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Gliese 876 b

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

   CAPTION: Gliese 876 b

     Extrasolar planet    Lists of extrasolar planets

                      Artist's concept of Gliese 876 b

                      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) 0.208 ± 0.012 AU
   Eccentricity       (e) 0.0249 ± 0.0026
   Orbital period     (P) 60.940 ± 0.013 d
   Inclination        (i)  ?°
   Longitude of
   periastron         (ω) 175.7 ± 6.0°
   Time of periastron (τ) 2,452,460.3 ± 1.0 JD
                Physical characteristics
   Mass               (m) >1.93 ± 0.27 M[J]
   Radius             (r)  ? R[J]
   Density            (ρ)  ? kg/ m^3
   Temperature        (T)  ? K
                 Discovery information
   Discovery date         1999
   Discoverer(s)          Marcy et al.,
                          Delfosse et al.
   Detection method       Radial velocity
   Discovery status       Confirmed

   Gliese 876 b is an extrasolar planet orbiting the red dwarf star Gliese
   876 every 60.940 days. Discovered in 1999, Gliese 876 b was the first
   planet to be discovered orbiting a red dwarf. It is the outermost known
   planet in its planetary system.

Discovery

   Gliese 876 b was discovered independently by two different teams, one
   led by Geoffrey Marcy and the other by Xavier Delfosse. Like the
   majority of known extrasolar planets, it was discovered by detecting
   variations in its star's radial velocity as a result of the planet's
   gravity. This was done by making sensitive measurements of the Doppler
   shift of the spectral lines of Gliese 876.

Orbit and mass

   Gliese 876 b is in a 2:1 orbital resonance with the inner planet Gliese
   876 c, which leads to strong gravitational interactions between the two
   planets. As a result, the orbital elements of the planet change fairly
   rapidly as the orbits precess. The planet's orbit has a low
   eccentricity, similar to the planets in our solar system. The semimajor
   axis of the orbit is only 0.208 AU, less than that of Mercury in our
   solar system. However Gliese 876 is such a faint star that this puts it
   in the outer part of the habitable zone.

   A limitation of the radial velocity method used to detect Gliese 876 b
   is that only a lower limit on the planet's mass can be obtained. This
   lower limit is around 1.93 times the mass of Jupiter. The true mass
   depends on the inclination of the orbit, which in general is unknown.
   In the case of a resonant system like Gliese 876, gravitational
   interactions between the planets can be used to find the true masses:
   the best fit to the radial velocity data is for an inclination of
   around 50° to the plane of the sky. If this is the case, the true mass
   is approximately 30% greater than this lower limit, at around 2.5
   Jupiter masses. On the other hand, astrometric measurements suggest the
   orbital inclination is around 84°, which suggests the mass is only
   slightly greater than the lower limit.

Characteristics

   Given the planet's high mass, it is likely that Gliese 876 b is a gas
   giant with no solid surface. Since the planet has only been 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, it is predicted that the atmosphere of Gliese 876 b is
   cloudless, though cooler regions of the planet may be able to form
   water clouds.

   Gliese 876 b lies within the habitable zone of Gliese 876 b as defined
   by the ability of an Earth-mass planet to retain liquid water at its
   surface. While the prospects for life on a gas giant are unknown, large
   moons may be able to support a habitable environment. Models of tidal
   interactions between a hypothetical moon, the planet and the star
   suggest that large moons should be able to survive in orbit around
   Gliese 876 b for the lifetime of the system. On the other hand, it is
   unclear whether such moons could form in the first place.

   Retrieved from " http://en.wikipedia.org/wiki/Gliese_876_b"
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