The hot yellow star HD 74156 is located at a distance of 64.56 parsecs (210 light years) in the direction of the constellation Hydra. It is notable as the host of three gas giants on eccentric orbits. As a G0-type star with a luminosity of 3.06 LSOL and a mass of 1.24 MSOL (Naef et al. 2004, Bean et al. 2008), the star seems unusually bright and massive for its spectral classification. Its brightness has prompted speculation that the star may have progressed to the subgiant phase (Naef et al. 2004), but other considerations indicate that it is simply mature rather than evolved.

Takeda and colleagues (2007) propose an age of 3.72 billion years and a radius of 1.5 RSOL. These values are comparable to those of Upsilon Andromedae, an F9 star that also hosts a three-planet system. At 3.12 billion years and 1.32 MSOL, Upsilon Andromedae has a radius of 1.6 RSOL (Takeda et al. 2007, Rivera & Haghighipour 2007). The two stars also have similar effective temperatures (6068 K for HD 74156 and 6213 K for Upsilon Andromedae) and metallicities (+0.13 and +0.15, respectively; all values from the Catalog of Nearby Exoplanets).

Radial velocity observations over the past decade have revealed the presence of three gas giant planets, making HD 74156 the most distant known multi-planet system. Although the planets in other multiple systems (including our own) tend to travel in relatively circular orbits, the three companions of HD 74156 are quite eccentric.

Architecture of the HD 74156 system. Colored circles indicate the relative sizes of the 3 planets, assuming the 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 c about 20 times heavier than planet d, all gas giants of Saturn mass or more are expected to have very similar radii.

The innermost ("b") is the most eccentric of all. With a period of 51.7 days and an eccentricity of 0.64, this planet must experience dramatic temperature variations over the course of a single orbit, perhaps resulting in a severe "annual" storm cycles. Although the planet's semimajor axis of 0.29 AU lies well outside the Hot Jupiter range, it is still tight enough to guarantee temperatures hotter than those on Venus (all values Bean et al. 2008). The minimum mass of planet b is 1.88 MJUP, just above the median for currently known gas giants (~1.7 MJUP; see Catalog of Nearby Exoplanets).

The second planet ("d") is the most recently announced, and some uncertainty lingers regarding its status. Bean and colleagues suggest a range of values, rather than a single one, for each of its major parameters. For mass their estimates range from 0.38 to 0.49 MJUP, with 0.40 MJUP as the preferred value; for period, from 337 to 350 days, with 346.6 days preferred; and for orbital eccentricity, from 0.14 to 0.62, with 0.25 preferred (all data Bean et al. 2008). These values imply a gas giant more massive than Saturn, orbiting at a semimajor axis of about 1 AU, with a periastron of 0.78 AU and an apastron of 1.3 AU.

The third planet ("c") is a Super Jupiter of 8.03 MJUP, with a semimajor axis of 3.85 AU. The formation of such massive planets remains poorly understood. Its wide semimajor axis corresponds to an orbital period of about 6.8 years, notably longer than the periods of most known exoplanets, while its eccentricity of 0.43 is higher than average. The planet's decentered orbit carries it from a periastron of 2.19 AU to an apastron of 5.5 AU. This range evidently straddles the system's ice line.

If we add up the minimum masses preferred by Bean and colleagues, the 3 planets of HD 74156 total about 10.3 MJUP. This is about 56% more than the total mass of the 3 detected planets of Upsilon Andromedae, and more than 6 times the total mass of all planets in the Solar System.

Last update September 2009