Mu Arae is a G3 star located at a distance of 15.3 parsecs (50 light years) in the constellation Ara. The first data on its planetary system appeared in 2001 (Butler et al. 2001), confirming a single planet more massive than Jupiter with a semimajor axis equivalent to that of Mars. Observations of increasing precision, along with continuing analyses, have since established the presence of at least three additional planets orbiting within 4 AU of the host star. Nevertheless, existing data do not yet permit orbital solutions that exhibit long-term stability (Barnes & Greenberg 2006b) – an indication that the parameters of the Mu Arae system remain poorly constrained.

Substantially more metallic than our Sun, with [Fe/H] = 0.29, Mu Arae is in other ways relatively Sun-like. It has no stellar companions, and its mass and age approach Solar values, at 1.15 MSOL and 6 billion years, respectively (McCarthy et al. 2004, Pepe et al. 2007). However, Mu Arae is quite bright for its mass and spectral type, with various sources providing a luminosity of 1.7 Solar (e.g., Kaler 2008). If we apply a scaling according to the square root of this value, the system’s current habitable or “liquid water” zone will center around 1.3 AU. If we apply a scaling according to stellar mass (Kennedy & Kenyon 2008b), its ice line will center around 3.1 AU.

According to current interpretations, the innermost planet (d) travels in an epistellar orbit with a period of just under 10 days and a minimum mass estimated as 0.033-0.047 MJUP (Pepe et al. 2007; Catalog of Nearby Exoplanets). This range is equivalent to 10-15 times the mass of Earth, indicating that Mu Arae d may be as heavy as Uranus.

The second planet (e) is a “Super Saturn” (0.55 MJUP) with a nearly circular orbit resembling that of Earth (period 310 days, semimajor axis 0.94 AU). On such an orbit, the second planet may be cool enough to sustain the formation of water clouds, and it is certainly far enough outside the reach of the host star’s tides to sustain rapid rotation. Thus we can picture a blue planet with extensive bands of white clouds. Given the planet’s mass and orbital radius, as well as its history of migration through the system’s original zone of rocky planetesimals, we can also expect this second planet to host a family of moons comparable in mass to those of Jupiter or Saturn.

The third planet (designated “b” as the first to be detected) has a mass of 1.67 MJUP, a period of 630 days, and a semimajor axis of 1.51 AU – as if a larger version of Jupiter had assumed the orbit of Mars. However, its estimated orbital eccentricity of 0.27 is higher than that of any planet in the Solar System, implying that Mu Arae b approaches its host star as close as 1.1 AU and then retreats as far as 1.9 AU over the course of a single revolution. Such an elliptical path is likely to cause large annual temperature variations. Nevertheless, planet b remains either inside or near the system’s habitable zone for its entire orbit (see, e.g., Jones et al. 2006). As with the second planet, we can expect this third planet to be girded by bands of white water clouds, and it may sustain an even more substantial family of moons.

Provided they are massive enough, these hypothetical satellites would be well placed to maintain surface bodies of liquid water. In fact, Mu Arae b is one of the few nearby extrasolar planets that seems likely to host Earthlike exomoons, since gas giants have rarely been found on orbits that remain confined to their systems’ habitable zones.

The fourth planet (c) is less well constrained than the other three. Pepe et al. (2007) provide a minimum mass of 1.8 MJUP a semimajor axis of 5.2 AU (identical to Jupiter’s), and an orbital eccentricity of 0.0985 (very close to Jupiter’s), while the Catalog of Nearby Exoplanets currently provides 1.18 MJUP, 3.78 AU, and 0.463 for the same parameters (April 2008). Each alternative would imply a substantially different system architecture, yet both may fall short of an accurate solution.

A recent study by Short et al. (2008) explores the uncertainties in the available data by presenting five different sets of system parameters, each of which is supported by the radial velocity observations and each of which implies orbits that are stable on time scales of (at least) hundreds of thousands of years.

Last updated April 2008