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Gliese 876 d

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

   CAPTION: Gliese 876 d

     Extrasolar planet           Lists of extrasolar planets

         Artist's rendition of Gliese 876 d as a terrestrial planet.

                             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) 0.0208 ± 0.0012 AU
   Eccentricity       (e) 0
   Orbital period     (P) 1.937760 ± 0.000070 d
   Inclination        (i)  ?°
   Longitude of
   periastron         (ω) 0°
   Time of periastron (τ) 2,452,488.33 ± 0.03 JD
                       Physical characteristics
   Mass               (m) >5.88 ± 0.99 M[E]
   Radius             (r)  ? R[J]
   Density            (ρ)  ? kg/ m^3
   Temperature        (T)  ? K
                        Discovery information
   Discovery date         2005
   Discoverer(s)          Rivera et al.
   Detection method       Radial velocity
   Discovery status       Confirmed

   Gliese 876 d is an extrasolar planet orbiting the red dwarf star Gliese
   876. At the time of its discovery in 2005, the planet had the lowest
   mass of any known extrasolar planet apart from the pulsar planets
   orbiting PSR B1257+12. Gliese 876 d takes less than two days to
   complete an orbit, at a distance only one-fiftieth of that between the
   Earth and the Sun and is the innermost known planet in its planetary
   system.

Discovery

   Like the majority of known extrasolar planets, Gliese 876 d was
   discovered by analysing changes in its star's radial velocity as a
   result of the planet's gravity. The radial velocity measurements were
   made by observing the Doppler shift in the star's spectral lines. At
   the time of discovery, Gliese 876 was known to host two extrasolar
   planets, designated Gliese 876 b and c, in a 2:1 orbital resonance.
   After the two planets were taken into account, the radial velocity
   still showed another period, at around 2 days, which could be
   interpreted as an additional planet with a mass at least 5.9 times that
   of Earth. The planet, designated Gliese 876 d, was announced in 2005 by
   a team led by Eugenio Rivera.

Orbit and mass

   Gliese 876 d is located in an orbit with a semimajor axis of only
   0.0208 AU (3.11 million km). At this distance from the star, tidal
   interactions would be expected to circularise the orbit, however
   orbital solutions to the radial velocities suggest that the value of
   the eccentricity could be as high as 0.22.

   A limitation of the radial velocity method used to detect Gliese 876 d
   is that only a lower limit on the mass can be obtained. In this case,
   the lower limit is 5.88 times the mass of Earth. The true mass depends
   on the inclination of the orbit, which in general is unknown. However,
   taking the gravitational interactions between the resonant outer
   planets into account and assuming that the system is coplanar, the
   inclination of the orbit of Gliese 876 d may be around 50° with respect
   to the plane of the sky, yielding a true mass of around 7.5 Earth
   masses. On the other hand, astrometric measurements of the outer planet
   Gliese 876 b suggest an inclination of around 84°, which would imply
   the true mass is only slightly greater than the lower limit.

Characteristics

   Since Gliese 876 d has only been detected indirectly by its
   gravitational effects on its star, properties such as its radius,
   composition and temperature are unknown, though the planet is likely to
   suffer high temperatures thanks to its proximity to the star. The low
   mass of the planet has led to suggestions that it may be a terrestrial
   planet. Assuming a density of around 8,000 kg/ m^3 to account for
   greater compression of material in a more massive planet than Earth, a
   terrestrial planet of 7.5 Earth masses would have a radius 73% greater
   than that of the Earth. This type of massive terrestrial planet could
   be formed in the inner part of the Gliese 876 system from material
   pushed towards the star by the inward migration of the gas giants.

   Alternatively the planet could have formed further from Gliese 876 and
   migrated inwards with the gas giants. This would result in a
   composition richer in volatile substances, such as water. In this
   model, the planet would have a pressurised ocean of liquid water
   separated from the silicate core by a layer of ice kept frozen by the
   high pressures in the planetary interior. Such a planet would have an
   atmosphere containing water vapor and free oxygen produced by the
   breakdown of water by ultraviolet radiation.

   Distinguishing between these two models would require more information
   about the planet's radius or composition. Unfortunately the planet does
   not appear to transit its star, which makes obtaining this information
   beyond our current observational capabilities.

   Retrieved from " http://en.wikipedia.org/wiki/Gliese_876_d"
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