16 Cygni 3

© James B. Kaler, UIUC -- more information




Photo from Stars, Planet Project, and
16 Cygni B (used with permission).




In 1996, astronomers announced the discovery of a Jupiter-like planet around this Sun-like star (Cochran et al, 1997 -- details below). (See an animation of the planetary and potentially habitable zone orbits of the 16 Cygni B system, with a table of basic orbital and physical characteristics.)

16 Cygni A or Aa

16 Cygni A is a yellow-orange main sequence dwarf star of spectral and luminosity type G1.5 Vb, with about 1.05 +/- 0.02 of a Solar-mass (Ramirez et al, 2011, Table 3, page 7; and Baliunas et al, 1997), 1.3 to 1.4 times its diameter (NASA Star and Exoplanet Database, and nearly 1.7 times its luminosity. One analysis of isochrones suggests that the star may be 9.5 billion years old, more than twice Sol's age (Guinan et al, 1999), while a more recent estimate indicates 6.8 +/- 0.4 billion years based on analysis of oscillations of stars A and B using asteroseismic observations from the Kepler Mission (Ken Croswell, 2012; and Metcalfe et al, 2012). Despite its presumed greater age, the star may be slightly more enriched (112 percent) as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Friel et al, 1993).

It has a widely separated, stellar companion "B", and possibly a closer companion "C" (at 70" from Star A, according to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 7503). Stars A and B are separated by about 840 AUs (39" at a distance of 70.5 light-years), while A and C may be separated by 80 to 150 AUs, if C is not a background star. Useful catalogue numbers and designations for the star include: 16 Cyg, HR 7503*, Gl 765.1 A, Hip 96895, HD 186408, BD+50 2847, SAO 31898, LTT 15750, USNO 889, Struve 46, and ADS 12815 A.

16 Cygni C or Ab

NASA -- larger image

Stellar companion "Ab" is a dim red dwarf, like
Gliese 623 A (M2.5 V) and B (M5.8Ve) at lower right.

A Planetary System?

© Tomislav Stimac -- larger image
(Artwork from Planets at Digital Eye, used with permission).
Planet "b" as a ringed gas giant, as imagined by Stimac (more).

In 1996, two team of astronomers announced the discovery of a Jupiter-class planet around Star B using highly sensitive radial-velocity methods (Cochran et al, 1997). Planet "b" has at least 1.68 times of Jupiter's mass. It moves around Star B at an average distance of 1.66 AUs (a semi-major axis between the orbital distances of Mars and the Main Asteroid Belt) in a highly elliptical orbit (e=0.68) that takes almost 2.2 years to complete (16 Cyg B and Multiple Planet Systems at exoplanets.org).

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

Despite an eccentric orbit, some hoped that planet b could have
a moon with a habitable "Spring," as imagined by Whatmough.

Subsequent astrometric analysis, however, suggests that planet b may have 14 times the mass of Jupiter with an inclination of 173.0° from Earth's line of sight (Han et al, 2000). Thus, the planet could be an extremely dim brown dwarf, substellar companion of 16 Cygni B. The authors consider their analysis to be preliminary, needing confirmation with additional astrometric as well as other observations.

© Christoph Kulmann -- larger image


Artwork from Exoplaneten.de
(used with permission).


"Springtime" on a large,
Earth-sized moon of planet 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.