HD 18438 b

Radial velocity variations of HD 18438 were first analyzed and reported in 2018, which found regular fluctuations with a period of 719.0 days (1.969 years), longer than the stellar rotation period of 562 days (1.54 years).^[4] This was then attributed to oscillations of the star itself, as they were similar to that of long secondary period (LSP) giant stars,^[4] though a definite conclusion could not be established.^[1]

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 803 days (2.20 years), were caused by an orbiting planetary (or brown dwarf) companion with a minimum mass of 21 M_J.^[1] This also agrees with a previous study^[5] 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 637 days (1.74 years) to better match the accumulated data.^[1]

HD 18438 b is a massive planet with a mass of 21 M_J and an estimated radius of 1.08 R_J.^[6] This is above the deuterium burning limit (~13 M_J), and thus according to some definitions, including the IAU standard, it may be classified as a brown dwarf.^[7] However, other organizations deem the object a planet, such as the NASA Exoplanet Science Institute, which includes bodies as massive as 30 M_J in the NASA Exoplanet Catalog.^[3][8]