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HD 217107

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

   CAPTION: HD 217107

                            Observation data
   Epoch J2000.0
   Constellation             Pisces
   Right ascension           22^h 58^m 15.54^s
   Declination               −2° 23′ 43.39″
   Apparent magnitude (V)    +6.17
                            Characteristics
   Spectral type             G8IV
   B-V colour index          0.744
   U-B colour index          0.42
   Variable type             none
                               Astrometry
   Radial velocity (R[v])    −14.0 km/s
   Proper motion (μ)         RA: −6.05 mas/ yr
                             Dec.: −16.03 mas/ yr
   Parallax (π)              50.71 ± 0.75 mas
   Distance                  64.3 ± 1 ly
                             (19.7 ± 0.3 pc)
   Absolute magnitude (M[V]) 4.70
                                Details
   Mass                      0.98 M[☉]
   Radius                    1.31 R[☉]
   Luminosity                0.97 L[☉]
   Temperature               5570 K
   Metallicity               0.29
   Rotation                  39 days
   Age                       7.7 ×10^9 years
                           Other designations
   HR 8734, BD -03°5539, HD 217107, SAO 146412, FK5 3836, HIP 113421.

   HD 217107 is a yellow subgiant star in the constellation Pisces. Its
   mass is very similar to the Sun's, although it is considerably older.
   Two planets have been discovered orbiting the star: one is extremely
   close to the star and completes an orbit every seven days, while the
   other is much more distant, taking 8 years to complete an orbit.

Distance, age and mass

   HD 217107 is fairly close to the Sun: the Hipparcos astrometric
   satellite measured its parallax as 50.71 milliarcseconds, which
   corresponds to a distance of 64 light years. Its apparent magnitude is
   6.17, making it just barely visible to the naked eye under favourable
   conditions.

   Spectroscopic observations show that its spectral type is G7 or G8,
   which means its temperature is about 5,000 K. Its mass is thought to be
   roughly the same as the Sun's, although its estimated age of 7.7
   billion years is rather older than the Sun's 4.6 billion years, and it
   is thought to be beginning to evolve away from the main sequence,
   having consumed almost all the hydrogen in its core in nuclear fusion
   reactions.

Planetary system

   A study of the radial velocity of HD 217107 carried out in 1998
   revealed that its motion along the line of sight varied over a 7.1 day
   cycle. The period and amplitude of this variation indicated that it was
   caused by a planetary companion in orbit around the star, with a
   minimum mass slightly greater than that of Jupiter . The companion
   planet was designated HD 217107 b.

   While most planets with orbital periods of less than 10 days have
   almost circular orbits, HD 217107 b has a somewhat eccentric orbit, and
   its discoverers hypothesised that this could be due to the
   gravitational influence of a second planet in the system at a distance
   of several astronomical units (AU). Confirmation of the existence of a
   second planet followed in 2005, when long term observations of the
   star's radial velocity variations revealed a variation on a period of
   about 8 years, caused by a planet with a mass at least twice that of
   Jupiter in a very eccentric orbit with a semimajor axis of about 4 AU.
   . The second planet was designated HD 217107 c.

        Companion
   (In order from star)     Mass
                          ( M[J])     Orbital period
                                          (days)      Semimajor axis
                                                          ( AU)     Eccentricity
            b           >1.37 ± 0.14 7.1269 ± 0.00022 0.074 ± 0.002 0.13 ± 0.02
            c             >2.1 ± 1     3150 ± 1000       4.3 ± 2    0.55 ± 0.20

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