BD+18 683 / Gl 176

JPL, NASA



Larger image.



Planetary candidate "b" was
initially thought to be
around the size of Neptune.

This star was probably first designated as BD+18 683 and added to 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). This 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's high proper motion was probably noticed first and designated as Ross 33 by Frank Elmore Ross (1874-1960), who reported on this star in February 1928 with his "Sixth List of New Proper-Motion Stars," Astronomical Journal (38:900). Today, many astronomers refer to this star by its earliest designation as Gl or Gliese 176 in the first Gliese Catalogue of Nearby Stars (CNS, now ARICNS database) of Wilhelm Gliese (1915-93), who was a longtime astronomer at the Astronomiches Rechen-Institut at Heidelberg (even when it was at Berlin).

The Star

Gliese 176 is a cool and dim, main sequence red dwarf (M2.5 Ve). The star has almost a half (49 +/- 0.014 percent) of Sol's mass (Endl et al, 2008), 52 to 53 percent of its diameter (NASA Star and Exoplanet Database, derived from Kenneth R. Lang, 1980; Pasinetti-Fracassini et al, 2001; and Johnson and Wright, 1983, page 655), and less than 0.8 percent of its visual and 3.5 ± 0.3 of its bolometric luminosity (NASA Star and Exoplanet Database, derived from Kenneth R. Lang, 1980). The star appears to be have around 79 percent as enriched as Sol in elements heavier than hydrogen (Endl et al, 2008). It has moderate magnetic activity and rotates on a 39-day period (Endl et al, 2008). The star was once suspected of having a spectroscopic companion. Some other useful star catalogue designations include: Gl 176, BD+18 683, Hip 21932, HD 285968, G 8-55, G 33-82, G 85-9, LHS 196, LTT 11495, LFT 372, MCC 107, Ross 33, and 2MASS J04425581+1857285.

Planet b

On September 6, 2007, a team of astronomers (Michael Endl, William D. Cochran, Robert A. Wittenmyer, and Alan P. Boss) submitted a paper on the discovery of a Neptune-class planet (of at least 24.5 Earth-masses or 0.0771 Jupiter-masses) in a tight inner orbit (a = 0.0727 ± 0.0007 AUs) with a period less than 10.24 days, based on data from the HRS spectrograph on the Hobby-Eberly Telescope (Endl et al, 2008; and their web page on GJ 176 -- more below). However, on September 8, 2008, a different team of astronomers (Thierry Forveille, Xavier Bonfils, Xavier Delfosse, Michael Gillon, Stephane Udry, François Bouchy, Christophe Lovis, Michel Mayor, Francesco Pepe, Christian Perrier, Didier Queloz, Nuno Santos, Jean-Loup Bertaux) submitted a paper (based on data from the ESO 3.6-meter telescope and its HARPS spectrograph) which claims to rule out the mass of a Neptune-sized inner planet but found evidence for a less massive "Super-Earth" companion in an tighter inner orbit (Forveille et al 2008). Planet "b" has at least 8.4 Earth-masses (or 0.026 Jupiter-masses). It revolves around Gliese 176 at an average distance of 0.066. AU, in a roughly circular orbit (e= 0. ± 0.0) which it completes in 8.8 days. With a semi-major axis of 0.066 AUs, it orbits so close to its host star that its orbital period lasts only 8.78 days, and so the planet must be very hot at around 450° Kelvin, 351° F, or 177° C (Forveille et al 2008).

Trent Schindler, NSF


Larger and jumbo illustrations
(more images and videos of Gl 876).


Planet "b" may actually be a "Super-Earth"
that is similar to Gl 876 "d," which may
rocky and have an atmosphere, clouds, and
the glow of molten areas on its surface,
as imagined by Schindler (more).

Based on its visual luminosity, a planet may be able to hold water on its surface beyond 0.09 AUs of Gliese 176. However, star-spot activity on Gliese 176 makes the detection of additional, smaller planets difficult (Forveille et al 2008). (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)