HD 190228

HD 190228 is a star with an orbiting substellar companion in the northern constellation of 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 205 ly from the Sun. The system is drifting closer with a radial velocity of −50 km/s.

The spectrum of HD 190228 presents as a subgiant star with a stellar classification of G5 IV, indicating it has exhausted the supply of hydrogen at its core and is evolving off the main sequence. The star is older than the Sun with an age over 5 billion years with a projected rotational velocity of 1.4 km/s. A metal-poor star, it has 18% more mass than the Sun and has grown to 2.4 times the Sun's girth. The star is radiating roughly 4.6 times the luminosity of the Sun from its photosphere at an effective temperature of 5,311 K.

Planetary system
In 2000, it was announced that a giant exoplanet 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 $0.793$ 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 an exoplanet. It takes 3.1 years to orbit the star, and its orbit is elliptical with an eccentricity of 0.55.