Editing Mu Arae (section)

== Planetary system ==
[[File:Exoplanets of Cervantes.ogg|thumb|Emulation in [[Celestia]] of the exoplanets of Cervantes based in the [[Sudarsky's gas giant classification]]:  Dulcinea, Rocinante, Quijote y Sancho.]]
[[File:Mu Arae system.jpg|thumb|right|The Mu Arae star with distance relationships for its four planets]]
[[File:MuAraeOuterOrbits.svg|thumb|right|220px|The orbits of the outer three planets in the Mu Arae system compared with those in the [[Solar System]]. Central star is not to scale. At the scale of this picture, the innermost planet would be located at the edge of the disc representing the central star.]]

=== Discovery ===
In 2001, an extrasolar planet was announced by the [[Anglo-Australian Planet Search]] team, together with the planet orbiting [[Epsilon Reticuli]]. The planet, designated [[Mu Arae b]], was thought to be in a highly [[eccentricity (orbit)|eccentric]] orbit of around 743 days.<ref name=Butler_2001/> The discovery was made by analysing variations in the star's [[radial velocity]] (measured by observing the [[Doppler shift]] of the star's [[spectral line]]s) as a result of being pulled around by the planet's [[gravity]].  Further observations revealed the presence of a second object in the system (now designated as [[Mu Arae e]]), which was published in 2004. At the time, the parameters of this planet were poorly constrained and it was thought to be in an orbit of around 8.2 years with a high eccentricity.<ref name="McCarthy2004"/> Later in 2004, a small inner planet designated [[Mu Arae c]] was announced with a mass comparable with that of [[Uranus]] in a 9-day orbit. This was the first of the class of planets known as "[[hot Neptune]]s" to be discovered. The discovery was made by making high-precision radial velocity measurements with the [[High Accuracy Radial Velocity Planet Searcher]] (HARPS) [[spectrograph]].<ref name="santos"/>

In 2006, two teams, one led by [[Krzysztof Goździewski]] and the other by [[Francesco Pepe]] independently announced four-planet models for the radial velocity measurements of the star, with a new planet ([[Mu Arae d]]) in a near-circular orbit lasting approximately 311 days.<ref name="gozdziewski"/><ref name="pepe"/> The new model gives revised parameters for the previously known planets, with lower eccentricity orbits than in the previous model and including a more robust characterization of the orbit of Mu Arae e. The discovery of the fourth planet made Mu Arae the second known four-planet extrasolar system, after [[55 Cancri]].

=== System architecture and habitability ===
The Mu Arae system consists of an inner Uranus-mass planet in a tight 9-day orbit and three massive planets, probably gas giants, on wide, near-circular orbits, which contrasts with the high-eccentricity orbits typically observed for long-period extrasolar planets. The Uranus-mass planet may be a [[chthonian planet]], the [[planetary core|core]] of a gas giant which has had its outer layers stripped away by stellar radiation.<ref name=Baraffe_et_al_2006/> Alternatively it may have formed in the inner regions of the Mu Arae system as a rocky "super-Earth".<ref name="santos"/>

The inner gas giants "d" and "b" are located close to the 2:1 [[orbital resonance]] which causes them to undergo strong interactions. The best-fit solution to the system is actually unstable:<ref name=Agnew_et_al_2019/><ref name="Benedict2022"/> simulations suggest the system is destroyed after 78 million years, which is significantly shorter than the estimated age of the star system. More stable solutions, including ones in which the two planets are actually in the resonance (similar to the situation in the [[Gliese 876]] system) can be found which give only a slightly worse fit to the data.<ref name="pepe"/> A 2022 study finds a stable orbital fit to the system, and estimates a lower limit on the system [[orbital inclination|inclination]] of about 20°.<ref name="Goździewski2022"/>

[[Astrometry|Astrometric]] observations using the [[Hubble Space Telescope]] have not detected any of the known planets, but have set upper limits on the masses of the outer three planets: planet b is {{Jupiter mass|<4.3|link=y}}, planet d is {{Jupiter mass|<7.0}}, and planet e is {{Jupiter mass|<4.4}}.<ref name="Benedict2022"/> Searches for [[circumstellar disc]]s show no evidence for a debris disc similar to the [[Kuiper belt]] around Mu Arae. If Mu Arae does have a Kuiper belt, it is too faint to be detected with current instruments.<ref name=Schütz_et_al_2004/>

The gas giant planet "b" is located in the liquid water habitable zone of Mu Arae. This would prevent an Earth-like planet from forming in the habitable zone, however large [[natural satellite|moons]] of the gas giant could [[Habitable exomoon|potentially support liquid water.]] On the other hand, it is unclear whether moons sufficiently massive to retain an atmosphere and liquid water could actually form around a gas giant planet, due to a theorized scaling law between the mass of a planet and its satellite system.<ref name=Canup_Ward_2006/> In addition, measurements of the star's [[ultraviolet]] [[flux]] suggest that any potentially [[planetary habitability|habitable]] planets or moons may not receive enough ultraviolet to trigger the formation of [[biomolecule]]s.<ref name="buccino"/> Planet "d" would receive a similar amount of ultraviolet to the Earth and thus lies in the [[Circumstellar_habitable_zone#Spectral_types_and_star-system_characteristics|ultraviolet habitable zone]]. However, it would be too hot for any moons to support surface liquid water.

{{OrbitboxPlanet begin
| table_ref = <ref name="Goździewski2022"/>
}}
{{OrbitboxPlanet
| exoplanet = [[Mu Arae c|c (Dulcinea)]]
| mass = {{val|0.032|0.002|p=≥}}
| period = {{val|9.638|0.001}}
| semimajor = {{val|0.092319|0.000005}}
| eccentricity = {{val|0.090|0.042}}
}}
{{OrbitboxPlanet
| exoplanet = [[Mu Arae d|d (Rocinante)]]
| mass = {{val|0.448|0.011|p=≥}}
| period = {{val|308.36|0.29}}
| semimajor = {{val|0.9347|0.0015}}
| eccentricity = {{val|0.055|0.014}}
}}
{{OrbitboxPlanet
| exoplanet = [[Mu Arae b|b (Quijote)]]
| mass = {{val|1.65|0.009|p=≥}}
| period = {{val|644.92|0.29}}
| semimajor = {{val|1.522|0.001}}
| eccentricity = {{val|0.041|0.009}}
}}
{{OrbitboxPlanet
| exoplanet = [[Mu Arae e|e (Sancho)]]
| mass = {{val|1.932|0.022|p=≥}}
| period = {{val|4019|24|fmt=commas}}
| semimajor = {{val|5.204|0.021}}
| eccentricity = {{val|0.049|0.011}}
}}
{{Orbitbox end}}