Canadian astronomer Philip Gregory has confirmed that there are three habitable zone planets orbiting the red dwarf star Gliese 667C. The star, which is part of a trinary star system, is only 22 light-years away and it features a planet that's only twice the mass of Earth — making it the lowest mass planet found in a habitable zone thus far.
Top image: Artist's impression of a sunset from the super-Earth Gliese 667Cc courtesy ESO/L. Calçada. The large sun is the red dwarf, 667C.
New results with old data
Astronomers have known about the Gliese 667 system for some time — including the fact that it features some interesting exoplanets. But now, armed with new telescopic techniques, scientists have been able to study the triple star system in much greater detail.
Specifically, Gregory re-analyzed data acquired by the High Accuracy Radial velocity Planet Searcher, HARPS, which is part of the European Southern Observatory's 3.6 metre telescope in Chile. But this time he performed a Bayesian analysis based on a fusion Markov chain Monte Carlo algorithm — a system that allowed him to sample probability distributions.
Looking at the new results, he confirmed the presence of at least six planets, including the three potentially habitable ones.
The triple star system
Gliese 667 consists of three stars: two K dwarfs which orbit each other quite closely (they're very similar our own sun), and a low mass M dwarf, what's more commonly referred to as a red dwarf.
It's this red dwarf, 667C, that's causing all the excitement. Even though it's part of the triple system, it's at a fair distance from the K dwarfs — about 200 AU (Earth-distances). And while it's only one-third the size of our own sun and is only 1% as bright, it hosts at least six planets — three of which sit comfortably within its habitable zone (HZ).
Gregory, who works out of the University of British Columbia's Physics and Astronomy Department, was able to confirm that the three planets spin around the red dwarf in 28, 31, and 39 days respectively. This indicates, quite obviously, that the planets are very close to the dwarf. But given its extremely low luminosity, the HZ range is much closer for an M dwarf than for a sun like ours.
Based on previous studies, the planets are likely super-Earths. But as Gregory's work now suggests, they are all capable of fostering large amounts of liquid water and complex and stable atmospheres (including crucial CO2 and O2 cycles).
Interestingly, there is a fourth planet worth discussing, the 91.3 day 66Cf. It lies just teasingly outside the edge of the HZ, but its eccentric orbit means that it spends the majority of its time outside the so-called Goldilocks Zone — that area of the solar system that's just right for life to emerge and flourish.
But it's the 39-day planet that has caught Gregory's attention. It's a potential super-Earth, like most terrestrial exoplanets that have been discovered so far — but it is now the tiniest of the super-Earths ever discovered, at roughly twice our planet's mass. This could bode well for its potential habitability.
The paper is still subject for review, and was recently submitted to the Monthly Notices of the Royal Astronomical Society for consideration: "Evidence for Multiple Planets in the Habitable Zone of Gliese 667C: A Bayesian Re-analysis of the HARPS data."
UPDATE #1: There are some questions about the scientific validity of this paper, and we are awaiting an update from the authors.
UPDATE #2: To get further clarification on potential problems with the study, we spoke to Abel Méndez, an Associate Professor of Physics and Astrobiology in the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo. He told us, "the planets as described [in the paper] are dynamically unstable. Therefore, one or more of the proposed planets does not exist or have different parameters."
Update #3: We also contacted Guilem Anglada-Escude, a postdoctoral researcher at the University of Goettingen in Germany. He told io9 that Gregory is not using all the available data, and that the system he proposes is unstable. In addition, says Anglada-Escude, "to promote signals to planet candidates one needs to eliminate astrophysical false positives (not done) and check that the system is physically feasible (not done, not stable)."
Based on what we're hearing from Anglada-Escude and Méndez, it's clear that Gregory will likely have to adjust his paper to answer these concerns. That's not to say he's incorrect — it's just that he'll have to resolve these pressing concerns.
Other sources and h/t: MIT Technology Review.