The yellow star 47 Ursae Majoris (“47 UMa”) is located at a distance of about 46 light years in the direction of the Big Dipper. Its two planets comprise one of the most intensively studied of all exoplanetary systems. The inner companion (“b”), a 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 planet (“c”) excited much speculation that the planet hunters had already found a “second Solar System” (Fischer et al. 2002, Ji et al. 2005).

This comparison was prompted by several apparent analogies between the two systems. According to early data (Fischer et al. 2002), both the Solar System and 47 UMa are dominated by a pair of gas giants traveling on almost circular orbits. The mass ratio of 47 UMa b to 47 UMa c was thought to be similar to that of Jupiter to Saturn (about 3:1); the same applied to the ratio of their semimajor axes (about 1:2). The latter correspondence implied that 47 UMa b and c, like Jupiter and Saturn, exhibit near-separatrix behavior, since they would be orbiting just outside a 2:1 mean motion resonance (Barnes & Greenberg 2006b). However, continuing observations have led to substantial revisions in the parameters of planet c, invalidating these intriguing parallels. The latest data are discussed below.

host star

47 UMa, 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, more than a billion years older, and endowed with a similar percentage of metals (Takeda et al. 2007, Catalog of Nearby Exoplanets [CNE]). Given these parameters, the system’s ice line and habitable zone will be similar to those of the Solar System.

planet b

Planet b, the inner giant, orbits at an average distance of 2.14 AU, with a period of 3 years and a nominal eccentricity of 0. Its minimum mass is estimated at 2.62 MJUP, with a maximum mass under 5 MJUP (CNE 2008, Butler & Marcy 1996). Notably, the 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 12% of exoplanets detected at this separation have orbital eccentricities smaller than 0.1 (Extrasolar Planets Encyclopaedia 2008), whereas all four of the Solar giants do.

Because all four 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.

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. 2007). Notably, in an article published early 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).

Still more recent data posted by the California Carnegie Planet Search team, which first reported the detection of planet c, represent a significant revision in the values published in 2002. The planet’s semimajor axis is now given as 3.39 AU (smaller by 10%), its period as about 6 years (shorter by one year), and its eccentricity as 0.22. Its minimum mass is estimated at only 0.46 MJUP (a reduction of 40%). Given the limitations of radial velocity observations (see Detection Methods), only two other exoplanets orbiting beyond 1 AU have been identified with minimum masses lower than 0.5 MJUP, and no others in the same mass range beyond 3 AU. 47 UMa c seems to be closer in mass to Saturn than to Jupiter, and it may host a family of satellites analogous to the Saturnian system.

system architecture

The present architecture of the 47 UMa system suggests that both gas giants formed beyond the system’s ice line – probably in the region between 3 and 10 AU (see Kennedy & Kenyon 2008) – and migrated inward to their present orbits. Their migration most likely resulted in a severe curtailment of terrestrial planet formation in the inner system. Simulations by Mandell and colleagues indicate that gas giants orbiting a Sun-like star closer than 2.5 AU restrict inner-system evolution to small semimajor axes (less than 1 AU), so we can expect the habitable zone of 47 UMa to be empty (Mandell et al. 2007). Perhaps a few rocky planets formed closer to the star, following orbits comparable to Mercury's.

An alternative view is presented by Rivera & Haghighipour, who find that the region between 0.9 and 1.5 AU around 47 UMa offers a broad range of stable orbits (Rivera & Haghighipour 2006). Thus, if an Earthlike planet somehow managed to accrete in the inner system and establish an orbit in this region – which in their calculations encompasses the system’s habitable zone – then 47 Ursae Majoris may indeed prove to be a second Solar System.

Last update March 2008