Editing Epsilon Eridani (section)

== Planetary system ==
{{OrbitboxPlanet begin
| table_ref = <ref name=apj690_2_1522 /><ref name="SuDe Buizer2017" /><ref name="Booth2023"/><ref name="Feng2023"/>
| name = Epsilon Eridani
}}
{{OrbitboxPlanet disk
| disk = Asteroid belt
| periapsis = ~1.5−2.0 (or 3–4)
}}
{{OrbitboxPlanet
| exoplanet = [[Epsilon Eridani b|b (AEgir)]]<ref name=naming_exoplanets />
| mass = {{val|0.76|0.14|0.11}}
| period = {{val|2688.60|16.17|16.51|fmt=commas}}
| semimajor = {{val|3.53|0.06}}
| eccentricity = {{val|0.26|0.04}}
| inclination = {{val|49.40|12.62|9.53}}
}}
{{OrbitboxPlanet disk
| disk = Asteroid belt
| periapsis = ~8
| apoapsis = 20
}}
{{OrbitboxPlanet disk
| disk = Main belt
| periapsis = 65
| apoapsis = 75
| inclination = {{nowrap|33.7° ± 0.5}}
}}
{{Orbitbox end}}

=== Debris disc ===
[[File:Epsilon Eridani ALMA Image.png|right|thumb|Image of epsilon Eridani's main belt taken by the Atacama Large Millimeter/submillimeter Array (ALMA) at a wavelength of 1.3mm. The star is seen at the centre and two other point sources (one coincident with the belt) are unrelated background galaxies.<ref name="Booth2023" /> |alt=The star is seen at the centre and the ring shows the main belt of the debris disc, which is located at 70 astronomical units from the star. The belt appears elliptical as it is slightly inclined from face-on. In addition to the star, two other point sources appear in the image (one coincident with the belt). These are background galaxies and not part of the epsilon Eridani system.]]

An infrared excess around Epsilon Eridani was detected by IRAS<ref name="pasp97_885" /> indicating the presence of circumstellar dust. Observations with the [[James Clerk Maxwell Telescope]] (JCMT) at a [[wavelength]] of 850&nbsp;μm show an extended flux of radiation out to an [[Angular diameter|angular radius]] of 35&nbsp;arcseconds around Epsilon Eridani, resolving the debris disc for the first time. Higher resolution images have since been taken with the [[Atacama Large Millimeter Array]], showing that the belt is located 70 au from the star with a width of just 11 au.<ref name="Booth2017" /><ref name="Booth2023" /> The disc is inclined 33.7° from face-on, making it appear elliptical.

Dust and possibly water ice from this belt migrates inward because of drag from the stellar wind and a process by which stellar radiation causes dust grains to slowly spiral toward Epsilon Eridani, known as the [[Poynting–Robertson effect]].<ref name=arxiv1011_4882 /> At the same time, these dust particles can be destroyed through mutual collisions. The time scale for all of the dust in the disk to be cleared away by these processes is less than Epsilon Eridani's estimated age. Hence, the current dust disk must have been created by collisions or other effects of larger parent bodies, and the disk represents a late stage in the planet-formation process. It would have required collisions between 11 Earth masses' worth of parent bodies to have maintained the disk in its current state over its estimated age.<ref name=apj690_2_1522 />

[[File:System Epsilon Eridani.JPG|right|thumb|Comparison of the planets and debris belts in the Solar System to the Epsilon Eridani system. At the top is the asteroid belt and the inner planets of the Solar System. Second from the top is the proposed inner asteroid belt and planet b of Epsilon Eridani. The lower illustrations show the corresponding features for the two stars' outer systems. |alt=The upper two illustrations show brown oval bands for the asteroid belts and oval lines for the known planet orbits, with the glowing star at the centre. The second brown band is narrower than the first. The lower two illustrations have grey bands for the comet belts, oval lines for the planetary orbits and the glowing stars at the centre. The lower grey band is much wider than the upper grey band.]]

The disk contains an estimated mass of dust equal to a sixth of the mass of the Moon, with individual dust grains exceeding 3.5&nbsp;μm in size at a temperature of about 55&nbsp;K. This dust is being generated by the collision of comets, which range up to 10 to 30&nbsp;km in diameter and have a combined mass of 5 to 9 times that of Earth. This is similar to the estimated 10 Earth masses in the primordial Kuiper belt.<ref name=apj619_2_L187 /><ref name=emp92_1_1 /> The disk around Epsilon Eridani contains less than {{nowrap|2.2 × 10<sup>17</sup> kg}} of [[carbon monoxide]]. This low level suggests a paucity of volatile-bearing comets and icy [[planetesimal]]s compared to the Kuiper belt.<ref name=mnras348_3_L39 />

The JCMT images show signs of clumpy structure in the belt that may be explained by gravitational perturbation from a planet, dubbed Epsilon Eridani c. The clumps in the dust are theorised to occur at orbits that have an integer resonance with the orbit of the suspected planet. For example, the region of the disk that completes two orbits for every three orbits of a planet is in a 3:2 [[orbital resonance]].<ref name=apjl_537_L147 /> The planet proposed to cause these perturbations is predicted to have a semimajor axis of between 40 and 50 au.<ref name=apj578_2_L149 /><ref name="Deller2005" /><ref name="Booth2023" /> However, the brightest clumps have since been identified as background sources and the existence of the remaining clumps remains debated.<ref name="Chavez2016" />

Dust is also present closer to the star. Observations from NASA's [[Spitzer Space Telescope]] suggest that Epsilon Eridani actually has two asteroid belts and a cloud of [[exozodiacal dust]]. The latter is an analogue of the [[zodiacal dust]] that occupies the plane of the [[Solar System]]. One belt sits at approximately the same position as the one in the Solar System, orbiting at a distance of {{nowrap|3.00 ± 0.75 au}} from Epsilon Eridani, and consists of [[silicate]] grains with a diameter of 3&nbsp;[[Micrometre|μm]] and a combined mass of about 10<sup>18</sup>&nbsp;kg. If the planet Epsilon Eridani b exists then this belt is unlikely to have had a source outside the orbit of the planet, so the dust may have been created by fragmentation and cratering of larger bodies such as [[asteroid]]s.<ref name=aaa499_2_L13 /> The second, denser belt, most likely also populated by asteroids, lies between the first belt and the outer comet disk. The structure of the belts and the dust disk suggests that more than two planets in the Epsilon Eridani system are needed to maintain this configuration.<ref name=apj690_2_1522 /><ref name=spitzer20081027 />

In an alternative scenario, the exozodiacal dust may be generated in the outer belt. This dust is then transported inward past the orbit of Epsilon Eridani b. When collisions between the dust grains are taken into account, the dust will reproduce the observed infrared spectrum and brightness. Outside the radius of ice [[Sublimation (phase transition)|sublimation]], located beyond 10&nbsp;au from Epsilon Eridani where the temperatures fall below 100&nbsp;K, the best fit to the observations occurs when a mix of ice and [[silicate]] dust is assumed. Inside this radius, the dust must consist of silicate grains that lack [[Volatile (astrogeology)|volatiles]].<ref name=arxiv1011_4882 />

The inner region around Epsilon Eridani, from a radius of 2.5&nbsp;AU inward, appears to be clear of dust down to the detection limit of the 6.5&nbsp;m [[MMT Observatory|MMT telescope]]. Grains of dust in this region are efficiently removed by drag from the stellar wind, while the presence of a planetary system may also help keep this area clear of debris. Still, this does not preclude the possibility that an inner asteroid belt may be present with a combined mass no greater than the asteroid belt in the Solar System.<ref name=apj693_2_1500 />

=== Long-period planets ===
[[File:NASA-JPL-Caltech - Double the Rubble (PIA11375) (pd).jpg|right|thumb|Artist's impression, showing two asteroid belts and a planet orbiting Epsilon Eridani|alt=A bright light source at right is encircled by comets and two oval belts of debris. At left is a yellow-orange crescent of a planet.]]
As one of the nearest Sun-like stars, Epsilon Eridani has been the target of many attempts to search for planetary companions.<ref name=apj544_2_L145 /><ref name=aaa488_2_771 /> Its chromospheric activity and variability mean that finding planets with the [[Methods of detecting extrasolar planets#Radial velocity|radial velocity method]] is difficult, because the stellar activity may create signals that mimic the presence of planets.<ref name=setiawan2008 /> Searches for exoplanets around Epsilon Eridani with [[direct imaging]] have been unsuccessful.<ref name=apj133_6_2442 /><ref name=apj688_1_583 />

Infrared observation has shown there are no bodies of three or more [[Jupiter mass]]es in this system, out to at least a distance of 500&nbsp;au from the host star.<ref name=aaa488_2_771 /> Planets with similar masses and temperatures as Jupiter should be detectable by Spitzer at distances beyond 80&nbsp;au. One roughly Jupiter-sized long-period planet has been detected and characterized by both the radial velocity and astrometry methods.<ref name="Feng2023"/> Planets more than 150% as massive as Jupiter can be ruled out at the inner edge of the debris disk at 30–35&nbsp;au.<ref name="Janson2015" />

==== Planet b (AEgir) ====
{{main|Epsilon Eridani b}}
[[Extrasolar planet#Nomenclature|Referred to]] as [[Epsilon Eridani b|Epsilon Eridani&nbsp;b]], this planet was announced in 2000, but the discovery remained controversial over roughly the next two decades. A comprehensive study in 2008 called the detection "tentative" and described the proposed planet as "long suspected but still unconfirmed".<ref name=apj690_2_1522 /> Many astronomers believed the evidence is sufficiently compelling that they regard the discovery as confirmed.<ref name=aaa488_2_771 /><ref name=arxiv1011_4882 /><ref name=aaa499_2_L13 /><ref name=apj688_1_583 /> The discovery was questioned in 2013 because a search program at [[La Silla Observatory]] did not confirm it exists.<ref name="aa552_A78_62" /> Further studies since 2018 have gradually reaffirmed the planet's existence through a combination of radial velocity and astrometry.<ref name="MawetHirsch2019"/><ref name="MakarovZacharias2021"/><ref name="Llop-Sayson2021"/><ref name="Benedict2022"/><ref name="Feng2023"/>

[[File:Epsilon Eridani b.jpg|thumb|left|Artist's impression of Epsilon Eridani&nbsp;b orbiting within a zone that has been cleared of dust. Around the planet are conjectured rings and moons.|alt=At left is a shadowed, spherical red object encircled by a ring, with a smaller crescent at lower centre portraying a moon. To the right is a luminous source bisected by a line representing a debris disk.]]

Published sources remain in disagreement as to the planet's basic parameters. Recent values for its orbital period range from 7.3 to 7.6 years,<ref name="Feng2023"/> estimates of the size of its elliptical orbit—the [[semimajor axis]]—range from 3.38 au to 3.53 au,<ref name=cne2008 /><ref name=apj646_505 /> and approximations of its [[orbital eccentricity]] range from 0.055 to 0.26.<ref name="Feng2023"/>

Initially, the planet's mass was unknown, but a lower limit could be estimated based on the orbital displacement of Epsilon Eridani. Only the component of the displacement along the line of sight to Earth was known, which yields a value for the formula [[Stellar rotation#Measurement|''m''&nbsp;sin&nbsp;''i'']], where ''m'' is the mass of the planet and ''i'' is the [[orbital inclination]]. Estimates for the value of {{nowrap|''m'' sin ''i''}} ranged from 0.60 [[Jupiter mass]]es to 1.06 Jupiter masses,<ref name=cne2008 /><ref name=apj646_505 /> which sets the lower limit for the mass of the planet (because the [[sine]] function has a maximum value of 1). Taking {{nowrap|''m'' sin ''i''}} in the middle of that range at 0.78, and estimating the inclination at 30° as was suggested by [[Hubble Space Telescope|Hubble]] astrometry, this yields a value of {{nowrap|1.55 ± 0.24}} Jupiter masses for the planet's mass.<ref name=aj132_2206 /> More recent astrometric studies have found lower masses, ranging from 0.63 to 0.78 Jupiter masses.<ref name="Feng2023"/>

Of all the measured parameters for this planet, the value for orbital eccentricity is the most uncertain. The eccentricity of 0.7 suggested by some older studies<ref name=aj132_2206 /> is inconsistent with the presence of the proposed asteroid belt at a distance of 3&nbsp;au. If the eccentricity was this high, the planet would pass through the asteroid belt and clear it out within about ten thousand years. If the belt has existed for longer than this period, which appears likely, it imposes an upper limit on Epsilon Eridani&nbsp;b's eccentricity of about 0.10–0.15.<ref name=aaa499_2_L13 /><ref name=spitzer20081027 /> If the dust disk is instead being generated from the outer debris disk, rather than from collisions in an asteroid belt, then no constraints on the planet's orbital eccentricity are needed to explain the dust distribution.<ref name=arxiv1011_4882 />

==== Potential habitability ====
Epsilon Eridani is a target for planet finding programs because it has properties that allow an Earth-like planet to form. Although this system was not chosen as a primary candidate for the now-canceled [[Terrestrial Planet Finder]], it was a target star for NASA's proposed [[Space Interferometry Mission]] to search for Earth-sized planets.<ref name=mccarcty2008 /> The proximity, Sun-like properties and suspected planets of Epsilon Eridani have also made it the subject of multiple studies on whether an [[interstellar probe]] can be sent to Epsilon Eridani.<ref name=jsr22_345 /><ref name=jbis29_94 /><ref name=mcnutt2000 />

The orbital radius at which the stellar flux from Epsilon Eridani matches the [[solar constant]]—where the emission matches the Sun's output at the orbital distance of the Earth—is 0.61&nbsp;au.<ref name=aaa511 /> That is within the maximum [[habitable zone]] of a conjectured Earth-like planet orbiting Epsilon Eridani, which currently stretches from about 0.5 to 1.0&nbsp;au. As Epsilon Eridani ages over a period of 20&nbsp;billion years, the net luminosity will increase, causing this zone to slowly expand outward to about 0.6–1.4&nbsp;au.<ref name=ijab2_289 /> The presence of a large planet with a highly [[elliptical orbit]] in proximity to Epsilon Eridani's habitable zone reduces the likelihood of a [[terrestrial planet]] having a stable orbit within the habitable zone.<ref name=jones_underwood_sleep2003 />

A young star such as Epsilon Eridani can produce large amounts of [[ultraviolet]] radiation that may be harmful to life, but on the other hand it is a cooler star than the Sun and so produces less ultraviolet radiation to start with.<ref name=asp2003 /><ref name="BuccinoLemarchand2006" /> The orbital radius where the UV flux matches that on the early Earth lies at just under 0.5&nbsp;au.<ref name=asp2003 /> Because that is actually slightly closer to the star than the habitable zone, this has led some researchers to conclude there is not enough energy from ultraviolet radiation reaching into the habitable zone for life to ever get started around the young Epsilon Eridani.<ref name="BuccinoLemarchand2006" />