Red dwarfs of spectral type M comprise about 75% of all stars, both in the Solar neighborhood and in the Milky Way Galaxy as a whole (Tarter et al. 2007). They range in mass from about 0.5 MSOL down to about 0.1 MSOL, with corresponding reductions in heat and brightness. The dim, lightweight character of red stars poses special problems to planet searches using either the standard radial velocity method or the increasingly successful transit method. As a result, search programs have only recently begun to include large numbers of M dwarfs in their target lists.

theoretical constraints

Strong theoretical arguments indicate that planets should form less frequently around M dwarfs than around stars of spectral classes F, G, and K (Laughlin et al. 2004, Ida & Lin 2005, Endl et al. 2006). Given the low mass of the host stars, the protoplanetary disks around M dwarfs generally contain less gas and dust than those around FGK stars. Moreover, at a given semimajor axis, objects orbiting M dwarfs travel much more slowly than objects orbiting more massive stars, meaning that collisions between planetesimals occur less often and accretion time scales are correspondingly longer. Thus fewer, less massive planets are able to form (Raymond et al. 2007).

Nevertheless, the sheer abundance of M stars throughout the galaxy ensures that a large portion of exoplanetary systems will be centered on red suns. Given their low energy output, the ice lines of M star systems are located very close to the primary, at about 0.4 AU (Ida & Lin 2005) as opposed to about 3 AU for G-type stars like our Sun. Because rocky, terrestrial-mass planets necessarily form within the ice line, while gas giants preferentially form just outside it (Ida & Lin 2005), we can expect limited formation of both types of planets in M star systems. Icy worlds like Uranus and Neptune seem the most likely planetary type, since their formation involves the accretion of ices over long periods of time, rather than oligarchic growth within a dense field of planetesimals (as with terrestrial planets) or rapid accumulation of gas (as with gas giants).

The reduced physical scale of M dwarf systems means that their habitable zones, like their ice lines, are located much closer to the primary than those of more massive stars. Mandell and colleagues provide inner and outer boundaries of 0.10 AU and 0.19 AU for the habitable zones of larger M dwarfs, with masses of about 0.4 MSOL (e.g., GJ 436). The corresponding boundaries shrink to about 0.024 AU and 0.045 AU for the smallest members of the class, with masses of about 0.1 MSOL (Mandell et al. 2007).

Such narrow boundaries, which imply belts ranging in width from 8.4 million miles down to 2 million miles, contrast strongly with the habitable zones of Sun-like stars. For example, the projected habitable zone of the Solar System extends from 0.8 AU to 1.5 AU, defining a region 65 million miles wide (Mandell et al. 2007).

Simple limitations of mass and space make habitable planets even less likely to form in M dwarf systems than in the systems of F, G, and K stars (Raymond et al. 2007). Moreover, the necessary proximity of such a planet to its host raises the likelihood that its rotation will be tidally locked. In that case, day and night will become geographical features rather than phases in a regular cycle, since the inner hemisphere experiences nonstop irradiation while the outer hemisphere remains eternally dark.

According to some models, tidal locking would result in complete loss of atmosphere through the freezing of volatiles on the dark side. This position has been challenged by recent demonstrations that atmospheric circulation would sufficiently moderate global temperatures to maintain a substantial envelope of gases (Tarter et al. 2007).

It remains possible in theory for a rocky planet orbiting in the habitable zone of an M dwarf to maintain Earthlike conditions in its so-called Twilight Zone, a narrow band separating the day side from the night side. In such a case, most of the day side would be burning, arid wasteland, while the night side would resemble Antarctica in winter. The Twilight Zone would sustain open bodies of water and, perhaps, both aquatic and telluric organisms. The width of any given planet's Twilight Zone is a function of its orbital eccentricity, such that circular orbits have narrow temperate regions while increasing eccentricity results in wider Twilight Zones (Dobrovolskis 2007). (See artist Lynette Cook’s depiction of such a world.)

Also theoretically possible is a so-called “Ocean Planet,” as described by Leger, Selsis, Valencia, and others (Leger et al. 2004, Valencia et al. 2007). Such a planet would have about 50% of its mass in metals and silicates, and 50% in ices and other volatiles, supporting a global ocean hundreds of kilometers deep. Ocean currents would doubtless be even more successful than winds in moderating temperatures, while the watery environment might be an effective incubator for life forms. (Readers of science fiction may be reminded of the sentient global ocean imagined in Stanislaw Lem’s novel Solaris, or the oceanic Eden in C.S. Lewis's Perelandra.)

Closer to the host star, however, such volatile-rich objects would become scalding Steam Planets, and farther away they would devolve into cloud-wrapped snowballs, smaller versions of Uranus and Neptune.

For Super Earths and other terrestrial planets in the habitable zone, one alternative to the scenario of tidal locking is spin-orbit alignment. A planet in a relatively eccentric orbit may escape synchronization and establish a rotational spin that is some multiple of its orbital period (Dobrovolskis 2007). This has actually happened to the planet Mercury, which has an eccentricity of 0.206, a rotational period of almost 59 days, an orbital period of 88 days, and a spin-orbit resonance of 3:2 (Correia & Laskar 2004).

systems detected so far

To date, eight planetary systems have been detected around M stars. Because all are located within about 10.5 parsecs (35 light years), this sample is strongly biased by distance from our Solar System. If exoplanetary systems that center on M dwarfs are as common within 30 parsecs (100 light years) as they are in the immediate Solar neighborhood, then a minimum of 130 additional M dwarf systems await discovery just within this limited volume of space. (See a comparative diagram of the eight systems, along with a table of physical and orbital characteristics.)

It is noteworthy that many of the detected systems deviate from theoretical predictions. Whereas four out of eight contain only lightweight planets in the mass range of Hot Neptunes and icy Super Earths, the other four (GJ 876, GJ 832, GJ 849, GJ 317) harbor gas giants similar in mass to those in most other extrasolar systems. One of the two giant planets orbiting GJ 876 contains more mass than all of the Solar System's planets combined, while each of the single detected giants of GJ 832, GJ 849, and GJ 317 bears an intriguing resemblance to our own Jupiter. The minimum masses of these planets range from 0.6 to 1.2 MJUP, comparable to the Solar System's gas giants, and all three orbit outside their systems' ice lines, again like Jupiter and Saturn. Less than 10% of exoplanets so far detected around Sun-like stars have comparable orbits.

The discovery of two terrestrial-mass planets orbiting at either edge of the habitable zone of GJ 581 (Udry et al. 2007) suggests that such planets may be common around M dwarfs, despite the limited parameter space in which their orbits may evolve. Given the low masses of red stars, their terrestrial planets are easier to find than Earth analogs around Sun-like stars, since Earthlike planets perturb low-mass stars far more strongly (and detectably) than they do higher-mass primaries. Good arguments indicate, nevertheless, that such planets will be very small if rocky and very icy if large (Raymond et al. 2007).

The links below lead to detailed discussions of five of the eight detected M dwarf systems, listed in order of increasing stellar mass.

Last update September 2008