e), which was published in 2004. At the time, the parameters of this planet were poorly constrained and it was thought to be in an orbit of around 8.2 years with a high eccentricity. Later in 2004, a small inner planet designated Mu Arae c was announced with a mass comparable with that of Uranus in a 9-day orbit. This was the first of the class of planets known as "hot Neptunes" to be discovered. The discovery was made by making high-precision radial velocity measurements with the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph. In 2006, two teams, one led by Krzysztof Goździewski and the other by Francesco Pepe independently announced four-planet models for the radial velocity measurements of the star, with a new planet (Mu Arae d) in a near-circular orbit lasting approximately 311 days. The new model gives revised parameters for the previously known planets, with lower eccentricity orbits than in the previous model and including a more robust characterization of the orbit of Mu Arae e. The discovery of the fourth planet made Mu Arae the second known four-planet extrasolar system, after 55 Cancri. System architecture and habitability The Mu Arae system consists of an inner Uranus-mass planet in a tight 9-day orbit and three massive planets, probably gas giants, on wide, near-circular orbits, which contrasts with the high-eccentricity orbits typically observed for long-period extrasolar planets. The Uranus-mass planet may be a chthonian planet, the core of a gas giant which has had its outer layers stripped away by stellar radiation. Alternatively it may have formed in the inner regions of the Mu Arae system as a rocky "super-Earth". The inner gas giants "d" and "b" are located close to the 2:1 orbital resonance which causes them to undergo strong interactions. The best-fit solution to the system is actually unstable: simulations suggest the system is destroyed after 78 million years, which is significantly shorter than the estimated age of the star system. More stable solutions, including ones in which the two planets are actually in the resonance (similar to the situation in the Gliese 876 system) can be found which give only a slightly worse fit to the data. A 2022 study finds a stable orbital fit to the system, and estimates a lower limit on the system inclination of about 20°. Astrometric observations using the Hubble Space Telescope have not detected any of the known