GJ 3021 / HD 1237 / CD-80 9

Many astronomers now refer to this star by its designation \in the famous 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 was included in the third version of the catalogue that was updated with Hartmut Jahreiss (also at (ARI) as "CNS3" in 1991. GJ 3021 is also listed as HD 1237 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.

GJ 3021, however, may have been first designated as CD-80 9, as it was found in 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. (More discussion on star names and catalogue numbers is available from Alan MacRobert at Sky and Telescope and from Professor James B. Kaler's Star Names.)

The Star

GJ 3021 is a yellow-orange dwarf star of spectral and luminosity type G6 V. The star has about 93 percent of Sol's mass, 94 percent of its diameter, and 66 percent of its visual -- 77 percent of its bolometric -- luminosity (Phillip J. Flower, 1996). The star may be 1.26 times as enriched than Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (exoplanets.org; and Santos et al, 2000). Chromospherically active (Henry et al, 1996) with a fast stellar rotation as well as metal rich, GJ 3021 is thought be a young star that may be only around 600 million years old (Observatoire de Genève; and Naef et al, 2001 -- in gzipped ps). Useful catalogue numbers and designations for the star include: GJ 3021, Hip 1292, HD 1237, CD-80 9, CP(D)-80 7, SAO 258219, and LTT 127.

Planet "b"

NASA

Cassini-Huygens Mission
to Saturn and Titan


Larger image.


Planetary candidate "b" appears to
be a gas giant that is larger than
Jupiter (shown here with Europa).

On January 25, 2000, a team of astronomers (Dominique Naef, Francisco Pepe, Michel Mayor, Nuno C. Santos, Didier Queloz, Stephane Udry, and M. Burnet) announced the discovery of a Jupiter-class planet around GJ 3021 using highly sensitive radial-velocity methods (press release, initial data, and exoplanets.org). According to the latest radial velocity data, planet "b" has at least 3.45 times Jupiter's mass. It moves around GJ 3021 at an average distance of only 0.51 AUs (a semi-major axis between the orbital distances of Mercury and Venus in the Solar System) in a highly elliptical orbit (e=0.505) that takes almost 134 days or more than one third of a year to complete (exoplanets.org).

Subsequent astrometric analysis, however, suggests that planet b may have as much as 16 times the mass of Jupiter with an inclination of 11.8° from Earth's line of sight (Han et al, 2000). Thus, planet is probably an extremely dim brown dwarf, substellar companion of GJ 3021. The authors consider their analysis to be preliminary, needing confirmation with additional astrometric as well as other observations.

The orbit of an Earth-like planet (with liquid water) around GJ 3021 may be centered around 0.79 AU -- between the orbital distances of Earth and Mars in the Solar System -- with an orbital period around 270 days (about three quarters of a year). However, the eccentric orbit of planet b at an average orbital distance of around 0.5 AU would disrupt the orbit of an Earth-type planet around GJ 3021's water zone. Astronomers would find it very difficult to detect an Earth-sized planet of this star using present methods. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)

Brown Dwarfs or Planets?

When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.

© American Scientist



Artwork by Linda Huff
(for Martin et al, 1997)
used with permission.

University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover a nearby brown dwarf, Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.

Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.