Gliese 876 d
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Gliese 876 d
Extrasolar planet List of extrasolar planets

An artist's impression of Gliese 876 d as a terrestrial planet.
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
Argument 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) 0.018±0.003 MJ
(>5.88±0.99 M⊕)
Discovery information
Discovery date 13 June 2005
Discoverer(s) Rivera et al.
Detection method Doppler spectroscopy
Discovery site California and
Carnegie Planet Search
 United States
Discovery status Published
Other designations
GJ 876 d, HIP 113020 d, IL Aquarii d, Ross 780 d
Database references
Extrasolar Planets
Encyclopaedia		 data
SIMBAD data

Gliese 876 d, also cataloged as IL Aquarii 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.

Contents

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

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 should in theory circularise 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. 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 had in 2002 suggested an inclination of around 84°, which (again assuming the system is coplanar) would imply the true mass of this inner planet d is only slightly greater than the lower limit.3 That team has not retracted their findings as of 2008.

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

Physical characteristics

The red dwarf star Gliese 876 and a its planet Gliese 876 d.

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 Venereal distribution of temperature and a maximum albedo of 0.8, the temperature was estimated 430–650 K.1

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 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. Unfortunately the planet does not appear to transit its star,1 which makes obtaining this information beyond our current observational capabilities.

See also

References

  1. ^ 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:10.1086/491669, http://www.journals.uchicago.edu/doi/full/10.1086/491669. 
  2. ^ Butler, R. et al. (2006). "Catalog of Nearby Exoplanets". The Astrophysical Journal 646: 505–522. doi:10.1086/504701, http://www.journals.uchicago.edu/doi/abs/10.1086/504701.  (web version)
  3. ^ 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:10.1086/322477, http://adsabs.harvard.edu/abs/2001ApJ...558..392R. 
  4. ^ Jackson, Brian; Richard Greenberg, Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". ApJ. arΧiv:0803.0026. 
  5. ^ 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 metre that the Earth does (1366 W/m^2), or 39,151 W/m^2.
  6. ^ 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:10.1051/0004-6361:20053453, http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2005A%26A...441..791F&db_key=AST&nosetcookie=1. 
  7. ^ 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. 
  8. ^ 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:10.1086/497094, http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2005ApJ...631L..85Z&db_key=AST&nosetcookie=1. 

External links

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