Confirmed: A Star System with Three Potentially Habitable Planets!

Late last year, Canadian astronomer Philip Gregory made the controversial claim that there are three habitable zone super-Earths orbiting the red dwarf star Gliese 667C. Now, in a separate study, a group of European astronomers are saying he was right.

When we reported on Gregory’s claim back in December, we spoke to Abel Méndez, an Associate Professor of Physics and Astrobiology. At the time, he told io9 that, "the planets as described [in the unpublished paper] are dynamically unstable. Therefore, one or more of the proposed planets does not exist or have different parameters."

We also spoke to Guilem Anglada-Escude, a postdoctoral researcher at the University of Goettingen in Germany who happens to be a scientist involved in the new study. He claimed that Gregory was not using all the available data, and that the system he proposed was unstable. He also said that, "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)."

But these concerns now appear to have been addressed.

'A physically meaningful dynamical solution'

Anglada-Escude, along with Mikko Tuomi of the University of Hertfordshire, have reexamined this complex and highly dynamic system — and they’ve largely reached the same conclusion as Gregory, but with more rigorous proofs.

I contacted Anglada-Escude and asked him to elaborate on the new study. He told me that they used newly measured radial velocities using the HARPS-TERRA software. This provided better precision and allowed the astronomers to secure the solutions more robustly. Moreover, the team also added observations from the HIRES/KEck and PFS/Magellan spectrographs to the picture. Also, spectra from VLT were used to refine the properties of the star.

"We extracted the signals with two independent data analysis methods," he told io9. "Bayesian analysis with some prior choices — which was very important to the study — and what we call log-likelihood periodograms. Both methods aim to identify the global best fit solution when one planet is added."

He told me that the first five signals coincided with five of Phil Gregory's claims, but not the last two. Also, his team checked whether or not correlations between activity indices were affecting the significances of the signals (in Gregory's paper, he only mentioned that one or two of the signals might be activity induced without doing any check).

"So, we can robustly promote six of the signals to fully flagged planet candidates," he says. "The dynamics of the system also took a long time to figure out properly. We directly integrated 80,000 possible orbits and ranked them according to stability. Only those with eccentricities close to zero correspond to physically viable systems, which validates the prior choices we made."

Painting a Picture of GJ 667C

Here’s what we now know about the star system.

Gliese 667C is the faintest of a triple star system that’s only 22 light-years away in the constellation of Scorpius. It’s a red dwarf (a.k.a. M-dwarf) that’s just over one-third the mass of the Sun. From the perspective of one of its several planets, the two other stars (both of which are K-dwarfs) would look like a pair of very bright stars visible in the daytime and night. They would likely provide as much illumination as the full Moon.

In addition to the three habitable zone super-Earths (i.e. rocky planets much bigger than ours), the astronomers have found evidence for an additional four planets around the star, raising the potential total to six (possibly seven). And in fact, one of the newly confirmed planets may also reside with the habitable zone — which could raise the total to four. That would make for a rather jam-packed habitable zone!

Because Gliese 667C is a low luminosity red dwarf, its habitable zone is situated at a distance comparable to that of Mercury’s orbit around our Sun.

The new discovery shows that dynamically packed systems of super-Earths are relatively common, and that red dwarfs have more small planets than earlier-type stars.

“These two trends together suggest that GJ 667C is one of the first members of an emerging population of M-stars with multiple low-mass planets in their habitable zones,” the authors note in their study.

It’s worth remembering that 'potentially habitable' simply means that liquid water can exist on these planets and that they can foster complex and stable atmospheres. But it’s not an indication that other features required to sustain life are present.

Read the entire study: “ “A dynamically-packed planetary system around GJ 667C with three super-Earths in its habitable zone.” The paper is set to appear in an upcoming edition of the journal Astronomy & Astrophysics.

Image: ESO.