Gliese 876 d
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| Extrasolar planet | List of extrasolar planets | |
|---|---|---|
 |
||
| Parent star | ||
| Star | Gliese 876 | |
| Constellation | Aquarius | |
| Right ascension | (α) | 22h 53m 16.73s |
| Declination | (δ) | −14° 15′ 49.3″ |
| Distance | 15.3 ly (4.7 pc) | |
| 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 |
| Angular distance | (θ) | 4.408 mas |
| Longitude of periastron |
(ω) | 0° |
| Time of periastron | (T0) | 2,452,488.33 ± 0.03 JD |
| Semi-amplitude | (K) | 6.46 ± 0.59 m/s |
| Physical characteristics | ||
| Mass | (m) | >5.88 ± 0.99 M⊕ |
| Discovery information | ||
| Discovery date | June 13, 2005 | |
| Discoverer(s) | Rivera et al. | |
| Detection method | Radial velocity | |
| Discovery site | Virginia, USA | |
| Discovery status | Published | |
| Database references | ||
| Exoplanet | [data] | |
Gliese 876 d is an extrasolar planet orbiting the red dwarf star Gliese 876. At the time of its discovery in June 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. Due to its low mass it can be categorized as a Super-Earth.
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[edit] 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.[1]
[edit] Orbit and mass
Gliese 876 d is located in an orbit with a semimajor axis of only 0.0208 AU (3.11 million km).[2] 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.[1]
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, the gravitational interactions between the resonant outer planets suggest that the inclination of the outer two planets is around 50° with respect to the plane of the sky. Assuming that Gliese 876 d orbits in the same plane as the outer two planets, this would imply a true mass of around 7.5 times that of the Earth.[1] On the other hand, astrometric measurements of the outer planet Gliese 876 b suggest an inclination of around 84°, which (again assuming the system is coplanar) would imply the true mass is only slightly greater than the lower limit.[3]
Due to its extremely eccentric orbit, models predict that tidal heating will play a significant role in the planet's geology. In fact, they predict that the planet may well be kept in a completely molten state. Predicted total heat flux is approximately 104-5 W/m2 at the planet's surface; for comparison the surface heat flux for Io is around 3 W/m2.[4]
[edit] 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 due 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/m3 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.[1] 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.[5]
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 water (in the form of a supercritical fluid) 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.[6]
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,[1] which makes obtaining this information beyond our current observational capabilities.
[edit] See also
[edit] References
- ^ a b c d e Rivera, E. et al. (2005). "A ~7.5 M⊕ Planet Orbiting the Nearby Star, GJ 876". The Astrophysical Journal 634 (1): 625–640. doi:.
- ^ Butler, R. et al. (2006). "Catalog of Nearby Exoplanets". The Astrophysical Journal 646: 505–522. doi:. (web version)
- ^ Rivera, E., Lissauer, J. (2001). "Dynamical Models of the Resonant Pair of Planets Orbiting the Star GJ 876". The Astrophysical Journal 558 (1): 392–402. doi:.
- ^ Jackson, Brian; Richard Greenberg, Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". ApJ. arXiv:0803.0026.
- ^ Fogg, M., Nelson, R. (2005). "Oligarchic and giant impact growth of terrestrial planets in the presence of gas giant planet migration". Astronomy and Astrophysics 441 (2): 791–806. doi:.
- ^ Zhou, J.-L. et al. (2005). "Origin and Ubiquity of Short-Period Earth-like Planets: Evidence for the Sequential Accretion Theory of Planet Formation". The Astrophysical Journal 631 (1): L85–L88. doi:.
