BD+29 2979 / HD 156668

Luis Calçada, ESO




Large and jumbo illustrations.




HD 156668 b is nearly as small as
CoRoT-7 b, the first confirmed
rocky planet detected outside
the Solar system around a main
sequence star (more)

Today, many astronomers refer to this star as HD 156668, as designated in the Henry Draper (1837-82) Catalogue with subsequent 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. However, it was probably first designated as BD+29 2979 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 high proper motion of this star may have been first detected in a series of proper motion surveys first published in 1957 of 9,867 stars in the Southern Hemisphere and updated in supplements. The star was designated in the Luyten Two-Tenths catalogue as LTT 15121 by Willem Jacob Luyten (1899-1994). It was also detected during the Lowell Proper Motion Survey of the Northern Hemisphere begun in 1957 by Henry Lee Giclas (1910-2007), Robert Burnham, Jr. (1931-93), and Norman G. Thomas at Lowell Observatory and designated as G 181-34 in the Giclas catalogues of faint proper motion stars (Giclas et al, 1971).

The Star

HD 156668 is a main-sequence, orange-red dwarf star of spectral and luminosity type K2-3 V. The star probably has a mass somewhat smaller (around 72 percent) than Sol's (NASA Star and Exoplanet Database, derived from David F. Gray, 1992), and it appears to have only around 75 percent of its diameter (NASA Star and Exoplanet Database, derived from Kenneth R. Lang, 1980). Most revealing, HD 156668 only has around 27 percent of Sol's bolometric luminosity (NASA Star and Exoplanet Database, derived from Kenneth R. Lang, 1980). It appears to be around 85 percent as enriched as Sol in iron relative to the abundance of hydrogen (Mishenina et al, 2008). Useful catalogue numbers and designations for the star include: G 181-34, Hip 84607, HD 156668, BD+29 2979, SAO 84984, LTT 15121, and 2MASS J17174049+2913378.

On January 6, 2010, a team of astronomers announced their discovery of a "super-Earth" class planet in a tight orbit around this star (Keck press release). Planet "b" has at least 4.15 Earth-masses and orbits HD 156668 at an average distance of around 0.05 AUs in a period of just over 4.6 days. The planet has a highly circular orbit (e~ 0). It was found using radial-velocity measurements obtained as part of the the NASA-UC Eta-Earth Survey by the California Planet Search (CPS) group, a systematic search for low-mass planets between three and 30 Earth-masses orbiting the nearest 230 G-, K- . and M-type stars suitable for high-precision Doppler observations at the Keck Observatory. [More details will be forthcoming in Howard et al, 2010].

Howard et al, 2010; Keck




Larger illustrations




Planetary candidate "b"
is a "super-Earth"
class planet with 4.15
Earth-masses in a
tight orbit (more).

Accounting for relatively infrared radiation, the orbit of an Earth-like planet with surface water would be between 0.70 and 0.93 AU around HD 156668 -- between the orbital distances of Venus and Earth in the Solar System (NASA Star and Exoplanet Database). Such an orbit would take around 250 and 390 days (0.69 to 1.06 years) to complete. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)