23 Librae

© James B. Kaler, UIUC -- more information
(Photo from Stars, Planet Project, and 23 Librae; used with permission)

The Star

23 Librae is a yellow-orange main sequence dwarf star of spectral and luminosity type G4-5 V. It has about 1.05 times the mass of Sol and 1.5 times its luminosity. The star is 1.7 times more enriched than Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (see HD 134987 at exoplanets.org). 23 Librae is older than Sol, at close to nine billion years. Useful catalogue numbers and designations for the star include: 23 Lib, HR 5657, Gl 579.4, Hip 74500, HD 134987, CD-24 11928, CP-24 5475, SAO 18327, and LTT 6066.

Planet "b"

In 1999, a team of astronomers (Steven S. Vogt, Geoffrey W. Marcy, R. Paul Butler, and Kevin Apps) announced the discovery of a Jupiter-class planet around 23 Librae using radial-velocity analysis (Vogt et al, 2000). Planet b has at least 1.6 times Jupiter's mass. It moves around 23 Librae at an average distance of only 0.81 AUs -- between the orbital distances of Venus and Earth in the Solar System -- in an elliptical orbit (e=0.24) (see HD 134987 at exoplanets.org). This orbit would take almost three-quarters of a year (260 days) to complete.

Subsequent astrometric analysis, however, suggests that planet b may have as much as 34 times the mass of Jupiter with an inclination of 2.7° from Earth's line of sight (Han et al, 2000). Thus, the "planet" could be an extremely dim brown dwarf, substellar companion of 23 Librae. 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 23 Librae may be centered around 1.2 AU -- between the orbital distances of Earth and Mars in the Solar System -- with an orbital period of about 1.3 years (486 days). However, the presence of planet b at its orbital distance of around 0.8 AU may have disrupted the development of an Earth-type planet in the 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.