In the north of the constellation of Cancer, just to the west of the magnitude 4 star iota Cancri, is 55 Cancri, also known as rho1 Cancri. Through binoculars or a small telescope, it resolves into two stars, appearing to be a wide binary. The second star is 53 Cancri, and is, in reality, a distant background star not associated with 55 Cancri. Nevertheless, 55 Cancri is a double star system. Star A is a G8 V yellow dwarf, older and dimmer than our sun. Star B, 1065 AU from the primary, is a faint M4 V red dwarf. It is around 55 Cancri A that a retinue of planets has been recently detected. Many stars with close in giant planets tend to have high metallicity, and 55 Cancri is no exception. Star A has a metallicity of +0.27, which is 186 times that of the sun. The star's atmosphere is also rich in carbon compounds, with a ratio of carbon to iron about 141 times that of the sun. Close in planets have also been associated, at least theoretically, with the phenomenon of superflares, although superflares have never been detected from 55 Cancri A. 55 Cancri was the first quadruple planetary system discovered, and continues to be a prime target for observation. More planets in this system are likely to be detected in coming years. In fact, the 0.3 to 5 AU gap between Planet c and Planet d is quite likely to contain at least one additional planet. Current theory on giant planet formation and migration suggests that material must have existed in the 0.3 AU to 5 AU range of 55 Cancri's protoplanetary disk to cause Planet e, b, and c to fall inward towards their star. Yet, those planets would not have been able to clear the disk of material in that region. This suggests that another planet or planets in this orbital range could have formed out of the remaining material. Current models suggest that the orbit of a terrestrial or sub-Saturn mass planet would be stable at around 1 AU, well within Star A's Habitable Zone. Simulations have shown that perturbations of the other planets would cause the orbital eccentricity of an Earth mass planet to oscillate over a 27,000 year period with an amplitude of 0.03, similar to the perturbations on the Earth's own orbit due to the outer gas giants. In 1998, the discovery of a dust disk around 55 Cancri was announced. The disk was found to extend from 27 to 40 AU from Star A, had an inclination of 25°, and seemed to have properties similar to the Sun's own Kuiper Belt. Assuming the planets orbited in the same plane as the disk, the disk's inclination could be used to pinpoint their masses. But doubt was soon cast on the disk detection. Later surveys suggested that emissions were lower by a factor of 100 than previously thought, and current evidence suggests that the original detection was spurious, caused by background emissions beyond 55 Cancri. The lack of a dust disk means that the assumed 25° inclination for the system is also in error. This has been borne out in recent analysis of HST Fine Guidance Sensor measurement data of 55 Cancri A. Theoretically the outer planet of the system should induce a wobble on the star large enough to be detected astrometrically. Although the HST data was not enough to cover a single 7 year orbit of Planet d, there was enough data to detect a small arc from which the inclination of the system could be calculated. The angle was found to be 53° ±6.8°, and allowed a new estimation of the masses of the planets in the system. 55 Cancri was one of the 5 targets of the 2003 "Cosmic Call" initiative.
View a VRML model of the 55 Cancri system. Please be patient while the file downloads. For a VRML tour of our galaxy's exoplanets, check out Extrasolar VR.
Skymap created with John Walker's online Your Sky Virtual Telescope
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