Rho Coronae Borealis

© James B. Kaler, UIUC -- more information




Photo from Stars, Planet Project,
and Rho Coronae Borealis
(used with permission).




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

The Star

Rho Coronae Borealis is a yellow-orange main sequence dwarf star of spectral and luminosity type G0-2 Va. The star has about 95 percent of Sol's mass (exoplanets.org), 1.31 times its diameter, and 1.61 to 1.77 times its luminosity (Noyes el al, 1997; and Extrasolar Planets Encyclopaedia). It may only be 51 to 65 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron and may be older than Sol at around six six billion years (Jones and Sleep, 2003). Useful catalogue numbers and designations for the star include: Rho CrB, 15 CrB, HR 5968*, Gl 606.2, Hip 78459, HD 143761, BD+33 2663, SAO 65024, Wo 9537, LHS 3145, LTT 14764, and LFT 1241.

Dust Disk

David Trilling, Robert Brown,
University of Arizona, IRTF
(Used with permission)



Rho Coronae Borealis has a
dust disk like that of 55 (Rho1)
Cancri A depicted here. (See a
more recent comparison image).



In October 1999, astronomers at the University of Arizona confirmed the existence of a circumstellar dust disk that may be similar in composition to the Edgeworth-Kuiper (E-K) Belt of the Solar System (Trilling et al, 2000). The disk may extend out to 85 AUs out from Rho Coronae Borealis, which is beyond the orbital distances of Neptune and Pluto from Sol). It is inclined at about 46° from Earth's line of sight.

Planet "b"

In 1997, a team of astronomers (Robert W. Noyes, Saurabh Jha, Sylvain G. Korzennik, Martin Krockenberger, and Peter Niesenson) announced the discovery of a Jupiter-class planet around Rho Coronae Borealis using radial-velocity methods (Noyes et al, 1997). Planet "b" has at least 99 percent of Jupiter's mass and the inclination of the dusk disk suggests a higher mass of 1.5 Jupiter masses (Trilling et al, 2000). The planet moves around Rho Coronae Borealis at an average distance of only 0.22 AUs (a semi-major axis well within Mercury's orbital distance) in a highly circular orbit (e=0.07) that takes only 39.8 days to complete.

NASA, Cassini-Huygens Mission
to Saturn and Titan


Larger image.



Planetary candidates "b" and
"c" are massive enough to be
gas giants like Jupiter (shown
here with Europa), brown dwarfs,
or even small red dwarf stars.

A residual drift in the radial velocity data over several years suggest the presence of an even larger planet "c" in an outer orbit from Rho Coronae Borealis (Multiple Planet Systems at exoplanets.org). Planet c may have more than four times the mass of Jupiter. Assuming an orbit that takes more than two years to complete and Rho Coronae Borealis's mass of 0.98 Solar, it may lie at an average orbital distance of 1.6 AUs.

Subsequent astrometric analysis, however, suggests that planet b may have as much as 115 times the mass of Jupiter with an inclination of 0.5° from Earth's line of sight (Han et al, 2000). Thus, the "planet" could be a dim red dwarf, stellar companion of Rho Coronae Borealis. The authors consider their analysis to be preliminary, needing confirmation with additional astrometric as well as other observations.

The orbit of an Earth-like rocky planet (with liquid water) around Rho Coronae Borealis may have to be between 0.94 and 1.85 AU (Jones and Sleep, 2003) -- from just inside the average orbital distance of Earth and beyond that of Mars -- with an orbital period around one to two Earth years. Although the development of an Earth-like planet in this zone could have been disrupted by the hypothesized inward migration of planet b, some astronomers between that a restricted but stable planetary orbit is possible (Jones and Sleep, 2003). Astronomers would find it very difficult to detect an Earth-type planet in the water zone 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.