55 (Rho1) Cancri 2

On July 11, 2011, a team of astronomers submitted a paper which revised the estimated habitable zone of 55 Cancri A to between 0.67 and 1.32 AUs. The paper also confirmed that planet "f" "spends the majority of duration of its elliptical orbit in the circumstellar habitable zone, where moderate green house heating, were, with moderate greenhouse heating, it could harbor liquid water." As the planet probably has around 57 Earth-masses, however, it is likely to a gas giant, although it could have a habitable moon given sufficient mass (von Braun et al, 2011; and Phil Plait, Discover - Bad Astronomy blog, July 20, 2011).

von Braun et al, 2011


Larger illustration.


New calculations confirm that giant
planet f "spends the majority of the
duration of its elliptical orbit in the
circumstellar habitable zone," where
an Earth-sized moon with moderate
green house heating could have
liquid water on its surface (more).

On April 28, 2011, a team of astronomers revealed new analysis of innermost planet "e" (or "a4") around 55 Cancri A using the transit method, which indicated that the planet is roughly twice as dense as Earth. Using the CSA's "MOST" (Microvariability and Oscillation of Stars) space telescope, the team determined that the planet is 63 percent larger (1.63 +/- 0.16) in diameter (21,000 kilometers or 13,000 miles) and eight times (8.57 +/- 0.64) as massive as Earth. As a result, the planet has been determined to be the densest planet detected thus far, where a Human being would weigh some three times as heavy on its surface as he or she would on Earth (U. of British Columbia media release; Winn et al, 2011; and John Voisey, Universe Today, May 1, 2011).

Jason Rowe, ARC, MOST, SETI,
Jaymie Matthews, UBC, CSA, NASA

Larger and jumbo illustrations.

The innermost planet around 55 Cancri A
has been determined to be roughly twice
as dense as Earth, 60 percent larger and
eight times as massive (more).

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

In 1996, astronomers announced the discovery of a Jupiter-like planet around the wide binary's Sun-like Star A (Butler et al, 1997 -- details below), with indications of an even larger planet in an outer orbit. By June 2002, two -- possibly three -- giant planets had been found (exoplanets.org; and Marcy et al, 2002, in pdf). On August 31, 2004, astronomers announced the discovery of a fourth, Neptune-sized inner planet (more below -- NASA/JPL press release and McDonald Observatory press release). On November 6, 2007, astronomers announced the discovery of a fifth planet ("f" or "A5") around half the mass of Saturn in the habitable zone of this star (NASA JPL press release and Fischer et al, 2007 -- more below).

---------------------------------------------- [Guide] -- [Larger] ----------------------------------------------

	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass* (Earths)	Diameter (Earths)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
55 Cancri A	0.0	...	...	...	301,000 -320,000	103-120	...	...	2.04
Planet "e" or "a4"	0.016	0.002	0.057	~53	8.6	2.05 +/- 0.15	0.76/1.06	~3	...
Planet "b" or "a1"	0.115	0.04	0.014	~53	323	13.4	...	...	...
Planet "c" or "a2"?	0.24	0.12	0.086	~53	66.7	<10?	...	...	...
Inner HZ Edge	0.67	0.58	0	~53	...	...	...	...	...
Planet "f" or "a5"?	0.78	0.71	0.4	~53	57	<10?	...	...	...
Outer HZ Edge	1.32	1.1.59	0	~53	...	...	...	...	...
Planet "d" or "a3"	5.77	14.7	0.025	~53	1,557	~11.2	...	...	...
Dust Disk?	27-50?	...	...	~53	0.0008-0.005	...	...	...	...

55 Cancri A

55 Cancri A is a yellow-orange main sequence dwarf star of spectral and luminosity type G8 V (that has been classed as orange as K0), with around 90.5 to 96 percent of Sol's mass (von Braun et al, 2011; Fischer et al, 2007; Takeda et al, 2007; and Winn et al, 2011), 94 to 110 percent of its diameter (von Braun et al, 2011; Demory et al, 2011, Table 1, page 3; van Belle and von Braun, 2009; and 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 of between two to eight billion years old, possibly around 5.5 billion years old based on chromospheric activity (Saffe et al, 2005, Table 4, page 8; Marcy et al, 2002, in pdf; and Baliunas et al, 1997). Based on isochrone fitting, however, the star appears to be 10.2 +/- 2.5 billion years old (von Braun et al, 2011). 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.

Habitable Zone

According to one type of model calculations performed for the NASA Exoplanet Archive, the inner edge of 55 Cnc A's habitable zone should be located around 0.57 AU from the star, while the outer edge edge lies around 1.13 AUs. A June 2011 study, however, argues that the habitable zone actually lies between 0.67 and 1.32 AUs from the star (von Braun et al, 2011), which updates earlier rough estimates of Star between 0.5 and 2 AUs -- possibly more restricted to 0.7 to 1.4 AUs -- from the 55 Cnc A (Fischer et al, 2007). Astronomers are hoping that someday they will be able to use NASA's indefinitely postponed Terrestrial Planet Finder (TPF) and the ESA's Darwin similarly postponed groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around 61 Virginis. As proposed, the TPF will include two complementary observatory groups: a visible-light coronagraph; and a "formation-flying" infrared interferometer, while Darwin would launch a flotilla of three mid-infrared telescopes and a fourth communications hub. (See a larger animation of the planetary orbits, dust disk, and potentially habitable zone around 55 Cancri A.)

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).

Planet "e" or "A4" - 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 8.57 +/- 0.64 Earth-masses (Winn et al, 2011), planetary candidate "e" (or "A4") orbits 55 Cancri A in just 17 hours and 41 minutes , or 0.74 days (Dawson and Fabrycky, 2010; and Ken Crosswell, New Scientist, May 28, 2010). Its scorchingly close, inner orbit has a semi-major axis of less than 0.016 AU (Winn et al, 2011). The planet's orbital eccentricity is now estimated to be mild, only e= 0.057 +0.064/- 0.041 (Demory et al, 2011). [More general discovery information and links are available at Astronomy Picture of the Day.]

JPL, NASA


Larger illustration.



As of 2011, planet "e" or "A4" has been
determined to be a rocky planet like Earth,
instead of gaseous like Neptune, with a
temperature of 1,800 - 2,700° C or 3,300
- 4,900° F (more from more).

Planet "b" or "A1" - 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.

Planet "c" or "A2" - 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 "f" or "A5" that
orbits within the habitable zone
of Star A was announced in
late 2007 (more).

Planet "f" or "A5" - 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 of 0.4 (von Braun et al, 2011).

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).

Planet "d" or "A3" - 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.





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

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.