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Gliese 876 c

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

   CAPTION: Gliese 876 c

     Extrasolar planet    Lists of extrasolar planets
                      Parent star
   Star                   Gliese 876
   Constellation          Aquarius
   Right ascension    (α) 22^h 53^m 16.73^s
   Declination        (δ) −14° 15′ 49.3″
   Spectral type          M3.5V
                   Orbital elements
   Semimajor axis     (a) 0.1303 ± 0.0075 AU
   Eccentricity       (e) 0.2243 ± 0.0013
   Orbital period     (P) 30.340 ± 0.013 d
   Inclination        (i)  ?°
   Longitude of
   periastron         (ω) 198.30 ± 0.90°
   Time of periastron (τ) 2,452,464.0 ± 0.1 JD
               Physical characteristics
   Mass               (m) >0.619 ± 0.088 M[J]
   Radius             (r)  ? R[J]
   Density            (ρ)  ? kg/ m^3
   Temperature        (T)  ? K
                 Discovery information
   Discovery date         2001
   Discoverer(s)          Marcy et al.
   Detection method       Radial velocity
   Discovery status       Confirmed

   Gliese 876 c is an extrasolar planet orbiting the red dwarf star Gliese
   876, taking 30.340 days to complete an orbit. The planet was discovered
   in 2001 and is the second planet in order of distance from its star.

Discovery

   At the time of discovery, Gliese 876 was already known to host an
   extrasolar planet designated Gliese 876 b. In 2001, further analysis of
   the star's radial velocity revealed the existence of a second planet in
   the system, which was designated Gliese 876 c. The orbital period of
   Gliese 876 c was found to be exactly half that of the outer planet,
   which meant that the radial velocity signature of the second planet was
   initially interpreted as a higher eccentricity of the orbit of Gliese
   876 b.

Orbit and mass

   Gliese 876 c is in a 1:2 orbital resonance with the outer planet Gliese
   876 b. This leads to strong gravitational interactions between the two
   planets, causing the orbital elements to change rapidly as the orbits
   precess. The orbit is more eccentric than any of the major planets in
   our solar system. The semimajor axis is only 0.1303 AU, around a third
   of the average distance between Mercury and the Sun. Despite this, it
   is located in the inner regions of the system's habitable zone, since
   Gliese 876 is such an intrinsically faint star.

   A limitation of the radial velocity method used to detect Gliese 876 c
   is that only a lower limit on the planet's mass can be obtained. In the
   case of Gliese 876 c, this lower limit is 62% of the mass of Jupiter.
   The true mass of the planet depends on the inclination of the orbit,
   which in general is unknown. However in a resonant system such as
   Gliese 876, gravitational interactions between the planets can be used
   to determine the true masses. Using this method, the orbital
   inclination is estimated to be around 50° with respect to the plane of
   the sky. In this case, the planet is around 30% more massive than the
   lower limit: around 0.81 Jupiter masses. On the other hand, astrometric
   measurements of the outer planet suggest that the inclination is around
   84°. Assuming the orbits are coplanar, this would mean the true mass is
   close to the lower limit.

Characteristics

   Based on its high mass, Gliese 876 c is likely to be a gas giant with
   no solid surface. Since it was detected indirectly through its
   gravitational effects on the star, properties such as its radius,
   composition and temperature are unknown. Assuming a composition similar
   to Jupiter and an environment close to chemical equilibrium, the planet
   is predicted to have a cloudless upper atmosphere.

   Gliese 876 c lies at the inner edge of the system's habitable zone.
   While the prospects for life on gas giants are unknown, it might be
   possible for a large moon of the planet to provide a habitable
   environment. Unfortunately tidal interactions between a hypothetical
   moon, the planet and the star could destroy moons massive enough to be
   habitable over the lifetime of the system. In addition it is unclear
   whether such moons could form in the first place.

   Retrieved from " http://en.wikipedia.org/wiki/Gliese_876_c"
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