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Gliese 876

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

   CAPTION: Gliese 876

                              Observation data
   Epoch J2000.0
   Constellation             Aquarius
   Right ascension           22^h 53^m 16.7^s
   Declination               -14° 15′ 49″
   Apparent magnitude (V)    10.18
                               Characteristics
   Spectral type             M3.5V
   B-V colour index          1.59
   U-B colour index          1.15
   Variable type             BY Draconis
                                 Astrometry
   Radial velocity (R[v])    -1.7 km/s
   Proper motion (μ)         RA: 960.33 mas/ yr
                             Dec.: -675.64 mas/ yr
   Parallax (π)              212.59 ± 1.96 mas
   Distance                  15.3 ± 0.1 ly
                             (4.7 ± 0.04 pc)
   Absolute magnitude (M[V]) 11.82
                                   Details
   Mass                      0.32 ± 0.03 M[☉]
   Radius                    0.36 R[☉]
   Luminosity                0.0124 L[☉]
   Temperature               3,200 ± 100 K
   Metallicity               107% solar
   Rotation                  96.7 days
   Age                       9.9 × 10^9 years
                             Other designations
   IL Aquarii, Ross 780, G 156-057, BD-15°6290, GCTP 5546.00, LHS 530, Vys
   337, HIP 113020

   Gliese 876 is a red dwarf star located approximately 15 light-years
   away in the constellation Aquarius. It has the variable star
   designation IL Aquarii. As of 2006 the star is known to host three
   extrasolar planets, including one with a mass less than half that of
   Neptune.

Distance and visibility

   Gliese 876 is located fairly close to our solar system. According to
   astrometric measurements made by the Hipparcos satellite, the star
   shows a parallax of 212.59 milliarcseconds, which corresponds to a
   distance of 4.70 parsecs (15.3 light years). Despite being located so
   close to us, the star is so faint that it is invisible to the naked eye
   and can only be seen using a telescope.

Stellar characteristics

   As a red dwarf star, Gliese 876 is much less massive than our Sun:
   estimates suggest it has only 32% of the mass of our local star. The
   surface temperature of Gliese 876 is cooler than our Sun and the star
   has a smaller radius. These factors combine to make the star only 1.24%
   as luminous as the Sun, though most of this is at infrared wavelengths.

   Estimating the age and metallicity of cool stars is difficult due to
   the formation of diatomic molecules in their atmospheres, which makes
   the spectrum extremely complex. Based on comparisons to red dwarfs in
   binary systems, Gliese 876 has a similar abundance of heavy elements to
   the Sun, estimated at around 107% the solar abundance of iron. Based on
   chromospheric activity the star is likely to be around 6,520 or 9,900
   million years old, depending on the theoretical model used.

   Like many low-mass stars, Gliese 876 is a variable star. It is
   classified as a BY Draconis variable and its brightness fluctuates by
   around 0.04 magnitudes. This type of variability is thought to be
   caused by large starspots moving in and out of view as the star
   rotates.

Planetary system

   Artist's impression of the outer planet Gliese 876 b.
   Enlarge
   Artist's impression of the outer planet Gliese 876 b.

   In 1998 an extrasolar planet was announced in orbit around Gliese 876
   by two independent teams led by Geoffrey Marcy and Xavier Delfosse. The
   planet was designated Gliese 876 b and was detected by making
   measurements of the star's radial velocity as the planet's gravity
   pulled it around. The planet, around twice the mass of Jupiter,
   revolves around its star in an orbit taking approximately 61 days to
   complete, at a distance of only 0.208 AU, less than the distance from
   the Sun to Mercury.

   In 2001 a second planet was detected in the system, inside the orbit of
   the previously-discovered planet. The 0.62 Jupiter-mass planet,
   designated Gliese 876 c is in a 2:1 orbital resonance with the outer
   planet, taking 30.340 days to orbit the star. This relationship between
   the orbital periods initially disguised the planet's radial velocity
   signature as an increased orbital eccentricity of the outer planet. The
   two planets undergo strong gravitational interactions as they orbit the
   star, causing the orbital elements to change rapidly.

   In 2005, further observations by a team led by Eugenio Rivera revealed
   a third planet in the system, inside the orbits of the two Jupiter-size
   planets. The planet, designated Gliese 876 d, has a minimum mass only
   5.88 times that of the Earth and may be a terrestrial planet. Based on
   the radial velocity measurements and modelling the interactions between
   the two giant planets, the system's inclination is estimated to be
   around 50° to the plane of the sky. If this is the case and the system
   is assumed to be coplanar, the planetary masses are around 30% greater
   than the lower limits established by the radial velocity method. This
   would make the inner planet have a true mass around 7.5 times that of
   Earth. On the other hand, astrometric methods suggest an inclination
   around 84° for the outermost planet, which would mean the true masses
   are only slightly greater than the lower limit.

   Both of the system's Jupiter-mass planets are located in the habitable
   zone of Gliese 876, which extends between 0.116 to 0.227 AU from the
   star. This leaves no room for an additional habitable Earth-size planet
   in the system. On the other hand, large moons of the gas giants, if
   they exist, may be able to support life.

                                  Companion
                          (In order from star) Mass
                           ( M[J]) Orbital period
                            (days) Semimajor axis
                             ( AU) Eccentricity
         d >0.0185 ± 0.0031 1.937760 ± 0.000070 0.0208 ± 0.0012 0
     c >0.619 ± 0.088 30.340 ± 0.013 0.1303 ± 0.0075 0.2243 ± 0.0013
       b >1.93 ± 0.27 60.940 ± 0.013 0.208 ± 0.012 0.0249 ± 0.0026

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