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 approximately Neptune mass, traveling in orbits of low eccentricity, as well as a narrow debris ring reminiscent of the Asteroid Belt (Lovis et al. 2006). This system is remarkable for at least two reasons: it is part of the small group of known exoplanetary systems without any known gas giants, and of the still smaller group with a field of colliding debris that produces warm dust.

Given its lightweight companions, HD 69830 conforms to the general rule that planet mass scales with star mass. Christophe Lovis and colleagues measure the star’s luminosity at 0.60 Solar, corresponding to a mass of 0.86 Msol (Lovis et al. 2006). These values are well below the median for exoplanetary host stars, just as the system's three planets are well below the median mass for exoplanetary companions.

Currently there is no consensus on the star’s age. Recent estimates span a wide range, from a low of 2 billion years (Wyatt et al. 2007, Smith et al. 2009) to a high of 12 billion years (Takeda et al. 2007). The low value would make HD 69830 a “young adult,” while the high value would place it among the earliest generations of stars to form after the coalescence of the Milky Way Galaxy – an astral Methuselah about to retire from its career of fusing hydrogen and puff up into a subgiant. The discovery team for the star’s three planets suggested 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). In light of the system's unique architecture, more precise data would be most welcome.

HD 69830's metallicity of +0.05 (Lovis et al. 2006) is typical of stars in the Solar neighborhood, but lower than the metal content of the average exoplanetary host star. In any case, this value provides no strong constraints on the system's evolution, since planets of all kinds, from massive gas giants down to Mars-size objects, are known to form in environments of similarly reduced metallicity.

neptunian triplets

Radial velocity measurements establish only the minimum masses of exoplanets, not their actual masses. Nevertheless, both observational and dynamical studies suggest that the true masses of HD 69830's three planets are close to their minimum values of 10-18 MEA (Smith et al. 2009, Payne et al. 2009). These estimates correspond to a viewing geometry in which we see the system approximately edge-on, rather than in a face-on or "overhead" view. In particular, Payne and colleagues suggest that the true masses of the three planets are less (perhaps much less) than five times the minimum. Even if their actual masses are triple the minimum values, the two inner planets would still fall in the mass range of the ice giants, while the larger, outer planet would become a kind of “tween,” not quite big enough to qualify as a gas giant but still much heavier than a typical ice giant.

Architecture of the HD 69830 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 modest cores. Semimajor axes are indicated in astronomical units (AU) on a logarithmic scale. White dots mark the ice line.

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 bulk composition, to be determined with reasonable precision. Pedro Figueira and colleagues have concluded that the composition of GJ 436 b must be 10%-20% gaseous hydrogen/helium, 45%-70% rock/metal, and 15%-40% ice (Figueira et al. 2009). By comparison, Neptune and Uranus are about 5%-15% hydrogen/helium, 25% rock/metal, and 60%-70% ice (Guillot 1999, Figueira et al. 2009). (See more extensive data on HD 69830 and other multiple exoplanetary systems here.)

1. The system’s inner planet, HD 69830 b, is a Hot Neptune with a minimum mass of 10.23 MEA, traveling in a star-grazing orbit at a semimajor axis of 0.08 AU. Given the uncertainties surrounding its true mass, the planet's composition is also uncertain. If the minimum value is correct, it could be primarily rocky like Earth and Venus, but if its actual mass falls in the range of 15 to 30 Mea, as seems more likely, it will be an ice giant with a substantial hydrogen envelope, like Uranus, Neptune, and GJ 436 b. Nevertheless, simulations of this planet's evolutionary history suggest that, regardless of its mass, its bulk composition is the most metallic of the HD 69830 trio (Alibert et al. 2006, Payne et al. 2009).
   
   
2. The second planet, HD 69830 c, is slightly more massive than the first. Its minimum value of about 12 MEA would make it very similar to Uranus, although its semimajor axis of 0.19 AU is less than half that of Mercury. Nevertheless, such an orbital separation is wide enough to prevent any significant mass loss through photoevaporation, so this second planet should retain its full inventory of volatiles.
   
   
3. The same applies to the third and largest planet, HD 69830 d. Its minimum mass of 18 MEA would make it slightly heavier than Neptune, and its semimajor axis of 0.63 AU is somewhat smaller than that of Venus. Even if planet d contains a rocky core as heavy as 10 MEA, it must also include a substantial percentage of ice. This planet is probably 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, since planets orbiting beyond about 0.1 AU steadily lose heat over the main sequence lifetimes of their host stars.
   
   If the actual masses of these three planets are significantly higher than their minimum masses, HD 69830 d is the only member of the trio that would exceed the likely range for ice giants. An increase by a factor of 3 would bring its mass to 55 MEA, similar to the minimum values for 55 Cancri c and f (54 MEA and 45 MEA, respectively). Both of these exoplanets seem to be intermediate between the ice giants and the gas giants. An increase by a factor of 4 would turn HD 69830 d into a lightweight gas giant of 73 MEA.
   
   Apart from the uncertainty over its mass, planet d is notable for its location near the system’s proposed habitable zone. Rugheimer and Haghighipour estimate the boundaries of HD 69830’s habitable zone as 0.74-0.89 AU (Rugheimer & Haghighipour 2007), whereas the separation of planet d from the central star varies from 0.59 AU to 0.67 AU on account of its orbital eccentricity (e=0.07). Perhaps, if this planet has a sufficiently massive moon, some combination of parameters might permit liquid water to exist somewhere on its surface.

icy debris

Even before the system’s three planets were detected, infrared observations established the presence of warm dust within a few AU of HD 69830, the likely result of collisions among asteroids or comets in a debris belt (Beichman et al. 2005). This finding was remarkable, both because warm dust is rare around mature stars and because the observations revealed no trace of cooler dust at larger semimajor axes, where many Sun-like stars harbor extensive debris. (See these artists' conceptions of the HD 69830 debris ring.)

Subsequent spectral analyses indicated that the composition of the colliding debris is very similar to asteroids orbiting the Sun beyond 3 AU, including silicate grains mixed with water ice (Lisse et al. 2007). Along with other data, the presence of ice constrains the dust to a narrow ring centered around an astrocentric radius of 1 AU, and thus orbiting about 30 million miles beyond planet c (Lisse et al. 2007, Smith et al. 2009).

The current consensus is that the copious dust around HD 69830 was created by a fairly recent collisional event – perhaps corresponding to the Late Heavy Bombardment in the Solar System, which lasted 10-150 million years – because the infrared excess could not simply represent the steady state evolution of a massive debris belt (Beichman et al. 2005, Lisse et al. 2007, Wyatt et al. 2007, Smith et al. 2009, Payne et al. 2009).

As Mark Wyatt's group argues, "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, their analysis is based on an estimate of only 2 billion years for the age of HD 69830. It is unclear how their conclusions would be affected if HD 69830 turns out to be substantially older, as many sources claim.

evolution and migration

So far, two teams have conducted extensive numerical simulations in an attempt to understand the evolutionary history of the HD 69830 system (Alibert et al. 2006, Payne et al. 2009). Both studies assume that the three planets formed between circumstellar radii of 3 AU and 8 AU, just beyond the system’s ice line. After accreting an appreciable fraction of their final masses, the planets traveled to their present orbits by Type I migration. To explain their current non-zero orbital eccentricities, Payne and colleagues suggest that at least one additional protoplanet formed in the system’s nursery, and that this object was eventually ejected from the system or accreted by planet c or (more likely) planet d.

As they migrated through the zone of planetesimals that must have occupied the inner system at primordial times, the three planets accreted some of these objects and scattered most of the rest into an external belt of relatively large asteroids traveling on eccentric orbits (Payne et al. 2009). This belt became the site of the collisions that produced the warm dust we now observe at about 1 AU.

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 (i.e., between the second and third planets) that could accommodate a low-mass, low-eccentricity body (Lovis et al. 2006). Additionally, Rugheimer and Haghighipour have argued that an Earth-mass planet could achieve orbital stability between 0.8 and 0.9 AU, within the proposed habitable zone (Rugheimer & Haghighipour 2007). However, their study did not comment on the likelihood that such a planet could form in the first place. In fact, the evolutionary scenarios developed by the teams of Yann Alibert and Matthew Payne appear to present formidable obstacles to the survival of such an object. In their scenarios, the inward migration of the three known planets would have swept away the planetesimals or protoplanets required to assemble this hypothetical planet.

Last update August 2009