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HD 69830 and planets
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.
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.
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.)
Moons and exomoons
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
Index of exoplanetary topics
Alibert Y, Baraffe I, Benz W, et al. (2006) Formation and structure of the three Neptune-mass planets system around HD 69830. Astronomy & Astrophysics, 455: L25-L28.
Beichman CA, Bryden G, Gautier TN, Stapelfeldt KR, Werner MW, Misselt K, Rieke G, Stansberry J, Trilling D. (2005) An excess due to small grains around the nearby K0 V star HD 69830: Asteroid or cometary debris? Astrophysical Journal, 626: 1061-1069. Abstract.
Figueira P, Pont F, Mordasini C, Alibert Y, Georgy C, Benz W. (2009) Bulk composition of the transiting hot Neptune around GJ 436. Astronomy & Astrophysics, 493: 671–676. Abstract.
Fortney JJ, Marley MS, Barnes JW. (2007) Planetary radii across five orders of magnitude in mass and stellar insolation: Application to transits. Astrophysical Journal, 659: 1661-1672. Abstract.
Gillon M, Pont F, Demory BO, Mallmann F, Mayor M, Mazeh T, Queloz D, Shporer A, Udry S, Vuissoz C. (2007) Detection of transits of the nearby hot Neptune GJ 436 b. Astronomy & Astrophysics, 472: L13-L16. Abstract.
Guillot T. (1999) Interiors of Giant Planets Inside and Outside the Solar System. Science, 286: 72-77.
Lisse CM, Beichman CA, Bryden G, Wyatt MC. (2007) On the nature of the dust in the debris disk around HD 69830. Astrophysical Journal, 658: 584-592. Abstract.
Lovis C, Mayor M, Pepe F, Alibert Y, Benz W, Bouchy F, et al. (2006) An extrasolar planetary system with three Neptune-mass planets. Nature, 44: 305-309 (doi:10.1038/nature04828).
Payne MJ, Ford EB, Wyatt MC, Booth M. (2009) Dynamical simulations of the planetary system HD 69830. Monthly Notices of the Royal Astronomical Society, 393: 1219-1234. Abstract.
Rugheimer S, Haghighipour N. (2007) Habitable planets in the planetary system of HD 69830. Bulletin of the American Astronomical Society, 38: 203. Full text.
Selsis F, Chazelas B, Borde P, et al. (2007) Could we identify hot Ocean-Planets with CoRoT, Kepler and Doppler velocimetry? Icarus, 191: 453-468. Abstract.
Smith R, Wyatt MC, Haniff CA. (2009) Resolving the hot dust around HD 69830 and Eta Corvi with MIDI and VISIR. Astrophysical Journal, in press.
Takeda G, Ford EB, Sills A, Rasio FA, Fischer DA, Valenti JA. (2007) Structure and evolution of nearby stars with planets II: Physical properties of ~1000 cool stars from the SPOCS catalog. Astrophysical Journal (Supplement), 168: 297-318. Online database: Structure & Evolution of Target Stars.
Wright JT, Upadhyay S, Marcy GW, Fischer DA, Ford EB, Johnson JA. (2009) Ten new and updated multi-planet systems, and a survey of exoplanetary systems. Astrophysical Journal, 693: 1084-1099. Abstract.
Wyatt MC, Smith R, Greaves JS, Beichman CA, Bryden G, Lisse CM. (2007) Transience of hot dust around Sun-like stars. Astrophysical Journal, 658:569-583. Abstract.
All text is copyright Raymond Harris 2006-2009