The star Gliese 581.
Digitized Sky Survey photo.
Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||15h 19m 26.8250s|
|Declination||−07° 43′ 20.209″|
|Apparent magnitude (V)||10.56 to 10.58a|
|B−V color index||1.61|
|Radial velocity (Rv)||−9.5 ± 0.5 km/s|
|Proper motion (μ)||RA: −1233.51 mas/yr
Dec.: −94.52 mas/yr
|Parallax (π)||160.91 ± 2.62 mas|
|Distance||20.3 ± 0.3 ly
(6.2 ± 0.1 pc)
|Absolute magnitude (MV)||11.6|
|Luminosity (bolometric)||0.013 L☉|
|Surface gravity (log g)||4.92±0.10|
|Temperature||3,480 ± 48 K|
|Metallicity [Fe/H]||−0.33 ± 0.12 dex|
|Age||7 to 11 Gyr|
Gliese 581 (pron.: //, also known as HO Librae) is a red dwarf with spectral type M3V, located about 22 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star to the Sun. Observations suggest that the star has a planetary system consisting of at least three, maybe four, and controversially six planets: Gliese 581 e, b, and c are certain; d is probable; and f and g are disputed.
The star first gained attention after Gliese 581 c, the first low-mass extrasolar planet found near a habitable zone, was discovered in April 2007. It has since been shown that under known terrestrial planet climate models, Gliese 581 c is likely to have a runaway greenhouse effect, and hence is probably too hot to be habitable, analogous to Venus. A subsequently proposed planet Gliese 581 d, may be just inside or just outside the outer boundary of the habitable zone (depending in part on the greenhouse properties of its atmosphere), analogous to Mars.
Excitement spiked again in September 2010 with the claimed discovery of Gliese 581 g, orbiting between c and d, believed to be the planet with the greatest likelihood of having conditions suitable for liquid water at its surface found to date because it is within the middle of the habitable zone. Its existence remains in dispute.
On 27 November 2012, the European Space Agency announced a comet belt, with at least ten times as many comets as the Solar system. This put constraints on possible planets beyond 0.75 AU.
The name Gliese 581 refers to the catalog number from the 1957 survey Gliese Catalogue of Nearby Stars of 965 stars located within 20 parsecs of the Earth. Other names of this star include BD-07° 4003 (BD catalogue, first known publication) and HO Librae (variable star designation). It does not have an individual name such as Sirius or Procyon. The star is a red dwarf with spectral type M3V, located 20.3 light-years away from Earth. It is located about two degrees north of Beta Librae, the brightest star in the constellation Libra. Its mass is estimated to be approximately a third that of the Sun, and it is the 89th closest known star system to the Sun.
An M-class dwarf star such as Gliese 581 has a much lower mass than the Sun, causing the core region of the star to fuse hydrogen at a significantly lower rate. From the apparent magnitude and distance, we can estimate an effective temperature of 3200 K and a visual luminosity of 0.2% of that of the Sun. However, a red dwarf such as Gliese 581 radiates primarily in the near infrared, with peak emission at a wavelength of roughly 830 nm (estimated using Wien's displacement law, which assumes the star radiates as a black body), so such an estimate will underestimate the star's total luminosity. (For comparison, the peak emission of the Sun is roughly 530 nm, in the middle of the visible part of the spectrum). When radiation over the entire spectrum is taken into account (not just the part that humans are able to see), something known as the bolometric correction, this star has a bolometric luminosity 1.3% of the Sun's total luminosity. A planet would need to be situated much closer to this star in order to receive a comparable amount of energy as the Earth. The region of space around a star where a planet would receive roughly the same energy as the Earth is sometimes termed the "Goldilocks Zone", or, more prosaically, the habitable zone. The extent of such a zone is not fixed and is highly specific for each planetary system.
Gliese 581 is classified as a variable star of the BY Draconis type, and has been given the variable star designation HO Librae. This is a star that exhibits variability because of the presence of star spots combined with the rotation of the star. However, the measured variability is close to the margin of error, and, if real, is most likely a long term variability. Its brightness is stable to 1%. Gliese 581 emits X-rays.
Planetary system 
Three to six planets are believed to be orbiting this star. Gliese 581 b, approximately Neptune-sized, was discovered in August 2005 and was the fifth planet to be discovered around a red dwarf. This inner planet is at least 16 times as massive as Earth (similar to Neptune's mass) and completes a full orbit of Gliese 581 in only 5.4 days.
Another planet, Gliese 581 c, was discovered in April 2007. In their 2007 paper, Udry et al. asserted that if Gliese 581 c has an Earth-type composition, it would have a radius of 1.5R⊕, which would have made it at the time "the most Earth-like of all known exoplanets".
A direct measurement of the radius cannot be taken because, viewed from Earth, the planet does not transit its star. With a minimum mass of roughly five times Earth—or one third that of Neptune—Gliese 581 c orbits just inside of the habitable zone of its parent star. The mean blackbody surface temperature has been estimated to lie between −3 °C (for a Venus-like albedo) and 40 °C (for an Earth-like albedo), however, the temperatures could be much higher (about 500 degrees Celsius) due to a runaway greenhouse effect akin to that of Venus. Some astronomers believe the system may have undergone planetary migration and Gliese 581 c may have formed beyond the frost line, with a composition similar to icy bodies like Ganymede. Gliese 581 c completes a full orbit in just under 13 days.
Observations of the star also posited a third planet, Gliese 581 d, with a mass of roughly 7 Earths, or half a Uranus, and an orbit of 66.8 Earth days. Its orbit would be just within the outer limit of the habitable zone of its star, which makes it a candidate for carbon-based life.
Discovery of a then-fourth planet, Gliese 581 e, was announced on 21 April 2009 and confirmed in September 2012. This planet, at a minimum mass of 1.9 Earths, was up to then the least massive confirmed exoplanet identified around a main-sequence star. It takes just under 3.15 days to orbit Gliese 581.
On 27 November 2012, the European Space Agency announced that the Herschel space observatory had discovered a comet belt "at 25 ± 12 AU to more than 60 AU". It must have "at least 10 times" as many comets as does the Solar system. This likely rules out Saturn-mass planets beyond 0.75 AU. However another (undiscovered) planet further out, say a Neptune-mass planet at 5 AU, might be required to keep the comet belt replenished.
4-planet solution (2009) 
Dynamical simulations of the (known) Gliese 581 system assuming that the orbits of planets b to d are coplanar show that the system becomes unstable if its component masses are more than 1.6 – 2 times the minimum mass. The upper limits on the masses of the planets are found to be 3.1, 30.4, 10.4 and 13.8 Earth masses for planets e, b, c and d respectively. This agrees with the system's maximum ecliptic as measured from its dust disc.
Two further years of HARPS observations doubled the amount of data, allowing the eccentricities of the planets to be redefined. This solution makes planet e significantly eccentric, which must impose a smaller upper limit on the inclination of the system; this is not discussed in the paper, however.
(in order from star)
|e||1.95 – <3.1 M⊕||0.028||3.14945 ± 0.00017||0.32 ± 0.09||—||—|
|b||15.86 – <30.4 M⊕||0.041||5.36865 ± 0.00009||0.031 ± 0.014||—||—|
|c||5.34 – <10.4 M⊕||0.073||12.9182 ± 0.0022||0.07 ± 0.06||—||—|
|d (unconfirmed)||6.06 – <13.8 M⊕||0.22||66.64 ± 0.08||0.25 ± 0.09||—||—|
|Comet belt||25 ± 12 AU–>60 AU||—||—|
6-planet solution (2010) 
On 29 September 2010, astronomers at Keck Observatory proposed two additional planets, Gliese 581 f and Gliese 581 g, both in nearly circular orbits based on analysis of a combination of data sets from the HIRES and HARPS instruments. Nicknamed Zarmina by one of its discoverers, the predicted mass of Gliese 581 g is 3 to 4 Earth-masses, with an orbital period of 37 days, and a distance that is well within the star's habitable zone though expected to be tidally locked, with one side of the planet always facing the star.
The approximate ratios of periods of adjacent orbits are (proceeding outward): 3:5, 2:5, 1:3, 1:2, 2:13.
The parameters listed below come from an orbital fit in which eccentricities were fixed at 0. Allowing eccentricities to vary did not significantly improve the fit.
According to a presentation by F. Pepe at IAU Symposium 276, planet g was not detected in a new analysis of data taken by the HARPS spectrograph, and both planets f and g are listed as unconfirmed at the Extrasolar Planets Encyclopaedia. Follow up studies published in September 2011 and in 2012 indicate that Gliese f and g do not exist, though the first such study was disputed by other groups.
(in order from star)
|e||≥1.7 M⊕||0.0284533 ± 0.0000023||3.14867 ± 0.00039||0||—||—|
|b||≥15.6 M⊕||0.0406163 ± 0.0000013||5.36841 ± 0.00026||0||—||—|
|c||≥5.6 M⊕||0.072993 ± 0.000022||12.9191 ± 0.0058||0||—||—|
|g (unconfirmed)||≥3.1 M⊕||0.14601 ± 0.00014||36.562 ± 0.052||0||—||—|
|d (unconfirmed)||≥5.6 M⊕||0.21847 ± 0.00028||66.87 ± 0.13||0||—||—|
|f (unconfirmed)||≥7.0 M⊕||0.758 ± 0.015||433 ± 13||0||—||—|
|Comet belt||25 ± 12 AU–>60 AU||—||—|
Radio signal sent from Earth 
A Message From Earth (AMFE) is a high-powered digital radio signal that was sent on 9 October 2008 toward Gliese 581 c. The signal is a digital time capsule containing 501 messages that were selected through a competition on the social networking site Bebo. The message was sent using the RT-70 radar telescope of Ukraine's National Space Agency. The signal will reach Gliese 581 in early 2029.
See also 
- Note a: Gliese 581 is classified as a BY Draconis variable in the General Catalogue of Variable Stars. This catalog gives a maximum magnitude of 10.56 and minimum of magnitude 10.58 for a relatively low 20 mmag (0.020 magnitudes) variability. For full data see data description and "The combined table of GCVS Vols I-III and NL 67-78 with improved coordinates, General Catalogue of Variable Stars". Sternberg Astronomical Institute. Retrieved 2009-04-27.
- Note b: In 1994 Edward Weis concluded that Gliese 581, like half the 43 dwarf M stars he studied over a multi-year period, showed long term variability (and page 1137, Fig 1 shows Gliese 581 had magnitude 10.58 in 1982 and between 10.57 and 10.56 from 1985 to 1990). Bonfils noted in 2005 that Gliese 581 "has been classified as a variable star (HO Lib), but its variability (Weis 1994) is only marginally significant. If real it would be on a time scale of several years, with short term variability being at most ∼0.006 mag." Measurements by MOST showed short term variability of about 5 mmag (half a percent) over a period of a few weeks.
- GJ 581 , SIMBAD. Retrieved 2008-08-21.
- Bonfils, X.; et al. (2005). "The HARPS search for southern extra-solar planets VI: A Neptune-mass planet around the nearby M dwarf Gl 581". Astronomy and Astrophysics Letters 443 (3): L15–L18. arXiv:astro-ph/0509211. Bibcode:2005A&A...443L..15B. doi:10.1051/0004-6361:200500193.
- van Leeuwen, F. (2007). "HIP 74995". Hipparcos, the New Reduction. Centre de données astronomiques de Strasbourg. Retrieved 2008-08-16.
- From apparent magnitude and parallax.
- "Star: Gliese 581". Extrasolar Planets Encyclopaedia. "Mass 0.31 MSun, Age 8+3
- Bean, J. L; Benedict, G. F.; Endl, M. (2006). "Metallicities of M Dwarf Planet Hosts from Spectral Synthesis". Astrophysical Journal Letters 653 (1): L65–L68. arXiv:astro-ph/0611060. Bibcode:2006ApJ...653L..65B. doi:10.1086/510527.
- Selsis 3.4 page 1382 "lower limit of the age that, considering the associated uncertainties, could be around 7 Gyr", "preliminary estimate", "should not be above 10-11 Gyr"
- "Entry 5594-1093-1". The Hipparcos and Tycho Catalogues. Centre de données astronomiques de Strasbourg. ID I/239.
- "The One Hundred Nearest Star Systems". Research Consortium on Nearby Stars, Georgia State University. 1 January 2009. Retrieved 2010-06-04.
- Roman Baluev (2012). The impact of red noise in radial velocity planet searches: Only three planets orbiting GJ581?. arXiv:1209.3154.
- "First Habitable Exoplanet? Climate Simulation Reveals New Candidate That Could Support Earth-Like Life". ScienceDaily. 16 May 2011. Retrieved 2011-05-16.
- Vogt, S. S.; et al. (2010). "The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581". arXiv:1009.5733 [astro-ph.EP].
- Mayor, M.; et al. (2009). "The HARPS search for southern extra-solar planets XVIII: An Earth-mass planet in the GJ 581 planetary system". Astronomy and Astrophysics 507: 487. arXiv:0906.2780. Bibcode:2009A&A...507..487M. doi:10.1051/0004-6361/200912172.
- "US scientists find potentially habitable planet near Earth". Yahoo News. 29 September 2010. Archived from the original on 5 October 2010. Retrieved 2010-09-30.
- Grossman, Lisa (2011-01-18). "New Study Finds No Sign of ‘First Habitable Exoplanet'". Wired.
- ESA Herschel (27 November 2012). "Do missing Jupiters mean massive comet belts?".
- Udry, S; et al. (2007). "The HARPS search for southern extra-solar planets XI: Super-Earths (5 and 8 M⊕) in a 3-planet system". Astronomy and Astrophysics Letters 469 (3): L43–L47. arXiv:0704.3841. Bibcode:2007A&A...469L..43U. doi:10.1051/0004-6361:20077612.
- Selsis, F.; et al. (2007). "Habitable planets around the star Gl 581?". Astronomy and Astrophysics 476 (3): 1373–1387. arXiv:0710.5294. Bibcode:2007A&A...476.1373S. doi:10.1051/0004-6361:20078091.
- Dragomir, D.; et al. (2012). "A Search for Transits of GJ 581e and Characterization of the Host Star Variability Using MOST Space Telescope Photometry". The Astrophysical Journal 759 (1): 2f. doi:10.1088/0004-637X/759/1/2.
- Schmitt, J. H. M. M; Fleming, T. A; Giampapa, M. S. (1995). "The X-Ray View of the Low-Mass Stars in the Solar Neighborhood". Astrophysical Journal 450 (9): 392–400. Bibcode:1995ApJ...450..392S. doi:10.1086/176149.
- J.-F. Lestrade et al. (2012). "A DEBRIS Disk Around The Planet Hosting M-star GJ581 Spatially Resolved with Herschel". Astronomy and Astrophysics. arXiv:1211.4898.
- von Bloh, W.; et al. (2008). "Habitability of Super-Earths: Gliese 581c & 581d". Proceedings of the International Astronomical Union (International Astronomical Union) 3 (S249): 503–506. arXiv:0712.3219. doi:10.1017/S1743921308017031.
- von Bloh, W.; et al. (2007). "The Habitability of Super-Earths in Gliese 581". Astronomy and Astrophysics (Astronomy & Astrophysics) 476 (3): 1365–1371. arXiv:0705.3758. Bibcode:2007A&A...476.1365V. doi:10.1051/0004-6361:20077939.
- "New 'super-Earth' found in space". BBC News. 25 April 2007. Retrieved 2008-10-20.
- Rincon, P.; Amos, J. (21 April 2009). "Lightest exoplanet is discovered". BBC News. Retrieved 2009-04-21.
- Until the confirmation of HD 10180 b.
- Forveille; Bonfils; Delfosse; Alonso; Udry; Bouchy; Gillon; Lovis et al. (2011). "Only 4 planets in the Gl~581 system". arXiv:1109.2505 [astro-ph.EP].
- "The astrophysicist who discovered Zarmina describes life on "second Earth"". 1 October 2010. Retrieved 2010-10-01.
- "Keck Observatory discovers the first Goldilocks exoplanet" (Press release). Keck Observatory. 29 September 2010. Retrieved 2010-09-29.
- pp. 15, 24, Vogt 2010.
- Ron Cowen (October 13, 2010). "Existence of habitable exoplanet questioned". Science News. Retrieved 2010-10-14.
- "Notes for star Gl 581". The Extrasolar Planets Encyclopaedia. Retrieved 2010-10-11.
- Lopez-Morales, M.; et al. (2006). "Limits to Transits of the Neptune-mass planet orbiting Gl 581". Publications of the Astronomical Society of the Pacific 118 (849): 1506–1509. arXiv:astro-ph/0609255. Bibcode:2006PASP..118.1506L. doi:10.1086/508904. "V* HO Lib ... BY Draconis" (page 2 of pre-print submitted 9 September 2006)
- "General Catalogue of Variable Stars Query results". Sternberg Astronomical Institute. Retrieved 2009-04-27.
- Weis, E. W. (1994). "Long term variability in dwarf M stars". American Astronomical Society 107 (3): 1138. Bibcode:1994AJ....107.1135W. doi:10.1086/116925.
- Bonfils page L15
- Matthews, J. M.; et al. (2007). "MOST Exoplanet System Photometry". p. 80. Retrieved 2009-04-27.
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