5 billion years.[1] The apparent magnitude of the star is 11.8, making it not visible to the naked eye.[3]
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