GJ 667C - Habitable candidates in a multi-planet system

Paper now available:

http://www.eso.org/public/archives/releases/sciencepapers/eso1328/eso1328a.pdf

Definitely a system to be further analyzed for habitability

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Planet c is slightly closer to the inner edge of the HZ than the Earth, and so we expect it to be warmer than the Earth, too. It receives 1230 W m−2 of stellar radiation, which is actually less than the Earth’s solar constant of 1360 W m−2. Assuming synchronous rotation and no obliquity, then the global climate depends strongly on the properties of the atmosphere. If the atmosphere is thin, then the heat absorbed at the sub-stellar point cannot be easily transported to the dark side or the poles. The surface temperature would be a strong function of the zenith angle of the host star GJ 667C. For thicker atmospheres, heat redistribution becomes more significant. With a rotation period of ∼ 28 days, the planet is likely to have Hadley cells that extend to the poles (at least if Titan, with a similar rotation period, is a guide), and hence jet streams and deserts would be unlikely. The location of land masses is also important. Should land be concentrated near the sub-stellar point, then silicate weathering is more effective, and cools the planet by drawing down CO2(Edson et al. 2012).

Planet f is a prime candidate for habitability and receives 788 W m−2 of radiation. It likely absorbs less energy than the Earth, and hence habitability requires more greenhouse gases, like CO2 or CH4. Therefore a habitable version of this planet has to have a thicker atmosphere than the Earth, and we can assume a relatively uniform surface temperature. Another possibility is an “eyeball” world in which the planet is synchronously rotating and ice-covered except for open ocean at the sub-stellar point (Pierrehumbert 2011). On the other hand, the lower albedo of ice in the IR may make near-global ice coverage difficult (Joshi & Haberle 2011; Shields 2013).

Planet e receives only a third the radiation the Earth does, and lies close to the maximum greenhouse limit. We therefore expect a habitable version of this planet to have > 2 bars of CO2. The planet might not be tidally locked, and may have an obliquity that evolves significantly due to perturbations from other planets. From this perspective planet e might be the most Earth-like, experiencing a day-night cycle and seasons.

Finally planet d is unlikely to be in the habitable zone, but it could potentially support sub-surface life. Internal energy generated by, e.g. , radiogenic heat could support liquid water below an ice layer, similar to Europa. Presumably the biologically generated gases could find their way through the ice and become detectable bio-signatures, but they might be a very small constituent of a large atmosphere, hampering remote detection. While its transit probability is rather low (∼0.5%), its apparent angular separation from the star is ∼ 40 milliarcseconds. This value is the baseline inner working angle for the Darwin/ESA high-contrast mission being considered by ESA (Cockell et al. 2009) so planet d could be a primary target for such a mission.

And this time they confirm the parameters correspond to stable orbits, interesting to see how it differs from the Gregory (2012) solution. Very nice indeed, what a fascinating system!

(Let's hope it isn't plagued by planet-counting controversies à la Gliese 581)

Wonderful !!!

Yaay it's Gliese 667 C mr. HZ again what else do you have to saay

…

…Oh my.



My oh my. 17 days, ~0.7 m/s, ~1.3 M_⊕… I hope this is a spur to better establish the very low-mass planet population around the early M dwarfs in the solar neighbourhood. Evidently, there is a lot being missed.

Of course, I am not one to miss the pileup in the HZ. c was impressive last year, but things are really looking packed now! The η⊕ population around early M dwarfs is getting coaxed out well, and apparently, weird.

There was a paper on arxiv a bit ago suggesting that, at least in some cases, the spacing of planets in compact low-mass systems are approximately constant. Using the semimajor axis values from table 4, for the inner six planets this seems to bear true:



But the outermost planet doesn't fit. Going a little more complex, though, finds reasonable fits for an exponent with g being the eighth planet:



It's very… Titius-Bode, But it works remarkably well, and has also been suggested in a few other cases (see the HD 10180 system discovery paper). What does it mean? Don't ask me. Apparently it's something to do with migration. That sounds about right.

Titius-Bode law is not "that" stupid
https://solar-flux.forumotion.com/t1300-exoplanet-predictions-based-on-the-generalised-titius-bode-relation

http://www.cieletespace.fr/node/10576

In English :

For Xavier Bonfils (Laboratoire d'Astrophysique de Grenoble) However, the conditional remains robust. He knows very well the observation data of Gliese 667C, for have collected them himself with his team through the harps (La Silla Observatory), and there have discovered the first planet in the habitable zone. "The method of analysis chosen by Guillem Anglada-Escudé and colleagues is more sensitive to false positives than the usual technique" prevents the astronomer. Technically, she returned to decide a priori that the orbits of the planets in the system are almost circular. And if this was not the case? The planets disappear is data... To get to the bottom, should be new observations. Unfortunately, it will take years to harvest. "Gliese 667C represents years and years of measurement, said Xavier Bonfils.". I am afraid that ambiguity hangs over these new planets for a long time...

The Planetary Habitability Laboratory has a page about this system, with some sunset graphics.

http://phl.upr.edu/press-releases/anearbystarwiththreepotentiallyhabitableworlds

Personally I prefer the picture of the rather desolate landscape with the three stars on the horizon from the press release.

Strange on EPE period of planet d is different than in submitted paper 106 instead 91

Yes, EPE is using the earlier orbital solution for planets b, c and d.

The slovenliness of new EPE is managed just leaves me upset. I think it's not Jean Schneider to update the new website.

http://arxiv.org/abs/1307.6984

Only 3 planets in GJ667C syst. ???  

Bayesian analysis of radial velocity data of GJ667C with correlated noise: evidence for no more than 3 planets

GJ667C is the least massive component of a triple star system which lies at a distance of about 6.8 pc (22.1 light-years) from Earth. GJ667C has received much attention recently due to the claims that it hosts up to seven planets including three super-Earths inside the habitable zone. We present a Bayesian technique for the analysis of radial velocity (RV) data-sets in the presence of correlated noise component ("red noise"), with unknown parameters. We also introduce hyper-parameters in our model in order to deal statistically with under or over-estimated error bars on measured RVs as well as inconsistencies between different data-sets. By applying this method to the RV data-set of GJ667C and show that this data-set contains a significant correlated (red) noise component with correlation timescale for HARPS data of order 9 days. Our analysis shows that the data only provides strong evidence for the presence of two planets: GJ667Cb and c with periods 7.19d and 28.13d respectively, with some hints towards the presence of a third planet with period 91d, whose confirmation needs additional RV observations. Previous claims of the detection of additional planets in this system are due the erroneous assumption of white noise. Using the standard white noise assumption, our method leads to the detection of up to five signals in this system. We also find that the measurement uncertainties from HARPS are under-estimated at the level of ~ 50%.

Oops I think we have the same story like in gliese 581.

Led_Zep wrote:

Only 3 planets in GJ667C syst. ???  

Maybe even only two: "some hints towards the presence of a third planet" is not a particularly strong detection!

Dynamical evolution and spin-orbit resonances of potentially habitable exoplanets. The case of GJ 667C
http://arxiv.org/abs/1311.4831

Considering the tidal history of the two-planet model, the b and c planets likely have experienced massive tidal heating during their captures into spin-orbit resonances.

Question is whether the planets are actually in such eccentric orbits or if this is a result of systematic errors.

Disentangling Planets and Stellar Activity for Gliese 667C

http://arxiv.org/abs/1409.0021

Gliese 667C is an M1.5V star with a multi-planet system, including planet candidates in the habitable zone (HZ). The exact number of planets in the system is unclear, because the existing radial velocity (RV) measurements are known to contain contributions from stellar magnetic activity. Following our analysis of Gliese 581 (Robertson et al. 2014), we have analyzed the effect of stellar activity on the HARPS/HARPS-TERRA RVs of GJ 667C, finding significant RV-activity correlation when using the width (FWHM) of the HARPS cross-correlation function to trace magnetic activity. When we correct for this correlation, we confirm the detections of the previously-observed planets b and c in the system, while simultaneously ascribing the RV signal near 90 days ("planet d") to an artifact of the stellar rotation. We are unable to confirm the existence of the additional RV periodicities described in Anglada-Escude et al. (2013) in our activity-corrected data

page 4 :
The results of this study imply an urgent mandate for
the acquisition of additional data for GJ 667C. In the
case of GJ 581, time-series photometry revealed very
low variability, indicating the magnetic activity creating
the RV signals of false-positive planets d and g may
be caused by localized active regions without optically
dark starspots. We are unaware of any such photometry
for GJ 667C, and therefore cannot conclude whether
the activity-induced RV shifts are caused by traditional
starspots or spotless active regions. Finally, more RV observations
are required to determine whether additional
low-mass planets truly exist in this system

Stellar activity correction really is becoming the theme in RV exoplanet searches, no?

Hopefully this kind of thing will eventually benefit the search for the really tiny variations associated with terrestrial-mass planets in the HZ of more luminous stars.