Gliese 876 d
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| Extrasolar planet | List of extrasolar planets | |
|---|---|---|
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| Parent star | ||
| Star | Gliese 876 | |
| Constellation | Aquarius | |
| Right ascension | (α) | 22h 53m 16.73s |
| Declination | (δ) | −14° 15′ 49.3″ |
| Apparent magnitude | (mV) | 10.17 |
| Distance | 15.3 ly (4.7 pc) |
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| Spectral type | M3.5V | |
| Orbital elements Epoch HJD 2,452,490.0 |
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| Semimajor axis | (a) | 0.0208069+0.0000004−0.0000004[1] AU |
| Eccentricity | (e) | 0.0[1][note 1] |
| Orbital period | (P) | 1.9379[note 2] d |
| Inclination | (i) | 47.2[1][note 3]° |
| Longitude of the node | (Ω) | 252.3[1][note 3]° |
| Argument of periastron |
(ω) | 0.0[1][note 1]° |
| Mean anomaly | (M) | 312.2+4.9−5.0[1]° |
| Physical characteristics | ||
| Mass | (m) | 8.41+0.78−0.75[1][note 3] M⊕ |
| Discovery information | ||
| Discovery date | June 13, 2005 | |
| Discoverer(s) | Rivera et al. | |
| Detection method | Doppler spectroscopy | |
| Discovery site | California and Carnegie Planet Search  United States |
|
| Discovery status | Published | |
| Database references | ||
| Extrasolar Planets Encyclopaedia |
data | |
| SIMBAD | data | |
Gliese 876 d is an extrasolar planet approximately 15 light-years away in the constellation of Aquarius (the Water-bearer). The planet was the third planet discovered orbiting the red dwarf star Gliese 876. At the time of its discovery, the planet had the lowest mass of any known extrasolar planet apart from the pulsar planets orbiting PSR B1257+12. Due to this 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 two days. The planet, designated Gliese 876 d, was announced in 2005 by a team led by Eugenio Rivera and was estimated to have a mass approximately 7.5 times that of Earth.[2]
[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).[3] At this distance from the star, tidal interactions should in theory circularize the orbit. According to the 2006 Catalog of Nearby Exoplanets, the planets are close enough to one another that they interact with each other much as Europa and Ganymede interact with Io. Gliese 876 d's orbit is, like Io's, not Keplerian.
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. This is because the measured mass value also depends on the orbital inclination, which in general is unknown. However, models incorporating astrometric measurements and the gravitational interactions between the resonant enables the orientation of the orbit to be determined. This reveals that the outer planets are nearly coplanar with an inclination of around 50° with respect to the plane of the sky. Assuming that Gliese 876 d orbits in the same plane as Gliese 876 b, the true mass of the planet is revealed to be 8.41 times the mass of Earth.[1]
Models predict that, if its non-Keplerian orbit could be averaged to a Keplerian eccentricity of 0.28, then tidal heating would play a significant role in the planet's geology to the point of keeping it completely molten. 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] This is similar to the energy it receives from its parent star of about 40,000 W/m2.[5]
[edit] Physical 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. On the assumption of a Venerean distribution of temperature and a maximum albedo of 0.8, the temperature was estimated at 430–650 K.[2]
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.[2] This would lead to a surface gravity of roughly 2.5 g. 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.[6]
Alternatively the planet could have formed further from Gliese 876, as a gas giant, and migrated inwards with the other gas giants. This would result in a composition richer in volatile substances, such as water. As it arrived in range, the star would have blown off the planet's hydrogen layer via coronal mass ejection.[7] 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.[8]
Distinguishing between these two models would require more information about the planet's radius or composition. The planet does not appear to transit its star,[2] which makes obtaining this information beyond current observational capabilities.
[edit] See also
[edit] Notes
- ^ a b The eccentricity and argument of periastron were fixed to zero in the orbital solution.
- ^ Orbital period calculated from the semimajor axis using Kepler's third law with a mass of 0.32 solar masses for Gliese 876.
- ^ a b c The orbital inclination and longitude of the node are assumed to be the same as for Gliese 876 b. This value of the inclination is used to derive the value for the planet's true mass.
[edit] References
- ^ a b c d e f g h Bean, Jacob L.; Andreas Seifahrt (2009). "The architecture of the GJ876 planetary system. Masses and orbital coplanarity for planets b and c". arΧiv: 0901.3144 [astro-ph].
- ^ a b c d Rivera, E. et al. (2005). "A ~7.5 M⊕ Planet Orbiting the Nearby Star, GJ 876". The Astrophysical Journal 634 (1): 625–640. doi:. http://www.journals.uchicago.edu/doi/full/10.1086/491669.
- ^ Butler et al. (2006). "Catalog of Nearby Exoplanets". The Astrophysical Journal 646 (1): 505–522. doi:. http://www.iop.org/EJ/article/0004-637X/646/1/505/64046.html. (web version)
- ^ Jackson, Brian; Richard Greenberg, Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". ApJ 681: 1631. doi:. arΧiv:0803.0026.
- ^ Star emits about 1.24% energy of the Sun, planet is at 0.0208 A.U. distance so receives 0.0124*48*48 times the energy per square meter that the Earth does (1366 W/m^2), or 39,151 W/m^2.
- ^ 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:. http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2005A%26A...441..791F&db_key=AST&nosetcookie=1.
- ^ H. Lammer et al. (2007). "The impact of nonthermal loss processes on planet masses from Neptunes to Jupiters". Geophysical Research Abstracts 9 (07850). http://www.cosis.net/abstracts/EGU2007/07850/EGU2007-J-07850.pdf?PHPSESSID=1eb3a7a98603083dda25d18001ea2a33.
- ^ 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:. http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2005ApJ...631L..85Z&db_key=AST&nosetcookie=1.
[edit] External links
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Wikimedia Commons has media related to: Gliese 876 d |
- "Artist renderings and data for the three planets orbiting GJ 876". http://exoplanets.org/gl876_web/gl876_graphics.html. Retrieved on 2008-06-21.
- "GJ 876 d -- Extra-solar Planet Candidate". SIMBAD. http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=GJ+876+d. Retrieved on 2008-06-21.
- "Gliese 876 d". Extrasolar Visions. http://www.extrasolar.net/planettour.asp?StarCatId=normal&PlanetId=296. Retrieved on 2008-06-21.
- "Smallest extrasolar planet found". BBC News. 2005-06-13. http://news.bbc.co.uk/2/hi/science/nature/4089534.stm. Retrieved on 2008-06-21.
- www.extrasolar.net
- GJ 876 d Catalog
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