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55 Cancri

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: 55 Cancri AB

                          Observation data
   Epoch J2000.0
   Constellation             Cancer
   55 Cancri A
   Right ascension           08^h 52^m 35.8^s
   Declination               +28° 19′ 51″
   Apparent magnitude (V)    5.95
   55 Cancri B
   Right ascension           08^h 52^m 40.9^s
   Declination               +28° 19′ 59″
   Apparent magnitude (V)    13.15
                          Characteristics
   Spectral type             G8V / M3.5-4V
   B-V colour index          0.86 / 1.66
   U-B colour index          0.65 / 1.21
   Variable type             none / ?
                             Astrometry
   Radial velocity (R[v])    27.3 km/s
   Proper motion (μ)         RA: -485.46 mas/ yr
                             Dec.: -234.40 mas/ yr
   Parallax (π)              79.80 ± 0.84 mas
   Distance                  40.9 ± 0.4 ly
                             (12.5 ± 0.1 pc)
   Absolute magnitude (M[V]) 5.46 / 12.66
                              Details
   Mass                      0.95 / 0.13 M[☉]
   Radius                    1.1 / 0.30 R[☉]
   Luminosity                0.61 / 0.0076 L[☉]
   Temperature               5240 / ? K
   Metallicity               186% / ?
   Rotation                  42 days / ?
   Age                       5.5 × 10^9 years
                         Other designations
   ρ^1 Cancri, Gl 324, BD +28°1660, HD 75732

   55 Cancri A

   HR 3522, LHS 2062, LTT 12310, GCTP 2117.00, SAO 80478, LFT 609,
   HIP 43587

   55 Cancri B LHS 2063, LTT 12311, LFT 610

   55 Cancri (abbreviated 55 Cnc) is a binary star located around 40
   light-years away in the constellation Cancer. It has the Bayer
   designation Rho^1 Cancri. The system contains a yellow dwarf star (55
   Cancri A) similar to our Sun and a red dwarf (55 Cancri B). The two
   components are separated by over 1000 times the distance from the Earth
   to the Sun.

   As of 2004, four extrasolar planets are known in orbit around 55 Cancri
   A. Three of the planets are comparable to Jupiter in mass, while the
   innermost planet has a mass similar to that of Neptune. The 55 Cancri
   system was the first known four-planet extrasolar planetary system.

   55 Cancri A is ranked 63rd in the list of top 100 target stars for the
   NASA Terrestrial Planet Finder mission.

Distance and visibility

   The 55 Cancri system is located fairly close to our solar system: the
   Hipparcos astrometry satellite measured the parallax of 55 Cancri A as
   79.80 milliarcseconds, corresponding to a distance of 12.5 parsecs. 55
   Cancri A has an apparent magnitude of 5.95, making it visible through
   binoculars. It is just visible to the naked eye under very dark skies.
   The red dwarf 55 Cancri B is of the 13th magnitude and only visible
   through a telescope.

System components

   55 Cancri A is a yellow dwarf star of spectral type G8V. It has a
   similar mass to our Sun, but is cooler and less luminous. The star is
   more enriched than our sun in elements heavier than helium: it is
   classified as a rare "super metal-rich" main sequence star and has
   around 186% the solar abundance of iron. This abundance of metal makes
   estimating the star's age and mass difficult, as evolutionary models
   are less well defined for such stars. One estimate based on
   chromospheric activity suggests an age of around 5,500 million years.

   55 Cancri B is a red dwarf star located at an estimated distance of
   1065 AU from the primary star, and is much less massive and luminous
   than our Sun. There are indications that this star may itself be a
   double star, though this is by no means certain.

Planetary system

   Comparison of the orbits of the inner planets of 55 Cancri (black) with
   the planets of our solar system.
   Enlarge
   Comparison of the orbits of the inner planets of 55 Cancri (black) with
   the planets of our solar system.

   In 1997, the discovery of a 51 Pegasi-like planet orbiting 55 Cancri A
   was announced, together with the planet of Tau Boötis and the inner
   planet of Upsilon Andromedae. The planet was discovered by measuring
   the star's radial velocity, which showed a periodicity of around 14.7
   days corresponding to a planet at least 78% of the mass of Jupiter.
   This planet was designated 55 Cancri b, though to distinguish it from
   the star 55 Cancri B it is occasionally referred to as 55 Cancri Ab.
   The radial velocity measurements still showed a drift unaccounted-for
   by this planet, which could be explained by the gravitational influence
   of a more distant object.

   In 1998 discovery a possible dust disk around 55 Cancri A was
   announced. Calculations gave the disk radius at least 40 AU, similar to
   the Kuiper belt in our solar system, with an inclination of 25° with
   respect to the plane of the sky. However, the discovery could not be
   verified and was later deemed to be spurious, caused instead by
   background radiation.
   Our solar system compared with the solar system of 55 Cancri
   Enlarge
   Our solar system compared with the solar system of 55 Cancri

   After making further radial velocity measurements, a planet orbiting at
   a distance of around 5 AU was announced in 2002. This planet received
   the designation 55 Cancri d. At the time of discovery, the planet was
   thought to be in an orbit of mild eccentricity (close to 0.1), however
   this value was increased by later measurements. Even after accounting
   for these two planets, a periodicity at 43 days remained, possibly due
   to a third planet. Measurements of the star suggested that this was
   close to the star's rotation period, which raised the possibility that
   the 43-day signal was caused by stellar activity. This possible planet
   received the designation 55 Cancri c.

   In 2004 a Neptune-mass planet designated 55 Cancri e was announced in a
   2.8-day orbit. This planet may either be a small gas giant or a large
   terrestrial planet. The measurements that led to the discovery of this
   planet also confirmed the existence of 55 Cancri c. In addition,
   astrometric measurements made by the Hubble Space Telescope led to an
   estimate of the inclination of the orbit of the outer planet: around
   53° with respect to the plane of the sky. Assuming the system is
   coplanar, this means the true masses of the planets are around 25%
   greater than the lower limits measured by the radial velocity method.
   55 Cnc planets
   Enlarge
   55 Cnc planets

   In 2005 the existence of planet e was questioned by Jack Wisdom in a
   reanalysis of the data. According to him, instead of the 2.8-day planet
   there is a planet with a mass similar to that of Neptune in a 261-day
   orbit (corresponding to 0.77 AU in distance). This analysis has not
   been confirmed.

   Without taking the unconfirmed 261-day planet into account, simulations
   suggest that there is a wide stable region between the orbits of
   planets c and d which could contain additional planets. Models predict
   that one or more terrestrial planets could form in this gap, though
   such planets would be undetectable with present-day technology. This is
   of astrobiological interest since this region includes 55 Cancri A's
   habitable zone.

        Companion
   (In order from star)     Mass
                           ( M[J])    Orbital period
                                          (days)     Semimajor axis
                                                         ( AU)     Eccentricity
            e           >0.045 ± 0.01  2.81 ± 0.002  0.038 ± 0.001 0.174 ± 0.127
            b           >0.784 ± 0.09 14.67 ± 0.0006 0.115 ± 0.003 0.0197 ± 0.01
            c           >0.217 ± 0.04 43.93 ± 0.021  0.240 ± 0.005  0.44 ± 0.08
            d            >3.92 ± 0.5  4517.4 ± 77.8   5.257 ± 0.9  0.327 ± 0.28

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