Gliese 876 d

Gliese 876 d is an exoplanet 15.2 light-years (4.7 parsecs) away in the constellation of Aquarius. The planet was the third planet discovered orbiting the red dwarf Gliese 876, and is the innermost planet in the system. It was the lowest-mass known exoplanet apart from the pulsar planets orbiting PSR B1257+12 at the time of its discovery. Due to its low mass, it can be categorized as a super-Earth.

Gliese 876 d
An artist's impression of Gliese 876 d as a terrestrial planet, with volcanic activity occurring on its night side
Discovery[1]
Discovered byRivera et al.
Discovery siteCalifornia and
Carnegie Planet Search
Discovery dateJune 13, 2005
Doppler spectroscopy
Orbital characteristics[2][3]
Epoch BJD 2,450,602.09311
0.021020525+0.000000047
−0.000000053 AU
Eccentricity0.035+0.033
−0.024
1.9377904+0.0000064
−0.0000073 d
104°+46.7°
−46.9°
Inclination56.7°+1.0°
−0.99°
120°+70°
−56°
Semi-amplitude6.0±0.19 m/s
StarGliese 876
Physical characteristics
Mass6.68±0.22 M🜨[3]
Temperature614 K (341 °C; 646 °F)

    Characteristics

    Mass, radius, and temperature

    The mass of any exoplanet from radial 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 the resonant outer planets enables the inclination of the orbits to be determined. This reveals that the outer planets are nearly coplanar 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 a terrestrial 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 inward migration 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 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.[6] 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.[7]

    Distinguishing between these two models would require more information about the planet's radius or composition. The planet does not transit its star,[1] which makes obtaining this information impossible with current observational capabilities.

    The equilibrium temperature of Gliese 876 d, is estimated to be around 614 K (341 °C; 646 °F).[8]

    Host star
    Main article: Gliese 876

    The planet orbits a (M-type) star named Gliese 876. The star has a mass of 0.33 M☉ and a radius of around 0.36 R☉. It has a surface temperature of 3350 K 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]

    Orbit

    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 should in theory circularize the orbit; however, measurements reveal that it has a high eccentricity of 0.207, comparable to that of Mercury 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 for Io 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]

    Discovery

    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 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]

    Notes
    1. Star emits about 1.24% energy of the Sun, planet is at 0.0208 AU distance so receives 0.0124*48*48 times the energy per square meter that the Earth does (1366 W/m2), or 39,151 W/m2.

    References
    1. Rivera, Eugenio J.; et al. (2005). "A ~7.5 M🜨 Planet Orbiting the Nearby Star, GJ 876". The Astrophysical Journal. 634 (1): 625–640. arXiv:astro-ph/0510508. Bibcode:2005ApJ...634..625R. doi:10.1086/491669. S2CID 14122053.
    2. Millholland, Sarah; et al. (2018). "New Constraints on Gliese 876—Exemplar of Mean-motion Resonance". The Astronomical Journal. 155 (3) 106. arXiv:1801.07831. Bibcode:2018AJ....155..106M. doi:10.3847/1538-3881/aaa894. S2CID 119011611.
    3. Moutou, C.; Delfosse, X.; et al. (July 2023). "Characterizing planetary systems with SPIRou: M-dwarf planet-search survey and the multiplanet systems GJ 876 and GJ 1148". Astronomy & Astrophysics. arXiv:2307.11569.
    4. Rivera, Eugenio J.; et al. (July 2010). "The Lick-Carnegie Exoplanet Survey: A Uranus-mass Fourth Planet for GJ 876 in an Extrasolar Laplace Configuration". The Astrophysical Journal. 719 (1): 890–899. arXiv:1006.4244. Bibcode:2010ApJ...719..890R. doi:10.1088/0004-637X/719/1/890. S2CID 118707953.
    5. Fogg, M. J.; Nelson, R. P. (2005). "Oligarchic and giant impact growth of terrestrial planets in the presence of gas giant planet migration". Astronomy and Astrophysics. 441 (2): 791–806. arXiv:astro-ph/0507180. Bibcode:2005A&A...441..791F. doi:10.1051/0004-6361:20053453. S2CID 15248175. Archived from the original on 2021-02-25. Retrieved 2011-12-26.
    6. Lammer, H.; et al. (2007). "The impact of nonthermal loss processes on planet masses from Neptunes to Jupiters" (PDF). Geophysical Research Abstracts. 9 (7850). Archived (PDF) from the original on 2019-12-15. Retrieved 2008-08-13.
    7. 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 Letters. 631 (1): L85–L88. arXiv:astro-ph/0508305. Bibcode:2005ApJ...631L..85Z. doi:10.1086/497094. S2CID 16632198.
    8. "Archived copy". Archived from the original on 2016-08-20. Retrieved 2016-08-04.{{cite web}}: CS1 maint: archived copy as title (link)
    9. Fraser Cain (16 September 2008). "How Old is the Sun?". Universe Today. Archived from the original on 18 August 2010. Retrieved 19 February 2011.
    10. Fraser Cain (September 15, 2008). "Temperature of the Sun". Universe Today. Archived from the original on 29 August 2010. Retrieved 19 February 2011.
    11. Jackson, Brian; et al. (2008). "Tidal Heating of Extra-Solar Planets". The Astrophysical Journal. 681 (2): 1631–1638. arXiv:0803.0026. Bibcode:2008ApJ...681.1631J. doi:10.1086/587641. S2CID 42315630.

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