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

The HARPS search for southern extra-solar planets XXXV. Super-Earths around the M-dwarf neighbors Gl433 and Gl667C
http://arxiv.org/abs/1202.2467

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M dwarfs have been found to often have super-Earth planets with short orbital periods. Such stars are thus preferential targets in searches for rocky or ocean planets in the solar neighbourhood. In a recent paper (Bonfils et al. 2011), we announced the discovery of respectively 1 and 2 low mass planets around the M1.5V stars Gl433 and Gl667C. We found those planets with the HARPS spectrograph on the ESO~3.6-m telescope at La Silla Observatory, from observations obtained during the Guaranteed Time Observing program of that instrument. We have obtained additional HARPS observations of those two stars, for a total of respectively 67 and 179 Radial Velocity measurements for Gl433 and Gl667C, and present here an orbital analysis of those extended data sets and our main conclusion about both planetary systems. One of the three planets, Gl667Cc, has a mass of only M2.sin(i)~4.25 M_earth and orbits in the central habitable zone of its host star. It receives just 10% less stellar energy from Gl667C than the Earth receives from the Sun. However planet evolution in habitable zone can be very different if the host star is a M dwarf or a solar-like star, without necessarily questioning the presence of water. The two other planets, Gl433b and Gl667Cb, both have M2.sin(i) of ~5.5 M_earth and periods of ~7 days. The Radial Velocity measurements of both stars contain longer time scale signals, which we fit as longer period Keplerians. For Gl433 that signal probably originates in a Magnetic Cycle, while a longer time span will be needed to conclude for Gl667C. The metallicity of Gl433 is close to solar, while Gl667C is metal poor with [Fe/H] ~ -0.6. This reinforces the recent conclusion that the occurence of Super-Earth planets does not strongly correlate with stellar metallicity.

Their best fit eccentricity exceeds the upper limit of the previous study... hmmmm. It is questionable just how much we should read into these eccentricity values for such low radial velocity amplitudes anyway.

It's also interesting that EPE has put GJ 433 "c" in the confirmed list - the paper states that they favour the magnetic cycle interpretation for the origin of the radial velocity variations...

Some modelling work suggests the atmosphere if GJ 667Cc might have gotten up and left already, at least if you assume the planet isn't a mini-Neptune.

https://solar-flux.forumotion.com/t1049-magnetic-field-protection-of-terrestrial-planets#7328

I do not believe in this modelling work as we do not know density of the planets as in case gj667Cc for example.From this paper most planet in HZ lost atmospheres because irradiation -little ridicoulous for me even 0.8 AU? .All planets within mass range 1-10 earth are water worlds or minineptunes or at least was born like these. I can not believe most of them lost their atmospheres ? what with kepler 11 planets?

Sirius_Alpha wrote:

the atmosphere if GJ 667Cc might have gotten up and left already ... if you assume the planet isn't a mini-Neptune.

Emphasis mine.

In our own Solar System,we have a example of Earth-size planet than did not lost the atmosphere,and did not have magnetic field to protect them,and receive much more irradiation that GJ667Cc

This Planet is Venus,Venus have atmosphere for billions years without need a magnetic field.(and probably the Venus atmosphere will still hold this atmospheric erosion for more billions and billions years)

So I don't think than this theoretic model can explain GL667Cc (if this planet really haven't a magnetic field,no one knows),Venus or any HZ planets without magnetic Field

Is rather very unlikely 4.5 earth mass planet without magnetic field. I guess also this planet gj667Cc is not completely rocky(without water)

On the other hand the solar wind conditions at Venus orbit are probably very different to those close-in to a red dwarf star. Plus Venus appears to have lost most of its water to atmospheric erosion anyway.

Nevertheless it would be good to get more actual data about the atmospheres (if present) of low-mass planets in a variety of systems.

They considered Venus.

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The case of Venus is particularly interesting order to analyse the rest of the planets. The dynamo of Venus probably shut down at t = 3 Gyr as a consequence of the drying of the mantle. A massive loss of water induced by a runaway greenhouse and unsufficient early magnetic protection played a role in the extinction of the early Venusian planetary magnetic field. The case of Gl 581 d, GJ 667Cc, and HD 85512 b, though similar to Venus is much more complex. On oe hand their masses are larger than Venus' and therefore their gravity could provide additional protection to the atmosphere mass-loss. On the other hand their dynamos shut down on timescales t ~ 3 - 5 Gyr exposing them to direct stellar wind. In G and K stars, this situation could not be consider[ed] a big threat since the stellar wind had also decreased in intensity when the planet lost its planetary magnetic field, this will also be the case for HD 85512 b. However, GJ 581d and GJ 667Cc are located inte HZ of dM stars where the stellar wid pressures even as late as 4 Gyr, are intense enough to massively erode their atmospheres or to make them lose their volatile content.

The key point is that red dwarf solar wind pressure keeps up longer than G or K type stars. Planets whose magnetic fields die before solar wind pressure dies down become vulnerable to atmospheric loss. In the case of Venus, the dynamo must have ceased after the solar wind pressure decreased.

From what I've seen, actual data on red dwarf stellar winds is somewhat lacking, turns out to be quite difficult to do the necessary measurements.

Again more data is needed. I'm not a particular fan of the Rare Earth hypothesis, but we should at least consider the possibility that habitable conditions are significantly rarer in the universe than have sometimes been portrayed.

On the other side most super earths are water worlds so removing excess of water and other shitty stuff(H,HE) is good thing

Evidence for Multiple Planets in the Habitable Zone of Gliese 667C: A Bayesian Re-analysis of the HARPS data
http://arxiv.org/abs/1212.4058

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A re-analysis of Gliese 667C HARPS precision radial velocity data was carried out with a Bayesian multi-planet Kepler periodogram (from 0 to 7 planets) based on a fusion Markov chain Monte Carlo algorithm. The most probable number of Keplerian signals detected is 6 with a Bayesian false alarm probability of 0.012. The residuals of the 6 planet model are shown to be consistent with white noise. The 6 Keplerian signals detected include two previously reported with periods of 7.198 (b) and 28.14 (c) days, plus additional periods of 30.82 (d), 38.82 (e), 53.22, and 91.3 (f) days. The 53 day signal is probably the second harmonic of the stellar rotation period and is likely the result of surface activity. Assuming the other five signals are planetary in origin, the 28.14, 30.82 and 38.82 day orbits are all in the central portion of the habitable zone while the 91.3 day orbit lies partly within the habitable zone. The M sin i values for planets b, c, d, e, and f are ~ 5.4, 4.8, 3.1, 2.4, and 5.4 times the mass of the Earth, respectively. If confirmed by further observations, planet e (38.82 day period) is the lowest mass planet in the habitable zone detected to date.

Wow that's a real pileup in the HZ if confirmed. Would like to see some stability simulations for that system!

Period 30.8 is little dodgy and no sign of candidate with 75 days

Presumably if these planets actually exist the orbits would have to be very close to circular: using the best-fit eccentricities planets c and "d" would have crossing orbits. The lower error bars seem to show that the orbits are compatible with circular...

It could be a nearby confirmation that packed, low-mass planetary systems are common. Even in multiple systems.

Paper is now updated with a different title which doesn't mention habitability:

"Additional Keplerian Signals in the HARPS data for Gliese 667C from a Bayesian Re-analysis"

I don't think the 6-planet version is stable, at least it doesn't seem to be when I run it in an n-body integrator (even after setting all the eccentricities to zero).

(Of course I could have written the integrator incorrectly, but it seemed to give sane results for a 1-planet system...)

Gregory might did mistake but 7,28,38,91 days periods looks good and maybe 53.What with 31 I do not know ? why is no sign planet at 75? In previous paper was just unconfirmed. There could be couple more planets farther out

At least 2 planets in hz and can be even 4 with 5 just slightly outside without taking into account 75 days signal from earlier paper sounds great.What a pity it did not occur in our solar system

tommi59 wrote:

why is no sign planet at 75? In previous paper was just unconfirmed.

Well Gregory's 91-day planet may be the same thing as the Anglada-Escudé et al. 75-day planet: from the abstract of arXiv:1202.0446:

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The 75-day signal is less certain, being significantly affected by aliasing interactions among a potential 91-day signal, and the likely rotation period of the star at 105 days detected in two activity indices.

According to this article on io9, the system is indeed unstable.

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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)."

ESO Press Release: Three Planets in Habitable Zone of Nearby Star

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A team of astronomers has combined new observations of Gliese 667C with existing data from HARPS at ESO’s 3.6-metre telescope in Chile, to reveal a system with at least six planets. A record-breaking three of these planets are super-Earths lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life. This is the first system found with a fully packed habitable zone.

Research paper

So 6 planets (perhaps 7) are confirmed for GJ 667 C. Accounting the planets orbit a star in triple system it quite seems a densely packed planetary system.