HD 18438 b
{{Use dmy dates|date=November 2023}}
{{Short description|Exoplanet in constellation Cepheus}}
{{Infobox planet
| name = HD 18438 b
| discoverer = Lee Byeong-Cheol et al.
| discovered = 15 March 2023 (accepted)
| discovery_method = Radial-velocity method
| alt_names =
| apsis = astron
| semimajor = {{val|2.1|0.1|ul=AU}}
| eccentricity = {{val|0.1|0.1}}
| period = {{val|803|5|ul=d}}
| inclination = {{val|88.49|0.78|0.49|u=deg}}
| semi-amplitude = {{val|305|18|ul=m/s}}
| star = HD 18438
| mean_radius =
| mass = >{{val|21|1|ul=Jupiter mass}}
| density =
| single_temperature =
}}
HD 18438 b is a substellar object, either a massive gas giant exoplanet or a brown dwarf, orbiting the star HD 18438, a red giant star located about {{convert|731|ly|pc}} away from Earth.{{Cite Exoplanet Archive|HD 18438|access-date=2023-10-31}}
Observational history
Radial velocity variations of HD 18438 were first analyzed and reported in 2018, which found regular fluctuations with a period of {{convert|719.0|day|year|abbr=off}}, longer than the stellar rotation period of {{convert|562|day|year|abbr=off}}. This was then attributed to oscillations of the star itself, as they were similar to that of long secondary period (LSP) giant stars, though a definite conclusion could not be established.
In 2023, however, with four more years of observations, a follow-up study ruled out the association of LSP with the radial velocity variations, and instead determined that the signals, with a revised period of {{convert|803|day|year|abbr=off}}, were caused by an orbiting planetary (or brown dwarf) companion with a minimum mass of 21 {{Jupiter mass}}. This also agrees with a previous study that provided mass estimates for a companion to HD 18438. In this re-analysis process, the rotation period of the host star was also revised to {{convert|637|day|year|abbr=off}} to better match the accumulated data.
Physical characteristics
HD 18438 b is a massive planet with a mass of 21 {{Jupiter mass|link=y}} and an estimated radius of 1.08 {{Jupiter radius|link=y}}. This is above the deuterium burning limit (~13 {{Jupiter mass}}), and thus according to some definitions, including the IAU standard, it may be classified as a brown dwarf. However, other organizations deem the object a planet, such as the NASA Exoplanet Science Institute, which includes bodies as massive as 30 {{Jupiter mass}} in the NASA Exoplanet Catalog.
Orbit
The planet orbits its host star at a distance of {{convert|2.1|AU|km mi|abbr=on}}, taking {{convert|803|d|yr|abbr=off}} to complete one orbit around the star. It has an orbital eccentricity of 0.1.
Host star
{{Main|HD 18438}}
HD 18438 is a red giant with a spectral type of M2.5 III, inflated to a diameter of 88 {{Solar radius}}. If HD 18438 b is to be considered a planet at all, this would make it the largest host to an exoplanet known to date as of September 2024. Its sheer size means that despite having an effective temperature significantly cooler than the Sun at {{convert|3860|K|C F}}, it shines 929 times as bright. With a mass of 1.84 {{Solar mass|link=y}} and an age of {{val|5.5|2.4|ul=Gyr}}, it is nearing the end of its lifespan, having long since left the main-sequence stage. The star has a low metallicity of {{val|-0.4|0.1}} dex, meaning it only has an iron content between 32% and 50% that of the Sun.