The red dwarf star GJ 581 made headlines in 2007 as the host of the most Earthlike planets yet detected: two terrestrial-mass bodies orbiting in the vicinity of the system's proposed habitable zone. The primary is an ordinary M3 star located at a distance of only 6.27 parsecs (20 light years) in the constellation Libra. It is one of the closest stars known to host a planetary system.

The mass of GJ 581 is about 31% Solar; its diameter is about 29% Solar. This is less than three times the diameter of Jupiter. With bolometric correction, the star’s luminosity is only 0.013 Solar, a typical value for M dwarfs. Its metallicity is -0.26, significantly less than the Sun’s and lower than most planet-bearing stars. Its age is estimated at 2 to 3 billion years, compared to 4.6 billion for the Solar System (all data Bonfils et al. 2005, Udry et al. 2007, Bailey et al. 2008).

three planets

The innermost companion of GJ 581 is a Warm Neptune. Its minimum mass of 15.7 MEA makes it a near-twin to Neptune in the Solar System, but its orbital period and semimajor axis are only 5.4 days and 0.041 AU, respectively. Like all giant planets in star-grazing orbits, planet b must have reached its present location by migrating inward from its place of origin beyond the system’s ice line, which is located at about 0.4 AU (Ida & Lin 2005). Having assembled at such a distance from a cool, low-metallicity star, GJ 581 b probably consists primarily of ices packed together at high pressure, with a modest rocky core and a hydrogen atmosphere. Such is the inferred composition of the Hot Neptune that transits GJ 436, an M dwarf very similar to GJ 581 (Gillon et al. 2007).

Two more planets travel in wider orbits. Both are in the mass range of GJ 876 d, and thus qualify as Super Earths. Planet c has a minimum mass of 5 MEA, a period of 12.9 days, and a semimajor axis of 0.073 AU. It is one of the smallest exoplanets yet detected.

Planet d is more remote and only slightly heavier, with a mass of about 7.7 MEA, a semimajor axis of 0.25 AU (a bit more than half the distance of Mercury from the Sun), and a period of about 84 days. This is almost exactly equal to Mercury's period, since planets travel more slowly around low-mass stars (like GJ 581) than around higher-mass stars (like our Sun). For more information, see the Table of M Dwarf Systems.

The orbital dynamics of these three planets have not yet been explored in detail. A naive look at the data suggests that planet c orbits just outside the 2:1 mean motion resonance with planet b (Udry et al. 2007). Whether the orbits of all three planets display near-separatrix behavior, as is typical of multiple-planet systems, has yet to be determined (see, e.g., Barnes & Greenberg 2006).

The orbit of the first planet has been circularized, while the second has an estimated eccentricity as high as 0.16. Although this is not extreme, it is more elliptical than the orbits of seven out of eight Solar planets. The eccentricity of the third planet may be still higher, at about 0.2, very close to Mercury's (all values Udry et al. 2007). Notably, Mercury has escaped tidal locking despite its proximity to a star three times as massive as GJ 581. It long ago arrived at a stable "spin-orbit resonance," such that it rotates three times for every two orbits around the Sun. Because this dynamic is closely related to Mercury's eccentricity, a similar resonance might have evolved in one or both of the outer planets of GJ 581.

However, W. von Bloh and colleagues have argued that planets c and d probably have much less elliptical orbits, consistent with an eccentricity near zero, and on this basis they conclude that both are tidally locked (von Bloh et al. 2007).

questions of habitability

Much speculation has centered on the question of whether either of the Super Earths, GJ 581 c or GJ 581 d, might be habitable. While the precise delineation of habitable zones remains an art rather than a science, the discovery team considered it likely that planet c orbits at the inner edge of this favored region. If it sustains a relatively transparent atmosphere, with no extreme greenhouse effect, liquid water might exist on its surface. When the planet's discovery was announced in 2007, astronomer Xavier Delfosse was widely quoted in the media (CNN, Time Magazine, Agence France-Presse) for observing, "On the treasure map of the universe, one would be tempted to mark this planet with an X."

Subsequent studies have been less optimistic. F. Selsis and colleagues conducted a detailed exploration of the system's potential habitability, foregrounding the complexity of such a undertaking and proposing numerous alternative models for the atmospheres of planets c and d (Selsis et al. 2007). They offered a range of boundaries for the habitable zone, among which the most conservative were 0.08 AU to 0.20 AU; these would exclude both planets. W. von Bloh and colleagues performed a competing analysis, favoring larger radii for the habitable zone. Their most generous boundaries were about 0.13 AU to 0.26 AU, although the outer value might be as small as 0.21 AU (von Bloh et al. 2007).

Both groups concluded that GJ 581 c is too hot to sustain liquid water, while the more massive third planet makes a much better candidate, provided its atmosphere creates a strong greenhouse effect.

The composition of these two planets remains unknown in the absence of any observational techniques that might shed light on their diameters or densities. Nevertheless, current theory indicates that protoplanetary disks – as well as the planets that evolve within them – share fundamental characteristics with their host stars (Greaves et al. 2007, Raymond et al. 2007, Johnson et al. 2007). Given a low-mass, low-metallicity star like GJ 581, we can expect a similarly low-mass, low-metal protoplanetary disk. Such an environment is not conducive to the formation of rocky planets as massive as GJ 581 c and d (Raymond et al. 2007, Thommes et al. 2008).

It seems likely, therefore, that the two planets consist of about 50% water ice and 50% metals and silicates, which is the suggested ratio for icy Super Earths (Leger et al. 2004, Selsis et al. 2007). They may have assembled as a result of the migration of planet b (the Warm Neptune), which spiraled inward from its point of origin to its present location near the star. This planet's passage through the primordial disk would have stirred up large quantities of icy planetesimals, which might then be available to accrete planets c and d in the inner regions of the disk (see Mandell et al. 2007). Thus the two smaller planets may have formed in the ice giant's wake. Alternatively, both may have formed near the system's ice line just like their larger companion, and then traveled to their present orbits by planet-planet scattering or Type I migration (Selsis et al. 2007, Kennedy & Kenyon 2008).

If both of them are indeed icy, planet c may be a "Steam Planet," with a scalding atmosphere of pure water vapor in direct contact with a layer of high-pressure, superheated ice. Planet d might be an "Ocean Planet," cool enough to sustain a global ocean 100 kilometers deep above its icy mantle.

Last update September 2008