Mu Arae d

Mu Arae d

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Mu Arae c, also known as HD 160691 c, is an extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

 orbiting around Mu Arae
Mu Arae
Mu Arae , often referred to by its designation in the Henry Draper catalogue HD 160691, is a main sequence G-type star around 50 light-years away from Earth in the constellation of Ara. The star has a planetary system with four known planets, three of them with masses comparable to that of Jupiter...

. Its discovery was announced on August 25, 2004. At the time, its minimum mass was reported at just 14 times that of Earth, although later work established a value of 10.5 Earth masses. It orbits very close to Mu Arae
Mu Arae
Mu Arae , often referred to by its designation in the Henry Draper catalogue HD 160691, is a main sequence G-type star around 50 light-years away from Earth in the constellation of Ara. The star has a planetary system with four known planets, three of them with masses comparable to that of Jupiter...

, completing one revolution every 9.6 days. The discovery was made with the aid of the High Accuracy Radial Velocity Planet Searcher
High Accuracy Radial Velocity Planet Searcher
The High Accuracy Radial velocity Planet Searcher is a high-precision echelle spectrograph installed in 2002 on ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003...

 (HARPS) spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...

, at the European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...

's La Silla Observatory
La Silla Observatory
La Silla Observatory is an astronomical observatory in Chile with three telescopes built and operated by the European Southern Observatory organisation, and several others are partly maintained by ESO...

 in Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...

. The data that revealed the presence of this planet was gathered on 8 nights of observations in June 2004.

Assuming its true mass is comparable to those of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

 and Gliese 436 b
Gliese 436 b
Gliese 436 b is a Neptune-sized extrasolar planet orbiting the red dwarf star Gliese 436. It was among the smallest known transiting planets in mass and radius until the much smaller Kepler discoveries started coming in 2010.-Discovery:...

, 14 Earth masses is theoretically the maximum size for a terrestrial planet. A rocky planet this size could certainly have formed, since Mu Arae has a higher metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

 than our Sun. Also, it is thought to have formed well inside the system's "snow line" at 3.2 AU. However various models of the system's formation have since converged that the planet attracted large amounts of volatiles before its star had cleared out the ice, and that it now has a core of only 6 Earth masses. Its core is likely enveloped in so much hot-ice and gas that the planet would behave more like Neptune.

Mu Arae c is too far from its sun to be subject to coronal mass ejections. There is disagreement as to whether it is and has always been a hot Neptune in mass (Lammer); or if it could have developed from a gas giant, losing most of its mass on the way (Baraffe).

If an eroded gas giant, the sun would have boiled the planet from a larger protoplanet, of 20 Earth masses up to half Jupiter's mass. If the latter, its current radius could be as high as 0.6 Jupiter.

The planet must be hot because of its closeness to Mu Arae. The discoverers chose for it an albedo of 0.35, lighter than albedos chosen to calculate the temperatures of hot Jupiters like, for instance, tau Boötis b. This was perhaps due to the discoverers' assumption that the planet would be a silicate super-terrestrial with neither clouds nor a deep Rayleigh-scattering atmosphere. If so the surface temperature would be about 900 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

.

None of the four planets orbiting Mu Arae are directly visible from Earth using currently available tools. All four were found using the radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...

method of extrasolar planet detection.