HD 128311 is notable for hosting two gas giant planets in a mean motion resonance within a few AU of the central star, as well as an extensive debris belt at a slightly larger radius (Trilling et al. 2008). The host is an orange star (type K0) located 16.57 parsecs (54 light years) away, less massive than our Sun (0.82 MSOL) and substantially younger. Takeda and colleagues suggest an age of 1.92 billion years (Takeda et al. 2007). Although both of its planets are larger than Jupiter, their orbits are roughly analogous to those of Earth and Mars.

The inner planet, HD 128311 b, has a minimum mass of 2.37 MJUP and a semimajor axis of 1.1 AU. Its period of about 456 Earth days (1.25 years) places the planet in a 2:1 mean motion resonance with the outer companion, HD 128311 c. This object has a minimum mass of 3.23 MJUP, a semimajor axis of 1.74 AU, and a period of about 912 Earth days (2.5 years). Evidently planet c dominates the interaction, guaranteeing the clockwork regularity of the two planets’ orbits. Both are eccentric (e = 0.22 and 0.24, respectively), so that the two planets regularly approach each other within a few tenths of an AU. (All values Catalog of Nearby Exoplanets 2008.)

Z. Sandor and P. Klagyivik have developed an animation that illustrates the regular oscillations of the two planets’ orbits over a period of 1700 years (see their Web page). Their animation shows that the oscillating orbit of the inner planet carries it from about 0.55 AU at its closest approach to the primary star to about 1.65 AU at its widest separation. This eccentricity would result in substantial temperature changes in the planet's deep atmosphere, as well as visible changes in the patterns of cloud cover.

Two other relatively nearby stars also host pairs of adjacent planets in a 2:1 mean motion resonance. These are HD 82943 (at 27 parsecs) and HD 73526 (at 99 parsecs). Both are G-type stars, warmer and more massive than HD 128311, but all three systems display a strong family resemblance. Each has two planets with minimum masses in the range of 1.5-3.5 MJUP and semimajor axes in the range of 0.65-1.80 AU – roughly equivalent to the region of the Solar System between the orbits of Venus and the Asteroid Belt. In each pair, the combination of similar semimajor axes and substantial orbital eccentricities means that the two planets regularly come into close proximity. One of these systems (HD 82943) even resembles HD 128311 in hosting an icy debris field like a larger version of our own Kuiper Belt (Trilling et al. 2008).

The debris belt of HD 128311, on the other hand, is more reminiscent of a pumped-up version of our Asteroid Belt, with an inner edge at about 5.1 AU, comparable to the orbit of Jupiter around the Sun (Trilling et al. 2008). Collisions among its rocky and icy components create extensive dust, whose infrared signature permits the detection of the belt. Debris fields are usually – though not always – associated with young stars (Wyatt et al. 2007). The simultaneous presence of planets and detectable debris is unusual, placing this system among a handful of others nearby: the K stars Epsilon Eridani and HD 69830, the G star 70 Virginis (Trilling et al. 2008), and the F star HD 10647 (Liseau 2008).

Debris belts identical to those in the Solar System are insufficiently dusty to be observed in nearby exosystems (see, e.g., Greaves et al. 2006). We can conclude that the dusty rings around nearby stars either [1] contain far more mass than the Kuiper Belt, ensuring frequent collisions and a steady supply of fine particles; or [2] have recently experienced a collision between two or more especially massive planetoids, creating a cloud of dust that will disperse over thousands or millions of years (see Wyatt et al. 2007, Trilling et al. 2007).

Some hundreds of millions of years after the eight major planets assembled, our own Solar System underwent a cataclysm known as the Late Heavy Bombardment (Gomes et al. 2005). Airless moons throughout the system preserve a record of intense meteor impacts, in the form of extensive cratering as on the surface of the Moon. This violent epoch probably resulted from gravitational perturbations of the Asteroid and Kuiper Belts by the giant planets, whose orbits had been evolving into their present configurations over secular time scales. Perhaps the system of HD 128311 is now in the throes of a similar cataclysm.

If the star's immediate environment is as violent as the evidence suggests, we have no reason to expect any forms of life to exist on its planets or moons - not even if they had Earthlike compositions and favorable temperatures. Living organisms could not evolve on Earth until the Late Heavy Bombardment ceased and the planets settled into their mature configurations. Nevertheless, it is noteworthy that the habitable zone of HD 128311 appears to occupy the region between 0.42 and 0.79 AU (see Mandell et al. 2007), where the inner planet spends a portion of its eccentric orbit. Since both planets seem capable of harboring massive rocky moons, life may someday emerge on the satellites of HD 128311 b.

Last update September 2008