HD 190228 is a star with a substellar companion in the constellation Vulpecula. Its apparent magnitude is 7.30 – too faint to be seen with the naked eye – and the absolute magnitude is 3.34. Based on parallax measurements, it is located at a distance of from the Sun. The star is older than the Sun with an age over 5 billion years and it is metal-poor.
Planetary system
In 2000, it was announced that a giant planet was orbiting the star with a minimum mass of 5 Jupiter masses, designated HD 190228 b. The planetary nature of the object was questioned because of the low metal content of the star: giant planets are more likely to be found around high-metallicity stars, so it was argued that the object was more likely to be a brown dwarf. A 2011 study using astrometric measurements from Hipparcos found that, with 95% confidence, HD 190228 b is in fact a brown dwarf of Jupiter masses in a nearly face-on orbit.
However, later studies in 2022 and 2023 using both Hipparcos and Gaia astrometry found much lower true masses, close to the minimum mass. While the former study notes that their inclination measurement is poorly constrained, and that further study should better constrain the mass, the latter says that the low-mass solution is preferred given the relatively large uncertainty in the Hipparcos data. Thus, it seems likely that HD 190228 b is a planet. It takes 3.1 years to orbit the star, and its orbit is elliptical with an eccentricity of 0.55.