14 Herculis

© James B. Kaler, UIUC -- more information
(Photo from Stars, Planet Project, and 14 Herculis; used with permission)

The Star

14 Herculis is an orange-red main-sequence dwarf star of spectral and luminosity type K0 V. The star may have 79 percent of Sol's mass (Observatoire de Genève), 88 percent of its diameter (Johnson and Wright, 1983, page 685), and 75 percent of its luminosity. The star appears to be around 3.2 times as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Gonzalez et al, 1999). It may be around six billion years old, roughly a billion and a half more than Sol. Useful catalogue numbers and designations for the star include: 14 Her, Gl 614, Hip 79248, HD 145675, BD+44 2549, SAO 45933, G 180-35, G 202-30, and LTT 14816.

The orbit of an Earth-like planet with surface water would be centered within 0.87 AU -- between the orbital distances of Venus and Earth in the Solar System -- and take around 331 days (over nine-tenths of a year) to complete. However, the eccentric orbit of a giant planetary companion "b" recently discovered around 14 Herculis would probably disturb the orbit of such an Earth-type planet. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.

Substellar Companion "b"

© Tomislav Stimac


Larger image.


Artwork from Planets
at Digital Eye
(used with permission).


A view of 14 Herculis and its
substellar companion "b" from
the surface of a satellite, as
imagined by Stimac (more).

On July 6, 1998, a team of astronomers (including Michel Mayor, Didier Queloz, and Stephane Udry) announced the discovery of a substellar companion "b," whose latest minimum mass estimate has been calculated as 5.04 times that of Jupiter (exoplanets.org) with a similar diameter. This object moves around 14 Herculis at an average separation of 2.81 AUs, which would be within the Main Asteroid Belt in the Solar System. Its orbit has a high eccentricity (e= 0.35) which takes around 4.6 years to complete (exoplanets.org). (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).


Some speculated that surface water
might exist on a large moon of "b"
warmed by tidal heating and a thick
atmosphere, as imagined by
Kulmann (more).

Astrometric analysis suggests that the object may actually have 13 times the mass of Jupiter with an inclination of 155.3° from Earth's line of sight (Han et al, 2001, in pdf). Thus, the object could be an extremely dim brown dwarf, substellar companion of 14 Herculis. The authors consider their analysis to be preliminary, needing confirmation with additional astrometric as well as other observations.

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.