Located at a distance of only 4.7 parsecs (15.4 light years) in the constellation Aquarius, Gliese 876 – usually abbreviated GJ 876 – is an ordinary red dwarf of spectral type M4. Roughly one-third the size and mass of our Sun, this star is notable as the first M dwarf to be identified as the host of a planetary system. Since 1998, three planets have been discovered in short-period orbits around the central star: two gas giants and one Hot Super Earth (Marcy et al. 1998, Marcy et al. 2001, Rivera et al. 2005).

Studies over the past dozen years have clarified the host star’s basic parameters. For the mass of GJ 876, Alexandre Correia and colleagues provide a value of 0.34 MSOL (Correia et al. 2010), almost identical to the 0.33 MSOL assumed by Eugenio Rivera and colleagues (Rivera et al. 2005). Its luminosity is estimated by Correia’s group as 1.3% Solar, which is typical for M dwarfs in this mass range. The star’s metal content or [Fe/H] has been a matter of disagreement, with estimates ranging from a low of -0.40 to a high of +0.37 (Bailey et al. 2009, Johnson & Apps 2009). In this case, the high-end value proposed by Johnson & Apps in their 2009 study has rapidly gained favor (see, e.g., Greg Laughlin’s blog entry for Red Dwarf Metallicities). Thus GJ 876 appears to be one of the most metal-rich M dwarfs known to harbor a planetary system.

The star’s age remains the most difficult parameter to constrain. Correia’s group observe that GJ 876 displays modest magnetic activity and rotates quite slowly, with a period of about 97 days (Rivera et al. 2005) – almost quadruple the Sun’s period of 25 days. Despite these indications of stellar maturity, they provide a minimum age of only 100 million years, and a maximum of 5 billion years (virtually the same age as our Sun), primarily on the basis of the star’s orbital motion around the Galactic Center.

system architecture

The architecture of the GJ 876 system is remarkable for two reasons. First, it remains the only M dwarf known to host two gas giants. Although additional two-giant systems are likely to be revealed around other red dwarfs in the near future, given the presence of long-term radial velocity trends for such stars as GJ 317 and GJ 849 (Wright et al. 2009), it is still true that 45% of the known M dwarf systems lack any gas giants whatsoever. This dearth contrasts sharply with the situation for host stars in the mass range of our Sun (0.9-1.1 MSOL), among which 95% harbor at least one gas giant.

Architecture of the GJ 876 system. Colored circles indicate the relative sizes of the 3 planets, assuming the actual masses provided by Correia et al. 2010, the mass-radius relationships provided by Fortney et al. 2007, and rock/metal cores. Semimajor axes are indicated in astronomical units (AU) on a logarithmic scale. White dots mark the ice line.

Second, the system’s two giants exhibit an unusual orbital behavior with the potential to reveal a great deal about their evolutionary origin. In astronomical terms, these two planets are “locked deep in a 2:1 mean motion resonance,” and are further engaged in a secular resonance. Several objects in the Solar System are also engaged in mean motion and secular resonances, including three of Jupiter’s largest moons as well as the planet/dwarf planet pair Neptune and Pluto. In the case of GJ 876, these resonances indicate that the outermost planet, “b,” completes one orbit for every two orbits of the middle planet, “c.” In addition, whenever planet b arrives at its nearest approach to the central star (i.e., periastron), it is also closely aligned with the periastron of planet c (Lee & Peale 2002, Beauge et al. 2006, Correia et al. 2010). Various authors have described the latter behavior in terms of “apsidal alignment” and “apsidal corotation” (Laughlin et al. 2005, Beauge et al. 2008).

GJ 876 was the first exoplanetary system in which a mean motion resonance was demonstrated, and it remains the best-studied example of this behavior outside our Solar System. The dynamic relationship shared by GJ 876 b and c could not exist if the two planets simply formed at their present locations, without any shared orbital evolution (Lee & Peale 2002). This system therefore furnishes excellent evidence for the hypothesis of Type II migration, which argues that gas giant planets are subject to incremental displacement from larger to smaller semimajor axes through interactions with the primordial gas nebula.

The system’s resonant architecture also implies that the two planets perturb each other’s motion so strongly that their orbits change rapidly over time (Laughlin & Chambers 2001). Continuing observations therefore result in ever more precise determinations of key system parameters. Now that investigations of GJ 876 have extended over two decades, using increasingly sensitive instrumentation, our understanding of this system has vastly improved. A number of recent studies agree that we see the orbits of the two gas giants at an angle of about 50 degrees against the plane of the sky (Rivera et al. 2005, Bean et al. 2009, Correia et al. 2010). This information enables us to calculate their true masses, a degree of precision otherwise unavailable for exoplanets except in the rare cases where we observe them in transit across the face of their host stars.

three planets

The three detected planets of GJ 876 orbit close to the central star in a region that extends from about 0.02 AU to 0.21 AU – a span of less than 29 million kilometers (18 million miles), substantially smaller than the average separation of Mercury from the Sun. All three planets most likely originated at greater distances and migrated inward to their present locations. The following discussion assumes the system parameters presented by Correia et al. (2010).

1. The innermost planet, GJ 876 d, has a semimajor axis of only 0.021 AU and an orbital period of less than two days. Its minimum mass of 6.3 Mea falls in the range of the Super Earths (2-10 Mea). In a system with so many firsts, planet “d” represents the earliest detection of such a low-mass object. Assuming that it orbits in the same plane as the two outer planets, its actual mass will be about 8 Mea. One likely outcome of its proximity to the host star will be a rotational period that has become tidally locked, so that one hemisphere always faces the star and the other lies in perpetual shadow.
   
   Given its star-grazing orbit, planet d’s effective temperature is expected to fall between 430 and 650 K. That would place its likely surface temperature in the range of our own Mercury and Venus, depending on planet d’s internal structure, atmospheric characteristics (if any), and true rotational period. Mercury, the innermost planet of the Solar System, is an airless sphere of metal and rock with a mean surface temperature of 445 K. Venus, although almost twice as far from the Sun, has a still higher surface temperature of 735 K on account of its dense atmosphere and runaway greenhouse effect.
   
   Although most planets detected so close to their central stars have circular orbits as a result of stellar tides, GJ 876 d has a notable eccentricity of 0.14. This may imply that the planet is subject to heating through gravitational stress, over and above its irradiation by the host star. In addition, its eccentricity opens the possibility that the planet’s rotation has escaped tidal locking and achieved a spin-orbit resonance like Mercury, with rotates 3 times for every 2 orbits of the Sun (Correia & Laskar 2004).
   
   The regime of Super Earths, of which GJ 876 d is the classic instance, includes at least two basic planetary types: objects composed almost entirely of rock and metal, like Venus and Earth, and objects with a rock/metal core and an extensive mantle of high-pressure phases of ice, like scaled-up versions of Ganymede and Titan. To date, transit studies have identified one likely example of each type among the known Hot Super Earths: the rocky planet CoRoT-7b (Queloz et al. 2009) and the icy planet GJ 1214 b (Charbonneau et al. 2009). Although numerous investigations have attempted to assign GJ 876 d to one class or the other (e.g., Lecavelier des Etangs 2006, Valencia et al. 2007), no conclusive constraints have yet been established.
   
   
2. The second planet, GJ 876 c, has a semimajor axis of 0.132 AU and an orbital period of 30.26 days. Its orbital eccentricity of 0.266 is unremarkable among exoplanets, but still higher than that of either of its planetary companions. At its widest separation from the host star (i.e., apastron), planet c is more than 70% farther away than at periastron, suggesting notable temperature variations over the course of a single orbit.
   
   Planet c’s minimum mass of 0.62 MJUP falls well within the gas giant range. The recent study by Correia and colleagues established its orbital inclination against the plane of the sky as about 48 degrees, enabling the planet’s true mass to be calculated at 0.83 Mjup. Planet c is thus intermediate in mass between Saturn and Jupiter, and significantly below the median for the full population of extrasolar gas giants (~1.75 MJUP). Given its proximity to the central star, this second planet almost certainly rotates far slower than the Solar System’s gas giants, both of which have rotational periods of about 10 hours. However, planet c’s eccentric orbit and mean motion resonance with planet d may have prevented it from spinning down to a tidal lock. Conceivably, it may retain a rotational period of a few days to a few weeks.
   
   
3. The outer planet, GJ 876 b, has a semimajor axis of 0.211 AU, an orbital period of about 61 days, and a small eccentricity of 0.03. However, considering the large eccentricity of its inner companion, planets b and c are separated by only 16 million km (10 million miles, 0.108 AU) at their closest approach. By comparison, Earth is about 42 million km (26 million miles, 0.28 AU) from Venus at their closest approach.
   
   Planet b’s minimum mass has been defined as 1.93 MJUP. Correia and colleagues established that its orbit is almost exactly coplanar with planet c’s, tracing an inclination of about 49 degrees against the plane of the sky. This determination enables planet b’s true mass to be calculated at 2.64 MJUP. It remains unknown whether the planet’s rotation has become tidally locked; nevertheless, tidal locking is the default assumption for all short-period exoplanets.

The orbits of both giant planets around GJ 876 approximately coincide with the system's habitable zone (Rivera & Haghighipour 2007), indicating that both may be cool enough to support water clouds in their dense atmospheres. The gravitational influence of the host star might have prevented either planet from maintaining a system of satellites or rings. However, one study suggests that GJ 876 b might sustain moons even as massive as Earth (Barnes & O'Brien 2002). Such heavy companions would be conceivable only if they were captured by the host planet, since theories of satellite formation place strict constraints on the maximum size of co-formed moons (Canup & Ward 2006). Objects in the mass range of the Galilean satellites of Jupiter (perhaps up to the mass of Mercury) may be more likely -- if indeed any moons at all are possible.

Two studies over the past several years have identified a region of dynamic stability just inside the orbit of GJ 876 c, where a terrestrial-mass planet might survive after being captured in a 2:1 mean motion resonance with planet c and a 4:1 resonance with planet b (Laughlin et al. 2005, Correia et al. 2010). With a proposed semimajor axis of 0.08 AU and an orbital period of 15 days, this hypothetical planet might be responsible for the unexpected eccentricity of GJ 876 d through gravitational perturbations of the innermost planet's orbit (Correia et al. 2010). Correia's group notes that such a planet would be detectable by a dedicated search using the most sensitive of currently available technologies.

from dusty clouds to gas giants (in progress)

The aggregate mass of the system’s three planets (3.5 MJUP) substantially exceeds that of the eight planets of the Solar System (less than 1.5 MJUP), even if we include Pluto and all 150+ moons. The best explanation for GJ 876's large planetic endowment seems to be an inordinately massive protoplanetary nebula, since heavyweight nebulae are predicted to yield heavyweight planets (Thommes et al. 2008). As Greg Laughlin and colleagues concluded, "Standard core accretion theory predicts that systems such as GJ 876 are drawn from the extreme high-mass end of the circumstellar disk mass distribution, and will thus be intrinsically rare" among M dwarf systems (Laughlin et al. 2004).

The innermost planet probably formed as a consequence of the inward migration of the two outer planets, which entered into the 2:1 mean motion resonance as they spiraled through the primordial gas disk of GJ 876. Gravitational perturbations by these two planets forced rocky planetesimals in the inner disk to clump inside the shrinking radius of the second planet’s orbit, where the planetesimals underwent ejections, collisions, and accretion (Fogg & Nelson 2005, Raymond et al. 2006b, Mandell et al. 2007). The ultimate result was the hot Super Earth that we now detect.

Last update February 2010