55 (Rho1) Cancri 2

© James B. Kaler, UIUC -- more information
(Photo from Stars, Planet Project, and 55 Cancri; used with permission)

55 Cancri A

55 Cancri A is a yellow-orange main sequence dwarf star of spectral and luminosity type G8/K0 V, with around 0.94 +/- 0.05 times the mass of Sol (Fischer et al, 2007), 1.1 times its diameter (Baliunas et al, 1997), and around 60 (57 to 62) percent of its bolometric luminosity (Fischer et al, 2007). The star is now estimated to around twice (204 percent) as enriched as Sol with elements heavier than hydrogen ("metallicity") based on its abundance of iron, where metal enrichment may have been primordial (Fischer et al, 2007; Marcy et al, 2002, in pdf; Baliunas et al, 1997), and Santos et al, 2001). Chromospherically "inactive" with a rotation period of 39 days (Fischer et al, 2007, it appears to be a middle-aged dwarf at between two to eight billion years old (e.g., five billion years old), see: Marcy et al, 2002, in pdf; and Baliunas et al, 1997)According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 3522, enhanced CN and C2 and perhaps CH has been detected in its spectrum. 55 Cancri A has a widely separated, dim companion Star ("B") located about 1,100 AUs (85" at 40.9 ly) away that seems to be gravitationally bound to it. Useful catalogue numbers and designations for the star include: 55 Cnc, Rho1 Cnc, Rho Cnc, HR 3522*, Gl 324, Hip 43587, HD 75732, BD+28 1660, SAO 80478, LTT 12310, LHS 2062, and LFT 609.

Dust Disk?

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



Disputed dust disk around 55 Cnc A
(more images). See more recent,
larger scaled image (Trilling et al, 2000).



In October 1998, astronomers at the University of Arizona announced the apparent confirmation of a circumstellar dust disk that may be similar in composition to the Edgeworth-Kuiper (E-K) Belt of the Solar System. Detected dust appeared to extend from 27 to beyond 44 AUs out from Star A (possibly with a radius of around 50 AUs, which would be beyond the orbital distances of Neptune and Pluto from Sol) and to be inclined at about 25° from Earth's line of sight. A subsequent study suggested that the sub-millimeter emissions were lower by a factor of 100 than previous thought, with a hole within around 10 AU of the star (Jayawardhana et al, 2000). Some astronomers now believe that the dust indications could be spurious, caused by background sub-millimeter emissions located near but not centered on 55 Cancri A (Marcy et al, 2002, in pdf; and Schneider et al, 2001).

As of November 6, 2007, astronomers have announced the discovery of five planetary candidates around Star A.

JPL, NASA



Larger illustration.



Very hot, inner planetary candidate "e"
or "A4" orbits 55 Cancri A within 0.04
AUs (more from Planet Quest).

On August 31, 2004, astronomers (Barbara E. McArthur, Michael Endl, William D. Cochran, and G. Fritz Benedict; using data from Debra A. Fischer, Geoffrey W. Marcy and R. Paul Butler, Dominique Naef, Michel Mayor, Didier Queloz, Stephane Udry, and Thomas E. Harrison) announced the discovery of a fourth, Neptune-sized inner planet (NASA/JPL press release; McDonald Observatory press release; and McArthur et al, 2004). With an updated estimate of at least 10.8 (but probably 13.5) Earth-masses (Fischer et al, 2007), planetary candidate "e" (or "A4") orbits 55 Cancri A in just under three days (2.81705 +/- 0.0001). Its scorchingly close, inner orbit has a semi-major axis of 0.038 +/- 0.001 AUs or approximately 5.6 million kilometers (3.5 million miles) -- less than a tenth of Mercury's orbital distance in the Solar System. The planet's orbital eccentricity is now estimated to be mild, only e= 0.07 +/- 0.06. Astronomers do not yet know if this planet is gaseous or rocky like Earth and Mars (more general discovery information and links at Astronomy Picture of the Day).

JPL, NASA


Larger illustration.



Planet "e" or "A4" may be rocky
or gaseous like Neptune, with a
temperature of around 1,500° C or
2,700° F (more from Planet Quest)

In 1996, a team of astronomers (including Eric Williams, Heather M. Hauser, and Phil Shirts) led by Geoffrey W. Marcy and R. Paul Butler announced the discovery of a Jupiter-class planet around Star A using highly sensitive radial-velocity methods (Marcy et al, 2002, in pdf; and Butler et al, 1997). Planet "b" or "A1" has at least 82.4 (but probably 103) percent of Jupiter's mass. It moves around Star A at an average distance of only 0.115 AUs (a semi-major axis well within Mercury's orbital distance) in a slightly elliptical orbit (e=0.02) that takes less than 14.7 days to complete (exoplanets.org). Assuming a Jupiter-like composition, its radius is projected to be about 1.2 times that of Jupiter, enlarged relative to Jupiter because of greater absorbed stellar radiation in its inner orbit, leading to derived temperature of 700° K (427° C or 800° F).

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

Planetary candidate "b" or "A1" depicted with lightning
flashes and two moons, as imagined by Whatmough.

Residual drift in the radial velocity data suggested the presence of another giant planet "c" or "A2" in a middle orbit, at about 0.24 AUs from Star A (like planet "b", still within orbital distance of Mercury in the Solar System). Planet A2 (or c) has at least a sixth (16.9 percent) but probably has a fifth (21 percent) of Jupiter's mass and an eccentric orbit (e= 0.34) that takes less than 44.3 days to complete (Fischer et al, 2007; and exoplanets.org).

NASA, JPL, Caltech





Larger illustration.





A fifth planet within
the habitable zone of
Star A was announced
in late 2007 (more).

On November 6, 2007, astronomers announced the discovery of a Jupiter-class, fifth planet ("f" or "A5"), that orbits at a distance of 0.781 AUs (between 0.73 and 0.84 AUs) from this star, within its "habitable zone" (NASA JPL press release and Fischer et al, 2007). The planet has at least 45 Earth-masses (0.144 Jupiter-masses) but probably 57 Earth-masses (0.18 Jupiter-masses) and may be substantially composed of hydrogen and helium, like Saturn in the Solar System (which has around 95 Earth-masses). It has an orbital period of around 260 days and an eccentricity fixed at 0.2 +/- 0.2.

NASA, JPL, Caltech





Larger illustration.





While three "inner"
planets lie within the
orbital distance of
Mercury, the outermost
planet has a Jupiter-like
orbit (more from JPL).

The residual drift data provided solid evidence of an even larger planet in an outer orbit, now estimated to be about 5.77 AUs from Star A (between the average orbital distances of Jupiter and the Main Asteroid Belt in the Solar System). Planet "A3" or "d" has at least 3.83 times (and probably 4.9 times) the mass of Jupiter and a mildly eccentric orbit (e= 0.16) that takes around 14.7 years to complete (Fischer et al, 2007 and exoplanets.org). Although one astrometric analysis suggested an orbital inclination of 179.6° from Earth's line of sight, subsequent astrometric analysis using the Fine Guidance Sensor on the Hubble Space Telescope indicated a orbital inclination of 53 +/- 6.8 degrees which allowed for better estimation of planetary masses (Fischer et al, 2007; McArthur et al, 2004; and Han et al, 2001, in pdf).

NASA, JPL, Caltech





Larger illustration.





The fifth planet could
have a habitable moon(s)
as it orbits within the
habitable zone (green
band) of 55 Cnc A (more).

Star A's "habitable zone" may lie between 0.5 and around 2 AUs from the star ((Fischer et al, 2007). Some astronomers believe that several smaller planets can exist within the large gap in orbital distance between the fourth and fifth planets from Star A (0.24 and 5.8 AUs), between orbital periods of 260 days (0.71 years) and 13 years. Despite recent findings, astronomers still 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, disputed dusk disk, and potentially habitable zone orbits of the 55 Cancri A system, with a table of basic orbital and physical characteristics.)

55 Cancri B

NASA -- larger image

55 Cancri B is a dim red dwarf star, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.