HD 69830 is a K0 star in the constellation Puppis, located at distance of 12.58 parsecs (41 light years). It has three detected planets of Neptune’s mass or less, traveling in relatively circular orbits, as well as a narrow debris ring reminiscent of the Asteroid Belt (Lovis 2006). It is notable as one of the few exoplanetary systems without any gas giants.

HD 69830 conforms to the general rule that planet mass scales with star mass. The Catalog of Nearby Exoplanets provides a mass of 0.871 Msol, well below the median for exoplanetary host stars.

Currently there is no consensus on the star's age. Takeda and colleagues provide a minimum estimate of 12 billion years, implying that HD 69830 belongs to one of the earliest generations of stars to form after the coalescence of the Milky Way Galaxy (Takeda et al. 2007). The discovery team for the system's three planets arrived at a less extreme -- and less exact -- estimate of 4-10 billion years, while characterizing the host as "an old main-sequence star" (Lovis et al. 2006). Their conclusions agree reasonably well with the analysis of Takeda and colleagues. However, a more recent article by M.C. Wyatt and colleagues assumes a still younger age of only 2 billion years (Wyatt et al. 2007a), presupposing a very different history from the one implied by Takeda's group.

HD 69830's metallicity of -0.06 is typical of field stars in the Solar neighborhood, but lower than that of the average exoplanetary host. In itself, this value provides no strong constraints on the system's evolution, since planets of all kinds, from massive gas giants down to Earth-size objects, are known to form in environments of similarly reduced metallicity.

neptunian triplets

Our current best guess is that all three planets of HD 69830 bear a strong family resemblance to GJ 436 b. This nearby Hot Neptune transits across the face of its host star, permitting the planet's mass and radius, and therefore its composition, to be determined with reasonable precision. Gillon and colleagues find that GJ 436 b has a mass of 22.6 MEARTH and a diameter of 50,400 km (31,250 mi). These values imply a modest rocky core, a very deep layer of methane-ammonia-water ice, and a thin atmospheric envelope of hydrogen and helium (Gillon et al. 2007). A similar structure can be assumed for the planets of HD 69830.

The system's inner planet, HD 69830 b, is a Hot Neptune or even a Hot Super Earth, with a minimum mass 10 times terrestrial, traveling in an epistellar orbit at a semimajor axis of 0.08 AU. Given the uncertainties surrounding its mass, the planet's composition remains unknown. It may be primarily rocky like Earth and Venus or primarily icy like GJ 436 b.

The second planet, HD 69830 c, is slightly more massive than the first. At 12 MEARTH, it is very similar to Uranus, while its semimajor axis of 0.19 AU is less than half that of Mercury. Nevertheless, this orbital separation is wide enough to prevent any significant mass loss through photoevaporation, so the second planet will retain its full inventory of volatile elements. In these circumstances a strong resemblance to GJ 436 b is very likely.

The same applies to the third and largest planet, HD 69830 d. At 18 MEARTH this object is slightly more massive than Neptune, traveling in an orbit whose semimajor axis of 0.63 AU is somewhat smaller than that of Venus. Even if the third planet were to contain a rocky core as heavy as 10 MEARTH, it must also include a very large percentage of ice. This planet may be cool enough to sustain water clouds, and may thus present a blue and white globe deceptively reminiscent of Earth. The likelihood of such an outcome increases along with the system's age.

icy debris

Outside the third planet's orbit lies a region of debris that is reminiscent in many ways of our system's Asteroid Belt. This debris was originally detected by infrared observations of the hot dust generated by collisions among its constituents. Recent spectral analyses indicate a composition similar to that of asteroids orbiting the Sun beyond 3 AU, including silicate grains mixed with water ice (Lisse et al. 2007). The survival of ice constrains the dust to orbits larger than 1 AU. (See these artists' conceptions of the HD 69830 debris ring.)

In a detailed investigation, Wyatt and colleagues argue that this hot dust has been scattered inward from planetesimals or asteroids on still wider orbits, implying a radius of about 4.5 AU for the system's debris belt (Wyatt et al. 2007a). Even with their low estimate for the age of HD 69830, this group finds that the quantity of dust presently observed cannot be the result of steady-state collisions within the debris ring. Instead, it must be the product of a recent collisional cascade analogous to the Late Heavy Bombardment of the Solar System, lasting only 10-150 million years (Wyatt et al. 2007a).

As Wyatt's group notes, "dynamical instability can occur up to several Gyr after the formation of [a] planetary system," with the delay "determined by the separation of the outer planet from the outer planetesimal belt [...] or from the separation between the planets [...] with larger separations resulting in longer timescales" (Wyatt et al. 2007a). However, it is unclear how their analysis would be affected if the system age determined by Takeda and colleagues proves correct.

inward migration

Regardless of this determination, all three planets of HD 69830 evidently migrated to their present positions from much wider orbits, possibly originating outside the system's ice line (Alibert 2006a). Such a history increases the probability that these planets are primarily icy in composition. In this regard the third planet is especially interesting, given its semimajor axis within the system’s habitable zone and its possible role in maintaining the inner edge of the debris belt. This planet "d" may harbor captured rocky moons enriched by volatiles from infalling dust, as well as rings made of disintegrated asteroids.

The original discovery team felt confident in excluding additional planets more massive than Saturn within 4 AU of the host star, while identifying a potentially stable orbital region between 0.3 and 0.5 AU that could accommodate a low-mass, low-eccentricity body (Lovis 2006).

Last update May 2007