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Mu Arae is located about 49.8 light-years from Sol. It lies in the northeastern corner (17:44:8.7-51:50:2.6, ICRS 2000.0) of Constellation Ara, the Altar -- southeast of Alpha Arae, south of Lambda Arae, southwest of Theta Arae, and northeast of Beta Arae. On December 11, 2000, astronomers announced the discovery of a Jupiter-like planet around this Sun-like star (AAO press release -- details below). By June 2002, a second giant planet in an outer orbit was also suspected (Jones et al, 2002, in pdf, which was ground in better observational data in 2004 (McCarthy et al, 2004). On August 25, 2004, astronomers announced the discovery of a third planet with around the mass of Uranus in an inner orbit (ESO press release). In August 2006, two teams of astronomers confirmed that they had discovered a fourth planet with at least half of Jupiter's mass (more below -- Gozdiewski et al, 2006; and Pepe et al, 2006). (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.) As Mu Arae has become one of the top 100 target stars for NASA's planned Terrestrial Planet Finder (TPF), images of this star and its position relative to the Milky Way in Earth's night sky are now available from the TPF-C team.
Mu Arae is a yellow-orange dwarf star of spectral and luminosity type G3 V-IV. The star has about 108 +/- 5 percent of Sol's mass (exoplanets.org; Giradi et al, 2002); it may have a similar diameter (102 percent) as Sol (Johnson and Wright, 1983, page 689) with around 1.7 times its luminosity. Its relative brightness compared to Sol and relative chromospheric inactivity suggest that the star is more highly evolved and probably older than Sol, at around 6.41 billion years (Saffe et al, 2005), as the accumulation of helium ash in its core is moving the star out of the main sequence into subgiant status. The star may be 1.8 to 1.9 times as enriched than Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (McCarthy et al, 2004; and exoplanets.org). Useful catalogue numbers and designations for the star include: Mu Ara, HR 6585, Gl 691, Hip 86796, HD 160691, CD-51 11094, CP(D)-51 10535, SAO 244981, FK5 662, and LTT 7053.
As of August 29, 2006, astronomers have announced the discovery of four planetary candidates around Mu Arae.
In August 2006, two teams of astronomers confirmed that they had discovered a fourth planet candidate "e" with at least half (52.19 percent) of Jupiter's mass (Gozdiewski et al, 2006; and Pepe et al, 2006). This planet has an average orbital distance of about 0.921 AUs, close to Earth's orbital distance from the Sun in the Solar System. It has a relatively circular orbit (e= 0.0666 ± 0.0122) which takes about 310.55 ± 0.83 days (85 percent of an Earth year) to complete.
On August 25, 2004, a team of astronomers (including Nuno C. Santos, François Bouchy, Jean-Pierre Sivan, Michel Mayor, Francisco Pepe, Didier Queloz, Stephane Udry, Christophe Lovis, Sylvie Vauclair, Michael Bazot, Gaspare Lo Curto, Dominique Naef, Xavier Delfosse, Willy Benz, Christoph Mordasini, and Jean-Louis Bertaux) announced the discovery of a third planetary candidate "d" (also called "c" by those unaware of now confirmed "c" candidate) in a very hot but circular, inner orbit (e~0) (ESO press release; Observatoire de Genève page on HD 160691; and Nuno et al, forthcoming, in pdf or ps). The planet is estimated to have at least 10.5 (revised from 14.0) times the mass of Earth (Pepe et al, 2006). Located at only 0.09 AUs from the star (less than one fourth of Mercury's orbital distance in the Solar System), the planet competes an orbit in 9.64 (revised from 9.55 +/- 0.03) days and may have a surface temperature of 1,160° Fahrenheit (or 900° Kevin). The astronomers believe that under the most likely planetary developmental scenario of inner migration from around 3 AUs under the influence of outer giant planet "b" now at 1.5 AUs, this planet is likely to have an "essentially rocky core" with an atmosphere of five to 10 percent of its total mass (Nuno et al, forthcoming, in pdf or ps).
On December 11, 2000, a team of astronomers (including Chris G. Tinney, Hugh R. A. Jones, Alan J. Penny, Kevin Apps, R. Paul Butler, Geoffrey W. Marcy, Steven S. Vogt, and Gregory W. Henry) announced the discovery of a Jupiter-class planet around Mu Arae using highly sensitive radial-velocity methods (Butler et al, 2001; and (AAO press release). According to the latest radial velocity data, planet "b" has at least 1.67 +/- 0.11 times Jupiter's mass. It moves around Mu Arae at an average distance of only 1.50 +/- 0.02 AUs (a semi-major axis around Mars's orbital distance) in an elliptical orbit (e= 0.31, revised from 0.20 +/- 0.03) that takes around 1.79 years (654.5, revised from 645.5 +/- 3 days) to complete (McCarthy et al, 2004; and exoplanets.org).
On June 13, 2002, astronomers announced the possible detection of a second giant planet of around 3.10 +/- 0.71 Jupiter-masses based on a residual trend in radial velocity measurements over more than 13 years, which was supported with better data by September 2004 (McCarthy et al, 2004). Planetary candidate "c" has a highly eccentric outer orbit (e~ 0.57 +/- 0.01) with a semi-major axis of 4.17 +/- 0.07AUs, which may take roughly 8.18 years (2986 +/- 30 days) to complete. It has a maximum separation of 0.43 arc-seconds (about 6.6 AUs) from Mu Arae, but this was appeared to have been insufficient for astronomers using the the Hubble space and Earth-based telescopes to take a direct image.
The orbit of an Earth-like planet (with liquid water) around Mu Arae may be centered around 1.3 AU -- between the orbital distances of Earth and Mars in the Solar System -- with an orbital period around 1.31 years (478 days). However, the eccentric orbit of planet b at an average orbital distance of around 1.5 AU would disrupt the orbit of an Earth-type planet around Mu Arae's water zone. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)
Astronomers are hoping to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around Mu Arae. As currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph to launch around 2014; and a "formation-flying" infrared interferometer to launch before 2020, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub beginning in 2015.
The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Mu Arae.
|Star System||Spectra &|
|CD-51 10924 AB||M0 V |
|CP-58 7399||M1 V||7.6|
|CP-58 7076||K0 V||8.0|
|CP-60 6718 AB||K0 V |
|CP-62 5888||K7 V||9.9|
|* plus bright stars *||. . .|
|Iota Pavonis AB||G0-3 V-IV |
|L 489-58 AB||sdG0 /VI |
|HR 6748||G3-5 V||16|
|HD 147513 / HR 6094 AB||G3-5 V |
|Beta Trianguli Australis A?||F2 IV-III||17|
|Zeta Trianguli Australis AB||F9-G0 V |
|Nu2 Lupi||G2-5 V||19|
|HR 7232||G5-8 V-IV||20|
The late John Whatmough created illustrated web pages on this system in Extrasolar Visions.
Up-to-date technical summaries on these stars can be found at: Jean Schneiders's Extrasolar Planets Encyclopaedia; the Astronomiches Rechen-Institut at Heidelberg's ARICNS, and the Nearby Stars Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.
Constellation Ara, the Altar, is located next to the celestial south pole. Ara is another of those constellations created by the Abbé [Abbot] Nicholas Louis de La Caille (1713-1762), who had the great honor of naming 15 of the 88 constellations by becoming the first astronomer to systematically observe the entire night sky by traveling to the Cape of Good Hope at the southern tip of Africa from 1750-54. For more information on stars and other objects in this Constellation and an illustration, go to Christine Kronberg's Ara. For another illustration, see David Haworth's Ara.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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