Revisiting HD 28185: The Surprising Discovery of an Outer Planet in a Saturn-like Orbit

The search for exoplanets often brings unexpected discoveries, and HD 28185 has proven to be no exception. Originally thought to host just one planet, recent research by Alexander Venner and his collaborators has led to a fascinating new conclusion: a second giant planet, rather than a brown dwarf, orbits this Sun-like star at a distance similar to Saturn’s orbit in our solar system. Using decades of radial velocity (RV) data and advanced astrometric methods, the team revised our understanding of this planetary system. Here’s a breakdown of the key findings and methods used in their study.

 Background and Earlier Discoveries

HD 28185 is a relatively bright, Sun-like star that first drew attention in the early 2000s. It was known to host a massive planet, HD 28185 b, which orbits the star with a period like Earth's year. This planet is about six times the mass of Jupiter and orbits in what is often referred to as the “temperate zone,” receiving a similar amount of light as Earth does from the Sun. Over time, however, astronomers noticed hints of a second companion in the system, originally thought to be a much larger object, possibly a brown dwarf—a star-like object that never gained enough mass to sustain nuclear fusion.

 New Observations and Methodology

Venner and his team revisited the HD 28185 system with new data spanning 22 years of observations and applied both RV measurements and astrometric data from the Hipparcos and Gaia missions. Radial velocity data tracks the wobble of a star caused by the gravitational pull of orbiting planets. Astrometry, on the other hand, measures how a star moves through space, allowing researchers to detect the subtle changes in motion caused by massive companions.

One key obstacle in exoplanet research is the “𝑚 sin 𝑖” problem: RV data alone can only provide the minimum mass of a planet because it depends on the unknown angle at which the planet orbits the star. Astrometry, by adding information on the planet's inclination, helps resolve this uncertainty. The team used these techniques to analyze not only HD 28185 b but also to clarify the nature of the suspected second companion.

 The Discovery of HD 28185 c

The breakthrough came when the researchers discovered that what was previously thought to be a brown dwarf is actually a massive planet, HD 28185 c, with a mass six times that of Jupiter. It has an orbital period of nearly 25 years and a semi-major axis of 8.5 astronomical units (AU)—very similar to Saturn’s orbit in our solar system. The planet’s orbit is slightly elliptical, with a modest eccentricity (0.15), placing it among the rare group of exoplanets with orbits like our solar system’s giant planets.

 Implications for Exoplanet Research

HD 28185 c represents an important addition to the growing number of distant giant planets detected in recent years. Its orbit lies at a boundary where detection methods like RV and direct imaging are often less effective. The fact that such a planet could be confirmed by using a combination of RV and astrometric data highlights how much our ability to detect and study distant exoplanets has improved.

This discovery adds to the understanding that giant planets like Jupiter and Saturn may be relatively common around Sun-like stars. While "hot Jupiters", giant planets with very short orbital periods, often make headlines, long-term studies like this one show that giant planets on wider orbits may be just as frequent—if not more so.

 Conclusion

The study of HD 28185 provides a perfect example of how modern techniques in astronomy can uncover new aspects of known systems. By reanalyzing the data with precision tools, Venner and his team have provided a clearer picture of the HD 28185 system, showing that it contains not just one but two giant planets, one of which follows an orbit remarkably similar to Saturn’s. This discovery pushes the boundaries of our knowledge of distant exoplanets and helps refine our understanding of planetary systems that may resemble our own.

Source: Venner et al.