Chara 2?

In late September 2003, astrobiologist Maggie Turnbull identified Chara as one of the best candidates for hosting Earth-type life in a shortlist of 30 stars (screened from the 5,000 or so stars that are estimated to be located within 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. On February 19, 2006, Turnbull named Chara as a nearby Sun-like star that is old enough to qualify as the top candidate for those listening for radio signals from intelligent civilizations (e.g., the SETI Institute).

Medialab, NASA
© ESA 2001



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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).

Chara

This star is a yellow-orange main sequence dwarf star of spectral and luminosity type G0 V, with less than 108 percent of Sol's mass, under 111 percent of its diameter, and about 1.2 times its luminosity. It may be 59 to 120 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 21). Chara's age can be revised to 4.05 billion years based a new metallicity measurement indicating 64 percent of Sol's iron abundance and nearly the same ratio (98 percent) of calcium to iron abundance (Ibukiyama and Arimoto, 2002), from a previous estimate of 9.8 billion years (Ng and Betelli, 1998). The star was once believed to have an unseen companion detected through spectroscopic analysis (more below).

The orbit of an Earth-like planet (with liquid water) around the star would have to be centered around 1.1 AU -- just beyond the orbital distance of Earth in the Solar System -- with an orbital period around 1.1 Earth years. Chara is a New Suspected Variable star designated NSV 5725. Useful star catalogue numbers for the star include: Bet CVn, 8 CVn, HR 4785, Gl 475, Hip 61317, HD 109358, BD+42 2321, SAO 44230, FK5 470, LHS 2579, LTT 13552, and LFT 924.

Chara B?

According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 4785, a possible companion object was detected as a single-line spectroscopic binary with an estimated orbital period of 2,430 days. However, that orbital estimate was questioned by Morbey and Griffin in 1987. Except for one instance in 1978 at a separation of 0.110", the object was unresolved by speckle interferometry (Hartkopf and McAlister, 1984) and has been noted as "spurious" by the Astronomiches Rechen-Institute at Heidelberg's ARICNS since at least 1998. On the other hand, the Nearby Stars Database (as of August 14, 2007) still notes a wide companion with a period of 2,429.9 days from the General Catalogue of Trigonometric Parallaxes, Fourth Edition, 1995 ("Yale Parallax Catalog" or YPC), with an apparent reference to the Center for High Angular Resolution Astronomy (CHARA).