70 Virginis

© James B. Kaler, UIUC -- more information
(Photo from Stars, Planet Project, and 70 Virginis; used with permission)

The Star

70 Virginis is a yellow-orange main sequence dwarf star of spectral and luminosity type G5 Va, but has been previously classified from G2.5 to G4. The star has about 1.10 percent of Sol's mass (70 Virginis at exoplanets.org), 2.0 to 2.5 times its diameter (Henry et al, 1997), and 2.9 times its luminosity. The star may be less enriched (92 percent) than Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (J.B. Heanshaw, 1974). It appears to be more highly evolved than Sol, perhaps as much as nine billion years old (Henry et al, 1997). Useful catalogue numbers and designations for the star include: 70 Vir, HR 5072*, Gl 512.1, Hip 65721, HD 117176, BD+14 2621, SAO 100582, FK5 1349, Wo 9446, LHS 2740, LTT 13918, and LFT 1009.

T. Pyle, SSC, JPL / CalTech, NASA -- larger illustration

Preliminary infrared observations indicate that 70 Virginis has a circumstellar
dust disk generated by collisions between Kuiper-Belt type objects (more from
SSC and Beichman et al, 2004).

Planet "b"

© Walter Myers -- larger image
(Artwork from Computergraphic Vistas, used with permission)

View of a ringed, planetary candidate "b" from
a rocky moon, as imagined by Myers (more).

In 1996, a team of astronomers (including Geoffrey W. Marcy and R. Paul Butler) announced the discovery of a Jupiter-class planet around 70 Virginis using highly sensitive radial-velocity methods (Marcy and Butler, 1996). Planet b has at least 7.4 times Jupiter's mass, with an upper mass limit of 38 Jupiter masses from analysis of HIPPARCOS astrometric data (Perryman et al. 1996). It moves around 70 Virginis at an average distance of only 0.48 AUs (a semi-major axis around Mercury's orbital distance) in a highly elliptical orbit (e=0.40) that takes almost 167 days to complete.

Subsequent astrometric analysis, however, suggests that planet b may have as much as 27 times the mass of Jupiter with an inclination of 16.1° from Earth's line of sight (Han et al, 2001, in pdf). Thus, planet is probably an extremely dim brown dwarf, substellar companion of 70 Virginis. The authors consider their analysis to be preliminary, needing confirmation with additional astrometric as well as other observations.

© John Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission)

After the initial discovery of b, some hoped that water
could exist on a large moon, as imagined by Whatmough.

The orbit of an Earth-like planet (with liquid water) around Star A may be centered around 1.7 AU -- between the orbital distances of Mars and the Main Asteroid Belt in the Solar System -- with an orbital period around 2.1 years. However, the presence of planet b at its orbital distance of nearly 0.5 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.)

© Christoph Kulmann -- larger image


Artwork from Exoplaneten.de
(used with permission).


Later, some speculated that surface
water might exist temporarily on a
large, tidally-locked moon of b, as
imagined by Kulmann (more).

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.