The number of potentially habitable planets continues to grow. This week, a team of astronomers provided an update on GJ 667C, a star known to host two super-Earths, based on past observations. Further observations, along with some refined statistical methods, now indicate that there are likely to be at least six planets in the system (and possibly a seventh), all packed in a region that's about half the distance from the Earth to the Sun. Although they're all much closer to the host star, the star is quite a bit dimmer, which also shifts the habitable zone such that two of the planets fall squarely within it.

GJ 667C is part of a three-star system in the direction of the constellation Scorpius. The stars orbit each other at a sufficient distance, however, that GJ 667C's companions don't interfere with the planetary orbits. Initial observations of the star were made with a spectrograph (the HARPS instrument), which detects subtle shifts in the wavelengths of the light emitted by the star. Some of these shifts are changes in the star's activity, but others are caused by its motion toward or away from Earth, which shift the light to higher or lower frequencies, respectively. One of the factors that can cause these shifts is gravitational pull of planets as their orbits take them ever so slightly closer to or farther from Earth.

GJ 667C is a type of star called an M-dwarf that is smaller than the Sun. Because of its small size, it's possible to detect even relatively light planets due to their pull on the host star. The ease of detecting planets was one of the reasons that the star was targeted for observations originally, and that paid off with the discovery of the exoplanets GJ 667Cb, a super-Earth close to the star (at 0.05 Astronomical Units) and GJ 667Cc, at about .12 Astronomical Units.

But a team of researchers has gone back and re-evaluated the system, combining some new observations from HARPS and other instruments with new methods of both modeling the star and identifying planets. The new model of the star allowed the authors to determine how variable it normally is; the answer turned out to be "not very." (In technical terms, the data "puts the star among the most inactive objects in the HARPS M-dwarf sample.")

With the background activity of the star controlled for, the authors looked at the remaining signals for indications of a planetary orbit. The method they used was fairly standard: search for a clear signal and once it was identified, remove it and look for a signal in what remained. As each planet was identified, the authors checked whether including it in their model of the exosolar system fit the actual data better. The two previously identified planets quickly popped out with a very significant signal by this test, as did a third (each of them improved the model by anywhere from 1012 up to 1037). And signals continued to pop out of the analysis, indicating three additional planets (GJ 667Ce-g), each of which improved the model by about a thousand-fold. There was also a hint of a planet close to the host star at just under 0.1 Astronomical Units, though it failed the authors' statistical tests.

The authors tested whether a system with planets in these apparent orbits would be stable, and it appears that it would be, provided none of the planets were much above their potential minimum masses. That would make most of them super-Earths, with the lone exception being an Earth-sized body.

Based on the brightness of GJ 667C, it's possible to calculate where the potential habitable zone would reside around the star. On the inner edge of the zone, enough water enters the atmosphere that it reaches altitudes where the incoming stellar radiation can dissociate it, allowing the hydrogen to escape into space. GJ 667Cc is right at this boundary, but its high mass means that it might be able to retain water in the atmosphere despite the heat. GJ 667Cf is squarely within the habitable zone, while GJ 667Ce is farther out, but still close enough that a healthy dose of greenhouse gasses like methane and carbon dioxide would warm it enough to keep water liquid.

All of these assumptions are based on the presence of an atmosphere and a reflectivity similar to that of Earth's. If those aren't present, then either of the two planets that are closer to the edge of the habitable zone could easily shift out of it. (Read this for more details of these complications.) Another consideration is that these planets are close enough to the host star that they could be tidally locked like our Moon, constantly showing a single face to the body they orbit. This can create a hot spot directly facing the host star, with progressively cooler zones farther from that, possibly allowing liquid water in a ring around one region of the planet. This configuration has been nicknamed an "eyeball Earth."

The authors expect that further observations should be able to tell us definitively whether there's a GJ 667Ch. But the planets are so tightly packed that trying to fit any additional planets inside the orbit of GJ 667Cg would destabilize the system. That doesn't rule out anything farther out, but those are getting far enough from the host star that, unless they're massive, they'll be difficult to detect. In general, the system continues what seems to be a trend with dwarf stars: it hosts a tightly packed system of rocky bodies a very short distance from the star. Given that these dwarfs emit far less intense light, that places the planets in and near the habitable zone with great frequency. Since planets near dwarfs are relatively easy to discover, habitable zone discoveries may become relatively commonplace in the future.

One minor negative in all of this is that the decision to name planets in order of their discovery is starting to create some very confusing exosolar systems. Assuming planet seven is confirmed at GJ 667C, then the order of planets will end up being b, h, c, f, e, d, and g, with g being farthest from the star. All of which makes keeping track of which planet is where rather challenging.

The arXiv. Abstract number: 1306.6074 (About the arXiv). To be published in Astronomy & Astrophysics.