The yellow star 47 Ursae Majoris (“47 UMa”) is located at a distance of about 14 parsecs (46 light years) in the direction of the Big Dipper. Its planets comprise one of the most intensively studied of all exoplanetary systems. The inner companion (“b”), a cool gas giant almost three times as massive as Jupiter, was confirmed in 1996 in the first crop of radial velocity detections (Butler & Marcy 1996). The announcement in 2001 of a second gas giant (“c”) on a still wider orbit excited much speculation that the planet hunters had already found a “second Solar System” (Fischer et al. 2002, Ji et al. 2005).

Continuing radial velocity observations, augmented by state-of-the-art statistical methods, have led to a series of revisions in our understanding of this system. Intriguing parallels proposed by earlier studies have been invalidated - for example, the notion that the masses and orbital periods of planets b and c exhibit a ratio very similar to those of Jupiter and Saturn. Current investigations suggest instead that 47 UMa is circled by at least 3 gas giants orbiting between 2 AU and 11 AU (Gregory & Fischer 2010). Such an architecture, with multiple giants located beyond the ice line, is unparalleled among the exoplanetary systems detected to date. Yet it is similar to the architecture of the Solar System, if we allow for a more massive population of outer planets.

47 Ursae Majoris, also known as HD 95128, is a single Sun-like star of spectral type G0. It is slightly hotter and more massive than our Sun, and endowed with a similar percentage of metals (Takeda et al. 2007, Wittenmyer et al. 2007). Given these parameters, the system’s ice line and habitable zone will be similar to those of the Solar System, with the ice line located at about 2.7 AU and the habitable zone centered around 1 AU. Recently published sources provide a range of stellar ages, from a low of 5.4 billion years to a high of 9.2 billion, with 7 billion years as a typical value (Fuhrmann et al. 1997, Laughlin et al. 2002, Takeda et al. 2007).

Architecture of the 47 Ursae Majoris system. Colored circles indicate the relative sizes of the 3 planets, assuming the minimum masses provided by Gregory & Fischer 2010 and the mass-radius relationships provided by Fortney et al. 2007. Semimajor axes are indicated in astronomical units (AU) on a logarithmic scale, where 1 = the distance of Earth from the Sun. White dots mark the ice line.

planet b

Planet b, the inner giant, orbits at a semimajor axis of 2.1 AU, with a period of almost 3 years and an eccentricity of only 0.03 (still smaller than Jupiter's value of 0.048). Its minimum mass is estimated at 2.53 MJUP, with a maximum mass under 5 MJUP (Butler & Marcy 1996, Gregory & Fischer 2010). Notably, the reported parameters for mass, semimajor axis, period, and eccentricity have changed very little since this planet was first announced, suggesting that our present characterization is robust.

Planet b is remarkable for the circularity of its orbit, which resembles those of the Solar System’s giant planets rather than most extrasolar planets orbiting beyond 1 AU. Only about 15% of exoplanets detected at this separation have orbital eccentricities smaller than 0.1 (Extrasolar Planets Encyclopaedia), whereas all four of the Solar giants do.

Because all four of our giants are also accompanied by co-formed satellites, 47 UMa b is likely to support a family of large moons, given its substantial mass and reasonably wide semimajor axis. Assuming the primary-to-satellite mass ratio proposed by Canup & Ward (2006), the largest of these moons might rival Mars. However, unless an additional source of heating (such as volcanic activity) is available for these hypothetical objects, their surface temperatures will be too low to support bodies of open water.

planet c

The parameters of planet c have been a topic of debate for some years (Naef et al. 2004, Rivera & Haghighipour 2006, Wittenmyer et al. 2009). Notably, in an article published in 2007, Wittenmyer and colleagues concluded, “The largest data set yet assembled on 47 UMa indicates no statistically significant signal attributable to a second planet in the system” (Wittenmyer et al. 2007).

Nevertheless, members of the scientific team that first reported this planet's detection continue to maintain the validity their findings (Wright et al. 2009). The latest published results (Gregory & Fischer 2010) indicate that planet c is a relatively lightweight gas giant with a minimum mass of 0.54 MJUP orbiting at a semimajor axis of 3.6 AU in a period of 6.55 years with an eccentricity of 0.098 (comparable to the eccentricity of Mars).

47 UMa c may be closer in mass to Saturn than to Jupiter, and it may host a family of satellites analogous to the Saturnian system. Given the limitations inherent in radial velocity searches (see Detection Methods), only one other exoplanet orbiting between 1 AU and 4 AU has been identified with a minimum mass lower than 0.6 MJUP.

planet d

The system's proposed third planet remains poorly constrained. Gregory & Fischer provide a minimum mass of 1.64 MJUP, a semimajor axis between 9 AU and 14 AU, a period in the vicinity of 25 to 50 years, and a relatively low eccentricity in the range of 0 to 0.25. To date, radial velocity searches have not proposed any other extrasolar systems with planets in the orbital space beyond 6 AU, and only four systems (all multiple) have been reported with planets between 5 AU and 6 AU. 47 UMa d is unique among candidate exoplanets.

system architecture

Simulations by Mandell and colleagues indicate that gas giants orbiting a Sun-like star closer than 2.5 AU severely curtail the formation of terrestrial planets in the inner system (Mandell et al. 2007), resulting in ever smaller numbers of rocky planets on short-period orbits. Nevertheless, several investigators have addressed the orbital stability of potential planets in the habitable zone of 47 UMa, which has been defined as the region between 0.9 and 1.5 AU. The prevailing conclusion is that stable orbits are possible for an object in the mass range of Earth and Mars at about 1 AU (Jones et al. 2001, Ji et al. 2005, Rivera & Haghighipour 2006, Wittenmyer et al. 2009). Thus, if an Earthlike planet somehow managed to accrete in the inner system and establish an orbit in this region, then 47 UMa may still prove to be a second Solar System.

Last revised March 2010