HR 8501 / HD 211415 AB

In late September 2003, astrobiologist Maggie Turnbull identified HR 8501 A as one of the best candidates for hosting Earth-type life. The star was chosen from 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 Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories (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 (Margaret C. Turnbull, 2004). On February 19, 2006, Turnbull named HR 8501 A as a Sun-like star that is old enough to qualify as a top-five candidate for those listening for radio signals from intelligent civilizations, such as the SETI Institute.

Medialab, NASA
© ESA 2001



Larger image.



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).

Today, some astronomers prefer to refer to Star A as HD 211415, as it is listed in the Henry Draper (1837-82) Catalogue with extension (HDE), a massive photographic stellar spectrum survey carried out by Annie Jump Cannon (1863-1941) and Edward Charles Pickering (1846-1919) from 1911 to 1915 under the sponsorship of a memorial fund created by Henry's wife, Anna Mary Palmer. As a relatively bright star in Earth's night sky, however, Star A is also catalogued as Harvard Revised (HR) 8501, a numbering system derived from the 1908 Revised Harvard Photometry catalogue of stars visible to many Humans with the naked eye. The HR system has been preserved through its successor, the Yale Bright Star Catalogue -- revised and expanded through the hard work of E. Dorrit Hoffleit and others.

Star A also has an older designation as CD-54 9222 from a catalogue that came from a visual survey of southern stars begun in 1892 at the Astronomical Observatory of Cordoba in Argentina under the direction of its second director John M. Thome (1843-1908). Thome died before the completion of this southern sky atlas in 1914, when 578,802 stars from declination -22° to -90° were published as the Cordoba Durchmusterung ("Survey"). The "CD" is an extension of an older catalogue by Friedrich Wilhelm August Argelander (1799-1875) in 1863 on the position and brightness of 324,198 stars between +90° and -2° declination that were measured over 11 years from Bonn, Germany, made with his assistants Eduard Schönfeld (1828-1891) and Aldalbert Krüger (1832-1896), which became famous as the Bonner Durchmusterung ("Bonn Survey") abbreviated as BD. The BD and CD were greatly expanded and extended into the modern age of photographic surveys with the subsequent creation of the Cape Photographic Durchmusterung from South Africa.

HR 8501 A

HR 8501 A is a yellow-orange main sequence dwarf star of spectral and luminosity type G1-3 V. Possibly a little cooler than Sol, the star probably has a mass similar to Sol's, around 96 percent of its diameter (Johnson and Wright, 1983, page 701), and nearly 1.09 times its luminosity. It may be about 49 to 54 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Margaret C. Turnbull, 2004; and Misha Haywood, 2001). Based on its chromospheric activity, the star may be young than than Sol at around 3.3 billion years of age (Margaret C. Turnbull, 2004). Useful catalogue numbers and designations for the star include: Gl 853 A, Hip 110109, HD 211415, CD-54 9222, SAO 247400, LHS 3790, LTT 8943, LFT 1702, LPM 848, and HDO 298 A.

Stars A and B had a separation exceeding 31 AUs (2.3") in 1900 and 41 AUs (3") in 1980, according to www.alcyone.de (HR 8501 = WDS 22183-5338, Washington Visual Double Star Catalog, 1996.0). According to one reference, they may move around each other at an average distance of about 46 AUs, a semi-major axis of a= 3.4" (Poveda el al, 1994, pp. 80-81; and O.J. Eggen, 1956, pp. 410 and 427).

An Earth-type planet could have liquid water in a stable orbit centered around 1.04 AU from Star A -- just beyond the orbital distance of Earth. Its orbital period would be a little longer than an Earth year, depending on the star's mass. Such a planet would be difficult to detect using present astronomical methods and equipment.

NASA -- larger image

HR 8501 B is a dim red dwarf star, like
Gliese 623 A (M2.5 V) and B (M5.8Ve) at lower right.

HR 8501 B

HR 8501 B is a red main sequence dwarf star of spectral and luminosity type M V. The star may only have less than half Sol's mass, 56 percent of its diameter (Johnson and Wright, 1983, page 701), and a visual luminosity less than 1.8 percent of Sol's. An Earth-type planet could have liquid water in a stable orbit centered around 0.013 AU from Star B -- well within the orbital distance of Mercury in the Solar System. Such a planet would have an orbital period of around 30-some days and would likely be tidally locked with respect to Star B. Such a planet would be difficult to detect using present astronomical methods and equipment.