 An artist's impression of Gliese 876 d as aterrestrial planet, with volcanic activity occurring on its night side | |
| Discovery[1] | |
|---|---|
| Discovered by | Riveraet al. |
| Discovery site | California and Carnegie Planet Search |
| Discovery date | June 13, 2005 |
| Doppler spectroscopy | |
| Orbital characteristics[2][3] | |
| EpochBJD2,450,602.09311 | |
| 0.021020525+0.000000047 −0.000000053 AU | |
| Eccentricity | 0.035+0.033 −0.024 |
| 1.9377904+0.0000064 −0.0000073 d | |
| 104°+46.7° −46.9° | |
| Inclination | 56.7°+1.0° −0.99° |
| 120°+70° −56° | |
| Semi-amplitude | 6.0±0.19 m/s |
| Star | Gliese 876 |
| Physical characteristics | |
| Mass | 6.68±0.22 M🜨[3] |
| Temperature | 614 K (341 °C; 646 °F) |
Gliese 876 d is anexoplanet 15.2light-years (4.7parsecs) away in theconstellation ofAquarius. The planet was the third planet discoveredorbiting thered dwarfGliese 876, and is the innermost planet in the system. It was the lowest-mass known exoplanet apart from thepulsar planets orbitingPSR B1257+12 at the time of its discovery. Due to its mass, it can be categorized as asuper-Earth.
The mass of any exoplanet fromradial velocity has one problem, in 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, in the case of Gliese 876, models incorporating the gravitational interactions between theresonant outer planets enables the inclination of the orbits to be determined. This reveals that the outer planets are nearlycoplanar with an inclination of around 59° with respect to the plane of the sky. Assuming that Gliese 876 d orbits in the same plane as the other planets, the true mass of the planet is revealed to be 6.83 times the mass of Earth.[4]
The low mass of the planet has led to suggestions that it may be aterrestrial planet. 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 inwardmigration of the gas giants.[5]
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 involatile substances, such aswater. As it arrived in range, the star would have blown off the planet's hydrogen layer viacoronal mass ejection.[6] In this model, the planet would have apressurisedocean of water (in the form of asupercritical fluid) separated from thesilicatecore by a layer ofice kept frozen by the high pressures in the planetary interior. Such a planet would have an atmosphere containingwater vapor and freeoxygen produced by the breakdown of water byultraviolet radiation.[7]
Distinguishing between these two models would require more information about the planet's radius or composition. The planet does nottransit its star,[1] which makes obtaining this information impossible with current observational capabilities.
Theequilibrium temperature of Gliese 876 d is estimated to be around 614 K (341 °C; 646 °F).[8]
The planet orbits a (M-type)star namedGliese 876. The star has a mass of 0.33M☉ and a radius of around 0.36R☉. It has a surface temperature of 3350K and is 2.55 billion years old. In comparison, the Sun is about 4.6 billion years old[9] and has a surface temperature of 5778 K.[10]
Gliese 876 d is located in an orbit with asemimajor axis of only 0.0208AU (3.11 million km). At this distance from the star,tidal interactions should in theory circularize the orbit; however, measurements reveal that it has a high eccentricity of 0.207, comparable to that ofMercury in the Solar System.[4]
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 forIo is around 3 W/m2.[11] This is similar to the radiative energy it receives from its parent star of about 40,000 W/m2.[note 1]
Gliese 876 d was discovered by analysing changes in its star'sradial velocity as a result of the planet'sgravity. The radial velocity measurements were made by observing theDoppler shift in the star'sspectral lines. At the time of discovery, Gliese 876 was known to host two extrasolar planets, designatedGliese 876 b andc, in a 2:1orbital 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 on June 13, 2005 by a team led by Eugenio Rivera and was estimated to have a mass approximately 7.5 times that of Earth.[1]
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