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 gas giant 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. Two are gas giants, but the inner planet falls in the range of the ice giants, like Neptune or GJ 436 b.

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.

In the following discussion, all planet parameters follow Wright et al. 2009 unless otherwise indicated.

Architecture of the Mu Arae system. Colored circles indicate the relative sizes of the 3 planets, assuming the orbital values and minimum masses provided by Wright et al. 2009, the mass-radius relationships provided by Fortney et al. 2007, and rock/metal cores. Semimajor axes are indicated in astronomical units (AU) on a logarithmic scale. White dots mark the ice line. Although these exoplanets are quite dissimilar in mass, with planet b almost 3 times heavier than planet e, all gas giants of Saturn mass or more are expected to have very similar radii. Note that Pepe et al. 2007 provide a different picture of the Mu Arae system (see discussion below); the present diagram follows Wright et al. 2009 for consistency with the other pages in this series.

According to current interpretations, the innermost planet (d) orbits at a semimajor axis of 0.09 AU with a period just under 10 days and a minimum mass estimated as 0.035 MJUP. This is just 11 times the mass of Earth, indicating that Mu Arae d may be as heavy as Uranus. Unlike most short-period planets, however, planet d has a moderately eccentric orbit, with e = 0.17.

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, e = 0.07). 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, it may even host a family of moons comparable in mass to those of our own 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 Wright et al. 2009 present a very different picture. Their most recent data indicate a mass of 1.18 MJUP, a semimajor axis of 3.78 AU, and a substantial eccentricity of 0.46, considerably higher than that of the third planet. Such an elliptical orbit would carry planet c from a periastron of 2.03 AU to an apastron of 5.53 AU, still wider than Jupiter's widest separation from our Sun. Again, this ellipticity would result in large annual temperature variations and corresponding changes in the color and appearance of the planet's cloud cover. Like Mu Arae's other two gas giants, this one also seems likely to sustain a family of moons.

According to current data, the orbits of the three outer planets approach one another very closely, such that the apastron of d is less than 0.10 AU smaller than the periastron of b, and the periastron of c is only 0.11 AU larger than the apastron of b. By comparison, Earth and Venus are separated by more than 0.20 AU even at their closest approach. The evident crowding of the Mu Arae system suggests that the orbits of these three gas giants may be steeply inclined relative to one another (Short et al. 2008), a configuration whose most likely origin is an epoch of planet-planet scattering early in the system's history. Such a history is also suggested by the eccentricities of planets b, c, and d.

Last updated April 2009