HD 40307 is a K2 star located 42 light years away in the direction of the southern constellation Dorado. It is notable for hosting three Super Earths but no gas giant planets. Such a system architecture is unusual among planetary systems so far known, as the vast majority (including our own Solar System) harbor at least one gas giant.

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 metal content is -0.31; and its luminosity is only 24% Solar, making it even dimmer than Epsilon Eridani (all values Mayor et al. 2008). 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 at 2.6 billion years (Mayor et al. 2008), old enough for its planetary system to have reached 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 values Mayor et al. 2008.)

1. At 4.2 MEA, the inner planet, “b,” has the lowest minimum mass of any object so far detected by radial velocity observations. It travels in a “hot” orbit with a semimajor axis of about 0.05 AU and a period of 4.3 days.
   
   
2. The middle planet, “c,” has a minimum mass of 6.8 MEA (similar to GJ 876 d, 55 Cancri e, and GJ 581 d), a semimajor axis of 0.08 AU, and a period of 9.6 days. Its small separation and short period place it on the cusp of the population of “hot” planets, which are often defined as those orbiting within 0.1 AU of their host star in 10 days or less.
   
   
3. The outer planet, “d,” rounds out the trio with a minimum mass of 9.2 MEA (similar to GJ 674 b and HD 69830 b), a semimajor axis of 0.134 AU, and a period of 20.5 days. Like its two companions, it has an orbital eccentricity approaching zero.

HD 40307 is thus the most compact of all known multiple-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. Thus the inner planet of GJ 876 has m sin i = 5.89 MEA, while its actual mass is estimated at 7.5 MEA (Rivera et al. 2005; i = 50 degrees). The inner planet of 55 Cancri has m sin i = 7.66 MEA, with a true mass in the vicinity of 9.6 MEA (Fischer et al. 2008, Table 4; i = 53 degrees).

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 cores. 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 about 10 times higher, while the stellar metallicity is much lower. It seems far more likely, therefore, that (at least) the two outer planets assembled beyond the ice line, where a large mass in volatiles would have been available.

According to Grant Kennedy and Scott Kenyon, such icy Super Earths would then travel inward to short-period orbits by Type I migration (Kennedy & Kenyon 2008b). These migrating planets (whether two or three in number) would shepherd rocky planetesimals within their shrinking semimajor axes, potentially resulting in the accretion of a rocky or mixed rock-ice Super Earth comparable to HD 40307 b. The two outer planets in this scenario would be at least 50% primordial ices, like the Steam Planets envisioned by F. Selsis and colleagues (2007), while the inner planet “b” would feature a larger proportion of metals and silicates.

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 -0.33, 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 -0.12 (metallicities Bean et al. 2006), 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 clearly falls along the same continuum in terms of system architecture, resembling most closely a scaled-down version of HD 69830. See also Crowded Orbits.

Last updated July 2008

References:

Bean JL, Benedict GF, Endl M. (2006) Metallicities of M dwarf planet hosts from spectral synthesis. Astrophysical Journal, 653: L65-L68. Abstract.
Fischer DA, Marcy GW, Butler RP, Vogt SS, Laughlin G, Henry GW, Abouav D, Peek K, Wright JT, Johnson JA, McCarthy C, Isaacson H. (2008) Five planets orbiting 55 Cancri. Astrophysical Journal, 675: 790-801. Abstract.
Kennedy GM, Kenyon SJ. (2008b) Planet formation around stars of various masses: Hot Super-Earths. Astrophysical Journal, in press.
Mayor M, Udry S, Lovis C, et al. (2008) The HARPS search for southern extra-solar planets. XIII. A planetary system with 3 Super-Earths (4.2, 6.9, & 9.2 Earth masses). Astronomy & Astrophysics, in press. Abstract.
Raymond SN, Scalo J, Meadows VS. (2007) A decreased probability of habitable planet formation around low-mass stars. Astrophysical Journal, 669: 606-614.
Rivera E, Lissauer JL, Butler RP, et al. (2005) A ~7.5 MEARTH planet orbiting the nearby star, GJ 876. Astrophysical Journal, 634: 625-640.
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.