HD 219134 b (or HR 8832 b) is one of at least five exoplanets orbiting

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 rel ...

, a

main-sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...

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. The exoplanet was initially detected by the instrument

HARPS-N HARPS-N, the ''High Accuracy Radial velocity Planet Searcher for the Northern hemisphere'' is a high-precision radial-velocity spectrograph, installed at the Italian Telescopio Nazionale Galileo, a 3.58-metre telescope located at the Roque de los ...

of the Italian

Telescopio Nazionale Galileo The Galileo National Telescope, ( it, Telescopio Nazionale Galileo; TNG; code: Z19) is a 3.58-meter Italian telescope, located at the Roque de los Muchachos Observatory on the island of La Palma in the Canary Islands, Spain. The TNG is operated ...

via the

radial velocity method Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in t ...

and subsequently observed by the Spitzer telescope as transiting in front of its star. The exoplanet has a mass of about 4.5 times that of Earth and orbits its host star every three days. In 2017, it was found that the planet likely hosts an atmosphere.

Characteristics

Mass, radius and temperature

HD 219134 b is a

super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...

, an exoplanet with a radius and mass bigger than Earth, but smaller than that of the ice giants Neptune and

Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...

. Using both the radial velocity and transit method, both its mass and radius have been well determined, allowing for accurate modelling of the planet's composition. HD 219134 b has a radius of 1.602 and a mass of 4.74 , giving it a density of about 6.4 g/cm³ and 1.85 times the gravity of Earth. This is consistent with a rocky, Earth-like composition. This is relatively unusual, as most planets of ≥1.6 are expected to be rich in volatile materials, such as water and gas. Despite its Earth-like composition, the planet's equilibrium temperature is around , far too hot for liquid water or life. Depending on the amount of cloud cover in the atmosphere of HD 219134 b, the actual temperature may be somewhat lower, but nowhere near the range for liquid water.

Host star

The planet orbits a ( K-type) star named

, orbited by a total of five to seven planets. The star has a mass of 0.81 and a radius of 0.778 . It has a temperature of 4699 K and is about 11.0 billion years old, making it one of the oldest stars. In comparison, the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

is 4.6 billion years old and has a temperature of 5778 K. The star's

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

, or how bright it appears from Earth's perspective, is 5. It can be seen with the naked eye.

Orbit

HD 219134 b orbits its host star with about 28% of the Sun's luminosity with an orbital period of 3 days and an orbital radius of about 0.03 times that of Earth's (compared to the distance of Mercury from the Sun, which is about 0.38 AU).

Atmosphere

In 2017, it was predicted that HD 219134 b and HD 219134 c likely have atmospheres that are secondary in nature, based on the compositions of the planets and the potential for atmospheric escape. For HD 219134 b, the predicted atmosphere was calculated to reach a height of about 0.18 (~1,150 km), below the predicted height of a primordial hydrogen atmosphere (0.28 ). This would indicate a secondary atmosphere produced by processes like volcanic activity and evaporation of volatile materials. However, the composition of volatile materials on HD 219134 b and c could not be accurately determined, yet it is believed that the former has a very thin gaseous envelope, far thinner than those of Uranus and Neptune.

References

External links

Cassiopeia Constellation at Constellation Guide

The Deep Photographic Guide to the Constellations: Cassiopeia
{{Sky, 23, 13, 16.98, +, 57, 10, 06.1, 100000 Cassiopeia (constellation) Exoplanets discovered in 2015 Terrestrial planets Transiting exoplanets Exoplanets detected by radial velocity Near-Earth-sized exoplanets Articles containing video clips