HD 172051 / HR 6998

JPL, CalTech, NASA


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Astronomers have identified HD
172051 as a prime target for the
Terrestrial Planet Finder (TPF),
now planned for launch between
2014 and 2020.

In late 2003, astronomers working on the European Space Agency's planned Darwin mission of three space telescopes and a communications hub announced that they had selected HD 172051 as their top target of nearby stars that closely resemble Sol and so may harbor Earth-type life. They plan to use the method of nulling interferometry -- combining the signal from a number of different telescopes in such a way that the light from the central star is cancelled out -- will be used to reveal dim planets. If the star is found to have an inner rocky planet in a favorable orbit (within its so-called habitable zone) using nulling interferometry, then analysis of light from the planet itself will be used by Darwin to look for the presence of water, oxygen and carbon dioxide in its atmosphere, which are associated with biological life on Earth (more from BBC).

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 this star as HD 172051, 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, the star is also catalogued as Harvard Revised (HR) 6998, 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.

On the other hand, the star has an older designation as BD-21 5081 from a catalogue that was originally published in 1863 by Friedrich Wilhelm August Argelander (1799-1875) on the position and brightness of 324,198 stars between +90° and -2° declination that were measured over 11 years from Bonn, Germany with his assistants Eduard Schönfeld (1828-1891) and Aldalbert Krüger (1832-1896). The catalogue became famous as the Bonner Durchmusterung ("Bonn Survey") and is typically abbreviated as BD. It was later expanded and extended during the early 20th Century with the Cordoba (observed from Argentina) then the Cape Photographic Durchmusterung (observed from South Africa).

The Star

HD 172051 is a yellow-orange main sequence dwarf star of spectral and luminosity type G5 V. The star may have a mass similar to that of Sol, 93 percent of its diameter (Johnson and Wright, 1983, page 691), and 65 percent of its luminosity. Useful catalogue numbers and designations for the star include: HR 6998, Gl 722, Hip 91438, HD 172051, BD-21 5081, and SAO 187086.

Hunt for Planetary Objects

For an Earth-type planet around HD 172051 to have liquid water at its surface, it would need a stable orbit centered around 0.81 AU -- between the orbital distances of Venus and Earth in the Solar System. Such a planet would have an orbital period less than one Earth year. 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 HD 172051. 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 HD 172051.