Gliese 876 d

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

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) 22h 53m 16.73s
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
Time of periastron (τ) 2,452,488.33 ± 0.03 JD
Physical characteristics
Mass (m) >5.88 ± 0.99 ME
Radius (r)  ? RJ
Density (ρ)  ? kg/ m3
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.


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.


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/ 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. 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.

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