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18 Scorpii is located about 45.7 light-years (ly) from Sol. It lies at the northern edge of (16:15:37.3-8:22:10.0, ICRS 2000.0) of Constellation Scorpius, the Scorpion -- northwest of Zeta Ophiuchi, south of Yed Posterior (Epsilon Ophiuchi) and Yed Prior (Delta Ophiuchi), north of Graffias or Acrab (Beta1,2 Scorpii), and east of Zubeneschamali (Beta Librae). In late September 2003, astrobiologist Maggie Turnbull from the University of Arizona in Tucson identified 37 Geminorum as one of the best candidates for hosting Earth-type life from a shortlist of 30 stars (screened from the 5,000 or so stars that are estimated to be located 100 ly of Earth) that were presented to a group of scientists from NASA's space-telescope project, the Terrestrial Planet Finder (TPF), which will search for habitable planets by using visible light with the "signature" of water and/or oxygen from an Earth-type planet after its scheduled launch around 2013, and the ESA's Darwin project involving six space telescopes (Astrobiology Magazine). The stars examined were selected from a larger list of 17,129 (of which 75 percent are located within around 450 ly, or 140 parsecs, of Sol) that were assembled into a Catalog of Nearby Habitable Stellar Systems (HabCat) by Turnbull and Jill Tarter of the SETI Institute (see: Margaret C. Turnbull, 2002, in pdf). Selection criteria for the 30-star shortlist included: X-ray luminosity, rotation, spectral types or color, kinematics, metallicity, and Strömgren photometry.
© ESA 2001
To find life around nearby stars,
the ESA's Darwin mission will look
for traces of water, oxygen, and
carbon dioxide in the atmospheres
of Earth-type planets found in
stellar habitable zones (more).
18 Scorpii is a yellow-orange main sequence dwarf star of spectral and luminosity type G1-5 V-Va. A little bigger and brighter than Sol, the star may have a mass similar to Sol's -- inferred mass of 1.0 +/- 0.03 Solar-mass (Guinan et al, 1999; and Porto de Mello and da Silva, 1997), 1.03 times its diameter (Johnson and Wright, 1983, page 685), and 1.05 times its luminosity. It may be 105 to to 112 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Porto de Mello and da Silva, 1997; and Cayrel de Strobel et al, 1991, 300). Similar to Sol in age, 18 Scorpii may be around 4.7 +/- 0.8 billion years old (Guinan et al, 1999). Photometric analysis over time suggests that seven to 15 percent of the star was covered with starspots during the period of observation; by comparison, Sol has sunspots covering less than 0.2 percent of its surface at starspot maximum in historical times (Guinan et al, 1999)
In recent years, some astronomers have regarded 18 Scorpii as the nearest "Solar twin" (Porto de Mello and da Silva, 1997). Observations through 2000, however, indicate that 18 Scorpii has a well-defined activity cycle which reached an apparent minimum in 1998 then showed a rapid rise through 2000. A comparison with contemporaneous Solar data using the same instrument suggests that 18 Scorpii's activity cycle may be of greater amplitude than Sol's and that its overall chromospheric activity level is noticeably greater than Sol's. Hence, this otherwise, "excellent solar photometric twin therefore may be a less perfect spectroscopic twin" (Hall and Lockwood, 2000). Moreover, 18 Scorpii has been given the variable star designations: CSV 101566, NSV 7577, and SV ZI 1223. Other useful catalogue numbers and designations for the star include: 18 Sco, HR 6060, Gl 616, Hip 79672, HD 146233, BD-07 4242, SAO 141066, LHS 3172, LTT 6482, LFT 1259, and LPM 594.
An Earth-type planet could have liquid water in a stable orbit centered around 1.02 AU from around 18 Scorpii -- around the orbital distance of Earth in the Solar System. Such a planet would have an orbital period of around one Earth year. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.
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 18 Scorpii.
|Star System||Spectra &|
|BD-07 4156||M0 V||3.1|
|HC+03 1919||M7 V||6.1|
|BK-05 9201||M1 V||7.1|
|O'Neil 723||M0 V||7.5|
|L 841-9||K-M2 V||8.0|
|HC+25 1902||K2 V||8.8|
|LP 684-17||M4.5-5 V||9.0|
|LP 805-10||M V||9.4|
|BK-02 75||M2 V||9.5|
|* plus bright stars *||. . .|
|HC-24 1124||F2 V||11|
|Psi Serpentis 3?||G2.5-5 V |
|DK-26 2485||G6 V||14|
|BK+00 2334||G2 V||14|
|BD+00 3593||G8 V||14|
|L. Serpentis 2||G0 V |
|L 989-20 AB||G-M3.5 V |
|HR 6516 AaB||G8-9 V-IV |
|BK+16 9747||G1 V||19|
|Gamma Serpentis 2?||F6 V |
|SS-36 2376||G0 V||20|
Up-to-date technical summaries on these stars can be found at: 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.
In Ancient Greek legend, Orion (the Hunter) boasted that his might and skill were so great that he could kill all the animals on the face of the Earth. Gaea, Goddess of Earth, was alarmed and sent a giant scorpion to kill him. After a brief battle, the scorpion managed to sting Orion on the heel (at the star Rigel), but the Gods decided to give both Orion and the scorpion honored places at opposite ends of the noight sky so that they would never engage in battle again. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Scorpius. For another illustration, see David Haworth's Scorpius.
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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