# Gliese 667

Observation data Characteristics Epoch J2000      Equinox J2000 A star chart of the constellation of Scorpius showing the position of Gliese 667 Constellation Scorpius Right ascension 17h 18m 57.16483s[1] Declination −34° 59′ 23.1416″[1] Apparent magnitude (V) 5.91/7.20/10.20[2] Spectral type K3V + K5V + M1.5V[2][3] U−B color index 0.83/???/1.17 B−V color index 1.03/???/1.57 Variable type A: suspected B: unknown C: flare star Radial velocity (Rv) 6.5[4] km/s Proper motion (μ) RA: 1129.76[1] mas/yr Dec.: −77.02[1] mas/yr Parallax (π) 146.29 ± 9.03[1] mas Distance 22 ± 1 ly (6.8 ± 0.4 pc) Absolute magnitude (MV) 7.07/8.02/11.03 Mass 0.73 / 0.69[5] M☉ Radius 0.76 / 0.70[2] R☉ Metallicity [Fe/H] –0.59[6] dex Mass 0.31[4] M☉ Radius 0.42[2] R☉ Luminosity 0.0137[4] L☉ Temperature 3,700 ± 100[4] K Metallicity [Fe/H] –0.59 ± 0.10[4] dex Rotation 105 days [4] Age 2-10[4] Gyr Orbit[7] Companion Gliese 667 B Period (P) 42.15 yr Semimajor axis (a) 1.81" Eccentricity (e) 0.58 Inclination (i) 128° Longitude of the node (Ω) 313° Periastron epoch (T) 1975.9 Argument of periastron (ω) (secondary) 247° 142 G. Scorpii, CD−34°11626, GJ 667, HD 156384, HIP 84709, HR 6426, LHS 442/442/443, SAO 208670. SIMBAD data Exoplanet Archive data ARICNS data Extrasolar Planets Encyclopaedia data

Gliese 667 (142 G. Scorpii) is a triple-star system in the constellation of Scorpius, none of which have masses greater than the Sun. The star system lies at a distance of about 6.8 pc (22.1 ly) from Earth. There is a 12th magnitude star close to the other three, but it is not gravitationally bound to the system. To the naked eye, the system appears to be a single faint star of magnitude 5.89.

The system has a relatively high proper motion, exceeding 1 second of arc per year.

## Star system

Mobile diagram of the GJ 667 system

The two brightest components of this system, GJ 667 A and GJ 667 B, are orbiting each other at an average angular separation of 1.81 arcseconds with a high eccentricity of 0.58. At the estimated distance of this system, this is equivalent to a physical separation of about 12.6 AU, or nearly 13 times the separation of the Earth from the Sun. Their eccentric orbit brings the pair as close as about 5 AU to each other, or as distant as 20 AU, corresponding to an eccentricity of 0.6.[note 1][8] This orbit takes approximately 42.15 years to complete and the orbital plane is inclined at an angle of 128° to the line of sight from the Earth. The third component, GJ 667 C, orbits the GJ 667 AB pair at an angular separation of about 30", which equates to a minimum separation of 230 AU.[4][9]

### Gliese 667 A

The largest component of this system, Gliese 667 A (GJ 667 A), is a K-type main-sequence star of stellar classification K3 V.[2] It has about 73%[5] of the mass of the Sun and 76%[2] of the Sun's radius, but is radiating only around 12 or 13% of the luminosity of the Sun.[10] The concentration of elements other than hydrogen and helium, what astronomers term the star's metallicity, is much lower than in the Sun with a relative abundance of around 26% solar.[6] The apparent visual magnitude of this star is 6.29, which, at the star's estimated distance, gives an absolute magnitude of around 7.07 (assuming negligible extinction from interstellar matter).

### Gliese 667 B

Like the primary, the secondary component Gliese 667 B (GJ 667 B) is a K-type main-sequence star, although it has a slightly later stellar classification of K5V. This component has a mass of about 69%[5] of the Sun, or 95% of the primary's mass, and it is radiating about 5% of the Sun's visual luminosity. The secondary's apparent magnitude is 7.24, giving it an absolute magnitude of around 8.02.

### Gliese 667 C

Gliese 667 C is the smallest stellar component of this system, with only around 31%[5] of the mass of the Sun and 42%[2] of the Sun's radius. It is a red dwarf with a stellar classification of M1.5. This star is radiating only 1.4% of the Sun's luminosity from its outer atmosphere at a relatively cool effective temperature of 3,700 K.[4] This temperature is what gives it the red-hued glow that is a characteristic of M-type stars.[11] The apparent magnitude of this component is 10.25, giving it an absolute magnitude of about 11.03. It is known to have a planetary system of two planets with a third planet as a strong possibility.

From the surface of Gliese 667 Cc, the second planet out that orbits along the middle of the habitable zone, Gliese 667 C would have an angular diameter of 1.24 degrees and would appear to be 2.3 times[note 2] the visual diameter of our Sun, as it appears from the surface of the Earth. Gliese 667 C would have a visual area 5.4 times greater than that of the Sun but would still only occupy 0.003 percent of Gliese 667 Cc's sky sphere or 0.006 percent of the visible sky when directly overhead.

## Planetary system

Artist's impression of Gliese 667 Cb with the Gliese 667 A/B binary in the background.

Two extrasolar planets, Gliese 667 Cb (GJ 667 Cb) and Gliese 667 Cc (GJ 667 Cc), have been confirmed orbiting Gliese 667 C by radial velocity measurements of GJ 667,[4] with an additional two or three unconfirmed signals. The two confirmed planets have masses of at least 5.68 and 4.54 times the mass of Earth, respectively, (and are thus classified as super-Earths). Planet Cb has an orbital period of approximately one week at a semimajor axis of 0.049 AU, while planet Cc orbits the star every four weeks at a distance of 0.123 AU.

An artist's impression of GJ 667 Cc, a potentially habitable planet orbiting a red dwarf constituent in a trinary star system.

Planet Cb was first announced by the HARPS group on 19 October 2009, together with 29 other planets, while 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.[12] However, the announcement of a refereed journal report came on 2 February 2012 by researchers at the Carnegie Institution for Science/University of Göttingen.[4][13] In this announcement, GJ 667 Cc was described as one of the best candidates yet found to harbor liquid water, and thus, potentially, support life on its surface.[14] A detailed orbital analysis and refined orbital parameters for Gliese 667 Cc were presented.[4] Based on GJ 667 C's bolometric luminosity, GJ 667 Cc would receive 90% of the light Earth does,[10] however much of that electromagnetic radiation would be in the invisible infrared light part of the spectrum. Based on black body temperature calculation, GJ 667 Cc should absorb more overall electromagnetic radiation, making it warmer (277.4 K) and placing it slightly closer to the "hot" edge of the habitable zone than Earth (254.3 K).[citation needed]

Preliminary radial-velocity measurements indicate 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).[14] However, this candidate is less certain due to the similarity of the period to very strong periodicities detected in several activity indices, meaning that the radial velocity signal could be caused by stellar parameters. Its phase sampling is also sparse, causing severe aliasing and potential confusion. Another likely period for this same candidate would 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 achieved, consistent with a roughly Saturn-mass planet, but a longer time baseline will be needed to differentiate between the two solutions.[4]

The Gliese 667 C : four-planet solution system[4]
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
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)

The HARPS data has another solution for Gliese 667 C. This is consistent with a system of up to 6 planets with orbital periods of 7.2, 28.1, 30.8, 38.8, 53.2 and 91.3 days. The 7.2 and 28.1 days signals correspond to the orbital periods of two previously known planets around the star. The 53.2-day periodicity corresponds to the 2nd harmonic of the star's rotation, so is assumed to be not from a planet.[9]

Given that, the five-planet solution from HARPS would be:

The Gliese 667 C : five-planet solution system[9]
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
b ≥5.4+0.3
−0.3
M
0.049+0.001
−0.001
7.1980+0.0009
−0.0008
0.068+0.037
−0.039
c ≥4.8+0.5
−0.5
M
0.123+0.003
−0.003
28.138+0.0023
−0.0023
0.083+0.035
−0.081
d (unconfirmed) ≥3.1+0.4
−0.5
M
0.130+0.003
−0.003
30.82+0.04
−0.04
0.16+0.08
−0.15
e (unconfirmed) ≥2.4+0.4
−0.4
M
0.152+0.003
−0.003
38.82+0.09
−0.09
0.35+0.21
−0.21
f (unconfirmed) ≥5.4+0.5
−0.6
M
0.268+0.006
−0.006
91.26+0.30
−0.28
0.36+0.10
−0.10

Guillem Anglada-Escudé has noted that this proposed orbital configuration is unstable, and that the possibility of astrophysical false-positives has not been eliminated.[15]

## References

1. van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357
2. Pasinetti Fracassini, L. E. et al. (February 2001), "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics", Astronomy and Astrophysics 367: 521–524, arXiv:astro-ph/0012289, Bibcode:2001A&A...367..521P, doi:10.1051/0004-6361:20000451 Note: see VizieR catalogue J/A+A/367/521.
3. ^ "Toward spectral classification of L and T dwarfs: infrared and optical spectroscopy and analysis", The Astrophysical Journal (The American Astronomical Society), January 2002, retrieved 2012-02-14
4. Anglada-Escude, Guillem et al. (February 2012), "A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone", The Astrophysical Journal Letters, accepted, arXiv:/1202.0446, Bibcode:2012arXiv1202.0446A
5. ^ a b c d Tokovinin, A. (September 2008), "Comparative statistics and origin of triple and quadruple stars", Monthly Notices of the Royal Astronomical Society 389 (2): 925–938, arXiv:0806.3263, Bibcode:2008MNRAS.389..925T, doi:10.1111/j.1365-2966.2008.13613.x
6. ^ a b Cayrel de Strobel, G.; Soubiran, C.; Ralite, N. (July 2001), "Catalogue of [Fe/H] determinations for FGK stars: 2001 edition", Astronomy and Astrophysics 373: 159–163, arXiv:astro-ph/0106438, Bibcode:2001A&A...373..159C, doi:10.1051/0004-6361:20010525
7. ^ Söderhjelm, Staffan (January 1999), "Visual binary orbits and masses POST HIPPARCOS", Astronomy and Astrophysics 341: 121–140, Bibcode:1999A&A...341..121S
8. ^ Bowman, Richard L. "Interactive Planetary Orbits - Kepler's Laws Calculations". Retrieved 23 February 2012.
9. ^ a b c Philip C. Gregory. Additional Keplerian Signals in the HARPS data for Gliese 667C from a Bayesian Re-analysis. arXiv:1212.4058.
10. ^ a b Sven Wedemeyer-Böhm. "Life on Gliese 667Cc?". Institute of Theoretical Astrophysics.
11. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
12. ^ Bonfils, X. et al. (November 2011), "The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample", Astronomy and Astrophysics, submitted, arXiv:/1111.5019, Bibcode:2011arXiv1111.5019B
13. ^ University of Göttingen. Presseinformation: Wissenschaftler entdecken möglicherweise bewohnbare Super-Erde - Göttinger Astrophysiker untersucht Planeten in 22 Lichtjahren Entfernung. Nr. 17/2012 - 02.02.2012. Announcement on university homepage, retrieved 2012-02-02
14. ^ a b Chow, Denise (February 2, 2012). "Newfound Alien Planet is Best Candidate Yet to Support Life, Scientists Say". Space.com. Retrieved February 3, 2012.
15. ^ George Dvorsky (18 December 2012). "Astronomer discovers three potentially habitable planets orbiting around one red dwarf". io9. Retrieved 21 March 2013.

## Notes

1. ^ Based on a calculated eccentricity value of $e={{r_a-r_p}\over{r_a+r_p}}$.
2. ^ $\frac {h} {{h}_{\odot}}={\left( \frac{{{T}_{\odot}}_{\rm eff}} {{T}_{\rm eff}} \right)^2} *\frac \sqrt{L} {a}$.[citation needed] where ${h}$ is the apparent visual diameter[clarification needed] of the star from the surface of the planet in orbit (GJ667Cc in this case), ${{h}_{\odot}}$ is the apparent visual diameter of the Sun (sol) from the surface of Earth, ${{T}_{\odot}}_{\rm eff}$ is the effective temperature of the Sun (sol), ${{T}_{\rm eff}}$ the effective temperature of the star, ${L}$ is the luminosity of the star as a fraction of the sun's luminosity and $a$ is the distance of the planet from the star in AU.