e x t r a s o l a r p l a n e t s
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HD 40307 is a K2 star located 42 light years away in the direction of the southern constellation Pictor. It is notable for hosting three planets in the Super Earth range, but no gas giants. Such a system architecture is unusual among planetary systems so far known, as the vast majority (including our own) harbor at least one gas giant planet. Among Sun-like stars with detected planets, HD 40307 falls near the bottom of the range in mass and especially metallicity. Its mass is 0.77 MSOL; its metallicity is -0.31; and its luminosity is only 24% Solar, making it even dimmer than Epsilon Eridani (all values Mayor et al. 2009). These parameters imply an original ice line located at about 2 AU, and a current liquid water zone centered around 0.5 AU. The star’s age is estimated as 2.6 to 7.7 billion years (Mayor et al. 2008, Barnes et al. 2009), so that its planetary system has had ample time to reach long-term physical and orbital stability. Given the absence of detectable debris, major asteroid and cometary impacts are probably rare. three planets The three planets orbiting HD 40307 are remarkable for their low mass, small semimajor axes, and circular orbits. All are located within an astrocentric radius that is less than half the size of the orbit of Mercury. (All values Mayor et al. 2009.)
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Packed orbits
System architectures 
Evolution of planetary systems 
Index of exoplanetary topics 
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HD 40307 is thus the most compact of all known multi-planet systems. Its nearest rival is GJ 876, whose three planets travel within an astrocentric radius of 0.21 AU. By comparison, the semimajor axis of Mercury is 0.39 AU. The orbital configurations of the HD 40307 triplets are particularly intriguing, insofar as they narrowly avoid a three-way mean motion resonance in a period ratio of 4:2:1 (a relationship that is actually observed among the inner Galilean moons of Jupiter). The low masses and short periods of these three planets raise interesting questions about their composition and formation history. The discovery team has provided minimum masses only, without specifying an upper limit, whether on dynamical grounds or otherwise. In two other systems harboring objects with minimum masses (m sin i) in the range of Super Earths, we are fortunate enough to have reasonable estimates of the planets’ orbital inclinations. Therefore, the true masses of these planets can be estimated. The inner planet of GJ 876 has m sin i = 5.89 MEA, corresponding to an actual mass of 7.5 MEA (Rivera et al. 2005), and the inner planet of 55 Cancri has m sin i = 10.8 MEA, with a true mass in the vicinity of 13.5 MEA (Fischer et al. 2008). If the masses of the HD 40307 triplets require a similar augmentation, as appears likely, the outer planet would become an ice giant similar to Uranus, while the middle planet would approach the boundary between ice giants and terrestrial-mass planets. The inner planet, however, would remain a Super Earth. evolutionary scenarios The widely accepted theory of planet formation by accretion implies that these three planets did not assemble in situ, for the simple reason that insufficient solids would have been available at their birthplace to form such massive objects. Sean Raymond and colleagues estimate that, within a radius of 2 AU, a typical K-type star of 0.8 MSOL and zero metallicity would have only 2 MEA in solid protoplanets available at primordial times for accretion into planets (Raymond et al. 2007). Yet the combined mass of the HD 40307 triplets is at least 20 MEA, even if we consider only the minimum values, while the stellar metallicity is much lower than zero. It seems far more likely, therefore, that these planets assembled beyond the ice line, where a large mass in frozen volatiles would have been available. According to Grant Kennedy and Scott Kenyon, the newly accreted objects would then travel inward to short-period orbits by Type I migration (Kennedy & Kenyon 2008b). In this scenario, all three planets would probably consist of about 50% primordial ices, like the Steam Planets envisioned by Franck Selsis and colleagues (2007). A substantial mass in volatiles is also predicted by recent simulations of the system’s dynamical history. The work of Rory Barnes and colleagues demonstrates that rocky planets could not have achieved the orbital configuration observed around HD 40307, whereas planets similar in composition to Uranus are fully consistent with the implied dynamics (Barnes et al. 2009). Regardless of its evolutionary history, the system of HD 40307 as currently understood bears a family resemblance to those of three other nearby stars, all of which are typical of the Solar neighborhood in terms of mass, metallicity, and spectral type: GJ 581, an M3 star with a mass of 0.32 MSOL and a metallicity of -0.26, hosting an ice giant and two Super Earths within 0.25 AU; GJ 876, an M4 star with a mass of 0.33 MSOL and a metallicity of +0.02 (metallicities Bailey et al. 2009), hosting a Super Earth and two gas giants within 0.21 AU; and HD 69830, a K0 star with a mass of 0.87 MSOL and a metallicity of -0.06 (Catalog of Nearby Exoplanets), hosting three ice giants within 0.63 AU. HD 40307 falls along the same continuum in terms of its crowded system architecture, resembling most closely a slightly scaled-down version of HD 69830. See also Packed Orbits.
Last updated August 2009
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All text is copyright Raymond Harris 2006-2009 |