Gliese 667 C
||It has been suggested that this article be merged with Gliese 667 C to Gliese 667. (Discuss) Proposed since July 2014.|
A star chart of the constellation of Scorpius showing the position of Gliese 667 (red circle at lower left)
Epoch J2000 Equinox J2000
|Right ascension||17h 18m 57.16483s|
|Declination||−34° 59′ 23.1416″|
|Apparent magnitude (V)||10.20|
|U−B color index||1.17|
|B−V color index||1.57|
|Variable type||flare star|
|Radial velocity (Rv)||6.5 km/s|
|Proper motion (μ)||RA: 1129.76 mas/yr
Dec.: −77.02 mas/yr
|Parallax (π)||146.29 ± 9.03 mas|
|Distance||22 ± 1 ly
(6.8 ± 0.4 pc)
|Absolute magnitude (MV)||11.03|
|Temperature||3,700 ± 100 K|
|Metallicity [Fe/H]||–0.59 ± 0.10 dex|
|Rotation||105 days |
Gliese 667 C, also known as GJ 667 C, is a star in the Gliese 667 triple star system. Located 6.8 parsecs (22 light-years) away from the Solar System in the constellation Scorpius, Gliese 667 C is a red dwarf with 1.4% of the luminosity of the Sun and a relatively cool effective temperature of 3,700 K at its surface. It is the smallest and least massive member of the Gliese 667 system. Gliese 667 C has been found to have a system of two confirmed extrasolar planets, all of which are sub-Neptunes or super-Earths detected using the radial velocity method. The outer planet Gliese 667 Cc orbits within the habitable zone. Additional planets have been proposed, including two that would be located in the habitable zone. These in fact may be artifacts caused by a failure to account for the correlated noise in the data
Gliese 667 C is a class M2V star that orbits at a projected separation of 230 AU from Gliese 667 AB, and is believed to be at least two billion years old.
Although M-class stars can remain active for much longer than the Sun, Gliese 667 C is now a relatively inactive star. This allowed investigation of its planetary system by examining small variations in its radial velocity. At the current time, it means that the planets would receive negligible levels of UV and X-ray radiation from chromospheric emission of the star and stellar flares.
The habitable zone of a star has been defined as a "Goldilocks region" of space which is neither too hot nor too cold for a planet with liquid water under an Earth-like atmosphere. The planet must be far enough away from its star to avoid a "moist greenhouse" in which water vapor retains so much heat that any ocean would boil and hydrogen would be lost to space, but close enough to avoid global ice coverage. The most recently published calculations of the habitable zone, when applied to the Gliese 667 C system, predict that the habitable zone extends from an inner edge around 0.095–0.126 AU to an outer edge of 0.241–0.251 AU. A broader definition of the zone might apply if planets with small but non-negligible amounts of water were detected.
As of February 2014, the most recent orbital solution contains two planets: one with a minimum mass around 5.6 Earth masses in a 7.2-day orbit, and a second planet of at least 3.7 Earth masses in a 28-day orbit. These planets are designated Gliese 667 Cb and Gliese 667 Cc respectively. The location of the second planet puts it close to the inner edge of the system's habitable zone, though it is unknown as to whether it actually supports life.
The radial velocity data for the star also contains periodic signals at 105 days and 91 days. The first of these is close to the rotation period of the star, and the stellar activity indicators also show variations at this period. It is likely that this period is due to processes intrinsic to the star rather than an orbiting planet. The activity indicators also show variability on a 91-day period, again suggesting that this period is the result of stellar variations rather than an additional planet.
(in order from star)
|b||≥5.661 ± 0.437 M⊕||0.050 ± 0.002||7.200 ± 0.001||0.122 ± 0.078||—||—|
|c||≥3.709 ± 0.682 M⊕||0.125 ± 0.004||28.143 ± 0.029||0.133 ± 0.098||—||—|
Gliese 667 Cb
Gliese 667 Cb was the first planet to be detected orbiting Gliese 667 C, and is the more massive of the two known planets. It was first announced in an ESO press release in 2009. Modelling the expected tidal forces experienced by this planet suggest that if it is terrestrial in nature, tidal heating would keep the planet completely molten.
Gliese 667 Cc
Gliese 667 Cc was first mentioned in a pre-print made public on 21 November 2011, claiming that a discovery paper from the same group was in preparation (the paper was finally published in a refereed journal in 2013). However, the announcement of a refereed journal report came on 2 February 2012 by researchers at the Carnegie Institution for Science and the University of Göttingen. In that announcement, Gliese 667 Cc was described as one of the best candidates yet found to harbor liquid water, and thus, potentially, support life on its surface. A detailed orbital analysis and refined orbital parameters for Gliese 667 Cc were also presented at the time.
Based on GJ 667 C's bolometric luminosity, GJ 667 Cc would receive 90% of the light Earth does; however much of that electromagnetic radiation would be in the invisible infrared. Based on blackbody temperature calculations, GJ 667 Cc should absorb more overall electromagnetic radiation and thereby have an equilibrium temperature of 277.4 K, as compared with 254.3 K for the Earth. If it is terrestrial in nature, the planet is expected to experience strong tidal heating that may lead to high levels of geological activity on the planet.
History of discovery
Gliese 667 Cb, the first planet of the system to be reported, was first announced by the European Southern Observatory's HARPS group on 19 October 2009, together with 29 other planets. The orbital parameters were subsequently given in a paper that appeared on the arXiv pre-print server in 2011, together with the parameters for the second planet Gliese 667 Cc. The paper was eventually published in 2013. In the meantime, a second study of the system
Preliminary radial-velocity measurements indicated the presence of an additional super-Earth candidate (Gliese 667 Cd), orbiting in an "extended habitable zone" where large quantities of CO2 and other greenhouse gases may make life possible (a planet similar to Gliese 581 d). However, this candidate was less certain at the time due to similarities of the period to rather clear signals detected in several activity indices. Its phase sampling was also sparse, causing severe aliasing and potential confusion. Another likely period for this same candidate was proposed to be 91 days. An additional, long-period signal was found in the data. While the trend is largely consistent with the star's orbit around the A/B primary, a minor curvature in the trend suggests that the object may have a shorter period. A preliminary solution of 7100 days was derived which consistent with a roughly Saturn-mass planet. Not enough data was available to decide conclusively the nature of such long-period trend.
In December 2012, a new solution was obtained using HARPS data only. Such solution contained 6 Doppler signals that were tentatively associated to 5 to 6 planet candidates. The 7.2 and 28.1 days signals correspond to the periods of two previously known planets around the star. Among the other signals, one was suspected to be generated by stellar activity and another one was proposed at a period of 30 days, too close to 28.1 days (period of the securely detected GJ 667Cc) to be in a stable orbit. In an informal interview, Guillem Anglada-Escudé, of the original discovery team, noted that the corresponding system of planets was unstable, and that the possibility of astrophysical false-positives had not been properly done to distinguish between activity induced and genuine Keplerian signals. The claim appeared as a submitted article (not yet accepted by June 2013) and the proposed solution lacked of several basic checks. Therefore, the claim was considered preliminary but suggestive.
On 25 June 2013 a new orbital solution was announced by the initial discovery team, with five strongly-detected planet candidates and two less certain objects. Of the five candidates, three including the previously-detected planet Gliese 667Cc would be located in the habitable zone of the star. The additional habitable zone planet candidates were proposed to have orbital periods of 39 and 62 days. The orbital solution retained the 90-day planet candidate orbiting beyond the outer edge of the habitable zone. With the new planets, Gliese 667C would be the star with most known low-mass planets in its habitable zone. The five first signals also match the five stronger signals reported by Phil Gregory in his preliminary solution proposed in December 2012 and listed in the six-planet solution section. The two additional candidates are less certain and rely on the assumption of dynamical stability of the system.
Further investigation of the data indicated significant levels of correlated noise. Using the erroneous assumption of white noise, the researchers could detect up to five planets. Using a model that accounted for correlated noise only two planets could be detected, together with long period signals attributed to stellar activity. The current model of the system therefore only includes two planets.
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|Wikimedia Commons has media related to Gliese 667 C.|
- "MLO 4". SolStation. Retrieved July 9, 2006.
- "Leave your message to the Gliese667ccians here ...". SolStation. Retrieved April 27, 2012.