Editing Epsilon Eridani (section)

=== 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" />