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HD 18438
HD 18438 is a red giant star in the deep northern constellation of Cepheus, located about 730 light-years (220 parsecs) from Earth. With an apparent magnitude of 5.49, it is visible by the naked eye under dark skies as a red-hued dot of light about 10 degrees away from the celestial north pole. It is part of a wide binary system with an F-type subgiant star. In 2023, HD 18438 was discovered to be orbited by a 21 MJ substellar object, potentially making it the largest host star to an exoplanet ever found as of September 2024.[2][6] Stellar characteristicsHD 18438 is a red giant on the asymptotic giant branch (AGB)[7] that has already gone through the helium shell flash, with a spectral type of M2.5 III. Thus, it has evolved past the main sequence after using up its core hydrogen. It has 1.84 times the mass of the Sun but 88.48 times its girth and, at an effective temperature of 3,860 K (3,590 °C; 6,490 °F) it radiates 929 times Sun's luminosity from its photosphere.[2] The star is aged 5.5±2.4 billion years, making it likely older than the Solar System. It has a low metallicity of −0.4±0.1 dex, which translates to an iron abundance 32–50% that of the Sun.[2] It is the largest host star to a confirmed exoplanet included in the NASA Exoplanet Archive as of September 2024.[6] However, some definitions[8] may classify the planet as a brown dwarf instead due to its high mass. Much like many AGB stars,[9] HD 18438 is suspected to be a variable star, its apparent magnitude fluctuating between 5.43 and 5.49.[3] Binary systemThe star has been noted to be part of a double star since at least 1932, receiving the designation ADS 2294 in the Aitken Double Star Catalogue,[1][10] with a 9th-magnitude F7IV[3] star later designated TYC 4516-2148-1.[1] Due to its similarities to HD 18438 in distance and proper motion, this star is thought to be a physical binary companion to HD 18438. Located at a wide separation of 1,100 AU from the primary star, TYC 4516-2148-1 is slightly more massive than the Sun, but is 2.5 times larger and 7.5 times as luminous. The star is similar to HD 18438 in that it is somewhat more massive than the Sun and is past the main-sequence stage, but its relationship with HD 18438 is poorly understood.[2] Substellar companionIn 2023, a team of South Korean astronomers reported the discovery of a substellar object, named HD 18438 b, which could either be described as a massive gas giant exoplanet or a low-mass brown dwarf. It is about 8% larger than the planet Jupiter and at least 21 times as massive, which places it above the deuterium burning limit (~13 MJ[8]). It revolves around the star in a roughly circular (eccentricity 0.1) orbit with a semi-major axis of 2.1 AU (310,000,000 km) once every 803 days (2.20 years).[2]
See alsoReferences
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