HD 219134 g, also known as HR 8832 g, is an unconfirmed

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

orbiting around the

K-type star In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

HD 219134 HD 219134 (also known as Gliese 892 or HR 8832) is a main-sequence star in the constellation of Cassiopeia. It is smaller and less luminous than the Sun, with a spectral class of K3V, which makes it an orange-hued star. HD 219134 is rela ...

in the constellation of Cassiopeia. It has a

minimum mass In astronomy, minimum mass is the lower-bound calculated mass of observed objects such as planets, stars and binary systems, nebulae, and black holes. Minimum mass is a widely cited statistic for extrasolar planets detected by the radial veloc ...

of 11 or 15 Earth masses, suggesting that it is likely a

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

-like ice giant. Unlike

HD 219134 b HD 219134 b (or HR 8832 b) is one of at least five exoplanets orbiting HD 219134, a main-sequence star in the constellation of Cassiopeia. HD 219134 b has a size of about 1.6 , and a density of 6.4 g/cm3 and orbits at 21.25 light-years away. ...

and HD 219134 c it is not observed to transit and thus its radius and density are unknown. If it has an Earth-like composition, it would have a radius 1.9 times that of Earth. However, since it is probably a Neptune-like planet, it is likely larger.

Characteristics

Mass, Radius, and Temperature

HD 219134 g is a sub-Neptune-mass exoplanet with a minimum mass of and an unknown radius, as it is not known to transit. However, due to the high metal content of the host star, the planet might not be an ice giant. For a rock-iron composition, HD 219134 g would be about 1.9 , which seems unlikely for a planet of this mass. A more plausible rock-water composition would put the planet at about 2.4 . Assuming an albedo of 0.3, it has an

equilibrium temperature The planetary equilibrium temperature is a theoretical temperature that a planet would be if it were a black body being heated only by its parent star. In this model, the presence or absence of an atmosphere (and therefore any greenhouse effect) is ...

of , but with an atmosphere it is likely to have a surface temperature much higher, that is if it has a rocky surface. So, it is rather unlikely to be habitable.

Orbit

HD 219134 g was initially estimated to take about 94.2 days to orbits its star at a distance of 0.3753 AU. This is comparable to Mercury's orbit of 88 days at about 0.38 AU. However, due to the lower luminosity of the host star, HD 219134 g is closer to

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...

' situation. The eccentricity of the planet's orbit is believed to be close to zero, indicating a very circular orbit. However, a 2020 study did not find evidence of a radial velocity signal at this period, but instead found a period of 192 days, corresponding to an orbital distance of 0.603 AU.

Host Star

The planet HD 219134 g orbits the K3V orange dwarf

, also known as HR 8832. It is 79% the radius and 80% the mass of the Sun with 26% the luminosity. It has a temperature of 4699 K and is around 11 billion years old. For comparison, the Sun has a temperature of 5778 K and is 4.55 billion years old. The apparent magnitude of the star, or how bright it appears from Earth, is around 5. Therefore, it is just visible to the unaided eye to most observers.

Habitability

The habitable zone for this star, as defined by Kopparapu et al. 2014, for a 5 Earth mass planet, would be between 0.499 AU and 0.947 AU. HD 219134 g may orbit slightly interior to the inner edge of the habitable zone based on its initially published parameters, or may orbit within the habitable zone based on a more recent estimated orbital period of 192 days and semi-major axis of 0.603 AU. This planet is significantly more massive than Earth and therefore it likely retains a dense atmosphere, comparable to the Solar System's

ice giant An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune. In astrophysics and planetary science t ...

References

Notes

{{2015 in space Cassiopeia (constellation) Exoplanets discovered in 2015 Exoplanets detected by radial velocity