Epsilon Eridani, also known as HD 22049, is the nearest non-binary Sun-like star, as well as the nearest exoplanetary host. Located at a distance of only 10.5 light years, this young orange dwarf has the further distinction of harboring a huge debris disk, which has a larger inner radius and a far greater mass in comets, planetoids, debris, and dust than our own Kuiper Belt (see also Debris Disk Systems).

The star belongs to spectral class K2, with a luminosity only 34% Solar (Kaler), and a mass and radius of 0.83 and 0.73 Solar, respectively (Di Folco et al. 2004, Benedict et al. 2006). These values correspond to a primordial ice line at about 2.5 AU and a current habitable zone centered around 0.6 AU. Epsilon Eridani’s metallicity is typical for stars in the Solar neighborhood, but lower than the average exoplanetary host, with [Fe/H] estimated between -0.06 and -0.09 (Di Folco et al. 2004). All sources agree that Epsilon Eridani is younger than 1 billion years, but beyond that, no consensus has emerged. Estimates of its age range from 200 to 850 million years. A careful review by Markus Janson and colleagues found no conclusive evidence to narrow this range, but on the basis of stellar rotation they favored a value of 440 million years (Janson et al. 2008).

At an equivalent stage in its youth, our Solar System was a chaotic field of eccentric asteroids and comets that ricocheted among the planets and created impact craters on every available surface. Similar circumstances may explain the extensive debris observed around Epsilon Eridani, except that the collisions in that system are occurring mostly in the outer disk rather than the region corresponding to our Asteroid Belt and inner planets.

eccentric planet

A single gas giant, “b,” has been identified by radial velocity searches. Its period is 6.85 years, one of the longest among extrasolar planets discovered to date. Its semimajor axis is 3.39 AU, and its eccentricity is extreme, at 0.7 (Benedict et al. 2006). These values correspond to a periastron of 1 AU and an apastron of 5.8 AU, equivalent to a journey from Earth's orbit, out past Jupiter, and back again every seven years. As a result we can expect large variations in temperature over a single orbit.

The planet’s mass is reasonably well constrained at 1.55 Mjup (Benedict et al. 2006), meaning that it outweighs all four of the Solar System's giants. Unlike the case for most exoplanets, this is an estimate of true mass rather than minimum mass, since the inclination of the planet’s orbit has been determined with some confidence to be about 25 to 30 degrees (Hatzes et al. 2000, Greaves et al. 2005, Benedict et al. 2006). (Note that the Catalog of Nearby Exoplanets provides conflicting values for most of these parameters.)

massive disk

On account of the low inclination of Epsilon Eridani’s orbital plane, we see its debris disk in an almost pole-on orientation (Greaves et al. 2005). Coincidentally, this is also true for two other nearby debris disk systems, Tau Ceti and Vega. Greaves and colleagues conclude that Epsilon Eridani’s dusty disk is an analog of our Kuiper Belt, with an inner edge at about 40 AU, a peak at 65 AU, and a steady attenuation out to the cut-off radius at 105 AU (Greaves et al. 2005). This debris field covers a much larger area and contains about 100 times more mass than our own outer disk. Like analogous structures around stars of all spectral types, Epsilon Eridani’s disk must include a substantial population of ice dwarf planets resembling Pluto and Eris (Kenyon and Bromley 2008), whose perturbations stir up the surrounding debris and cause dust-producing collisions.

system architecture

The morphology of Epsilon Eridani’s dust disk, with its extensive inner clearing, large outer radius, and clumpy distribution, cannot be fully explained by the presence of the detected planet at 3.39 AU (Greaves et al. 2005). Many investigators have therefore proposed that an additional giant planet orbits at 40 AU, near the inner edge of the disk, just as Neptune (whose semimajor axis is about 30 AU, corresponding to the sunward edge of the Kuiper Belt) is typically invoked to explain the morphology of our own outer disk (Ida et al. 2000, Greaves et al. 2005, Thommes et al. 2008). Radial velocity data for Epsilon Eridani do indeed hint at the presence of an outer planet with an orbital period well in excess of 30 years (Janson et al. 2008). On the other hand, as Janson and colleagues argue, imaging surveys rule out the presence of any large gas giants on wide orbits. Whatever additional planets may exist, “none of them can be more massive than 3 Mjup” (Janson et al. 2008).

Given the highly elliptical orbit of the known gas giant, the outlook is dim for the survival of terrestrial-mass planets in the system’s liquid water zone, around 0.6 AU. If any rocky protoplanets managed to form, they would have been dispersed and ejected by perturbations from planet b. Perhaps one or more low-mass rocky or icy objects still lurk within a few million miles of the star, awaiting discovery by a new generation of search methods.

Planet b’s eccentricity has further implications for the system’s past history and present architecture. It is widely acknowledged that eccentric orbits result from episodes of violent planet-planet interaction, which tend to deliver the system’s most massive planet to a relatively small but elliptical orbit (a description that matches Epsilon Eridani b) while scattering less massive planets to larger orbits, or ejecting them altogether from the central star’s influence (see, e.g., Adams & Laughlin 2003, Barnes & Quinn 2004, Ford & Rasio 2007).

Last updated July 2008