HD 181433 is a K-type star located at a distance of about 85 light years (26.15 parsecs) in the southern constellation Pavo, the Peacock. It is notable as the host of a multi-planet system consisting of a short-period Super Earth (or low-mass ice giant) plus two small gas giants, all traveling on eccentric orbits.

The star's basic parameters seem poorly constrained. The SIMBAD database suggests a spectral type of K3 III, indicating a red giant. Bouchy and colleagues, however, classify its type as K3 IV, indicating a subgiant. At the same time, they provide a stellar mass of 0.78 Msol, a luminosity 31% Solar, and an effective temperature of 4960 K (Bouchy et al. 2009), consistent with a main-sequence orange dwarf not too different from Epsilon Eridani. According to Bouchy's group, the star is rich in heavy elements, with a metallicity of +0.33. No information is available on its age. (A smallish K-type star that had already evolved into a subgiant or giant would be older than 10 billion years.)

Notably, the metallicity of HD 181433 is high, even among stars hosting gas giants; it exceeds the metal content of 80% of all stars hosting two or more planets. According to prevailing theories of planet formation, stellar enhancement in metals is associated with the rapid assembly of gas giants, and also potentially with Type II migration and planet-planet scattering (Thommes, Matsumura & Rasio 2008). The known planets of HD 181433 are all lighter than the median exoplanet mass (currently about 1.5 Mjup, or 1.75 Mjup if we consider only gas giants), perhaps because their host is also relatively lightweight among Sun-like stars with planets. Nevertheless, their highly eccentric orbits are evidence of efficient accretion of mass in a concentrated region of the protoplanetary nebula. This conclusion is prompted by prevailing theories that orbital eccentricity results from a past era of disruptive interactions among newly-formed planets (Weidenschilling & Marzari 1996, Ford & Rasio 2008).

system architecture

The HD 181433 is remarkable insofar as all 3 of its planets have orbital eccentricities of 0.28 or higher (see data at Multi-Planet Systems or Extrasolar Planets Encyclopaedia). Architecturally similar systems include HD 74156, whose 3 gas giants have eccentricities of at least 0.25, with a maximum of 0.68, and HD 169830, whose 2 well-spaced planets have e = 0.31 and e = 0.33. All of these systems must have been shaped by gravitational interactions between planets over tens of millions of years, possibly including ejections of planetary neighbors.

The innermost planet, a Super Earth designated HD 181433 b, orbits within the "hot zone," with a semimajor axis of 0.08 AU and a period just under 10 days. Although this planet may have reached the vicinity of its host star through Type I migration, as would be expected for objects in the mass range of terrestrial to ice giant planets, some other mechanism seems to be required to explain its large eccentricity of 0.38. Gravitational perturbations from other planets in the system are the likely suspects.

The two outer planets, whose minimum masses fall between those of Jupiter and Saturn, present a case of potentially tangled orbits. As currently understood, their eccentricities cause them to crisscross the system's ice line (located at about 2 AU) over the course of a single period, such that the planet with the larger semimajor axis (d) is sometimes closer to the central star than the planet with the smaller semimajor axis (c).

The middle planet, c, which is slightly heavier than its outer companion, travels from 1.27 AU at periastron to 2.25 AU at apastron. The outer planet, d, seems to be even more eccentric, traveling from 1.56 AU at periastron to 4.44 AU at apastron. How these planets avoid orbit crossings, collision, or ejection is still unexplained. The discovery team noted that the orbit of planet d remains poorly understood (Bouchy et al. 2009), so further observations may lead to revisions in our assessment of the system architecture. Perhaps residual data points deriving from an additional, undetected planet or planets have led to unrealistically high estimates of the outer planets' orbital eccentricities.

Last revised January 2010