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| An artist's impression of GJ 667 Cc, a potentially habitable planet orbiting a red dwarf constituent in a ternary star system. |
Gliese 667 C, also known as GJ 667 C, is a component of the Gliese 667 triple star system. Located 6.8 parsecs (22 light-years) away from theSolar 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 five confirmed extrasolar planets, all of which are super-Earths detected using the radial velocity method. Of these, three planets orbit in the habitable zone of their host star, allowing them to possess liquid water at their surfaces and thereby making them possible locations for the emergence of extraterrestrial life. Two other unconfirmed planets may exist in the system, though neither is considered to be habitable.
Habitable zone:
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.
Early planetary solutions:
Gliese 667 Cb, the first planet of the system to be reported, was first announced by the HARPS group on 19 October 2009, together with 29 other planets. 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. 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 thatelectromagnetic 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.
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.
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b | ≥5.68 ± 0.23 M⊕ | 0.049 | 7.20066 ± 0.00067 | 0.172 ± 0.043 | — | — |
| c | ≥4.54 ± 0.38 M⊕ | 0.123 ± 0.02 | 28.155 ± 0.017 | <0.27 | — | — |
| d (unconfirmed) | ≥5.65 ± 0.54 M⊕ | 0.235 | 74.79 ± 0.13 / 91 ± 0.5 | 0 (fixed) | — | — |
| (trend) | ≥0.25 ± 0.12 MJ | 2.577 | 7100 ± 3000 | 0 (fixed) | — | — |
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.
Source : Wikipedia
By Science and Universe