Epsilon Indi

Epsilon Indi, Latinized from ε Indi, is a star system located at a distance of approximately 12 light-years from Earth in the southern constellation of Indus. The star has an orange hue and is faintly visible to the naked eye with an apparent visual magnitude of 4.674.^[2] It consists of a K-type main-sequence star, ε Indi A, and two brown dwarfs, ε Indi Ba and ε Indi Bb, in a wide orbit around it.^[14] The brown dwarfs were discovered in 2003. ε Indi Ba is an early T dwarf (T1) and ε Indi Bb a late T dwarf (T6) separated by 0.6 arcseconds, with a projected distance of 1460 AU from their primary star.

ε Indi A has one known planet, ε Indi Ab, with a mass of 6.31 Jupiter masses in an elliptical orbit with a period of about 171.3 years. ε Indi Ab is the second-closest Jovian exoplanet, after ε Eridani b. The ε Indi system provides a benchmark case for the study of the formation of gas giants and brown dwarfs.^[11]

The constellation Indus (the Indian) first appeared in Johann Bayer's celestial atlas Uranometria in 1603. The 1801 star atlas Uranographia, by German astronomer Johann Elert Bode, places ε Indi as one of the arrows being held in the left hand of the Indian.^[15]

In 1847, Heinrich Louis d'Arrest compared the position of this star in several catalogues dating back to 1750, and discovered that it possessed a measureable proper motion. That is, he found that the star had changed position across the celestial sphere over time.^[16] In 1882–3, the parallax of ε Indi was measured by astronomers David Gill and William L. Elkin at the Cape of Good Hope. They derived a parallax estimate of 0.22 ± 0.03 arcseconds.^[17] In 1923, Harlow Shapley of the Harvard Observatory derived a parallax of 0.45 arcseconds.^[18]

In 1972, the Copernicus satellite was used to examine this star for the emission of ultraviolet laser signals. Again, the result was negative.^[19] ε Indi leads a list, compiled by Margaret Turnbull and Jill Tarter of the Carnegie Institution in Washington, of 17,129 nearby stars most likely to have planets that could support complex life.^[20]

The star is among five nearby paradigms as K-type stars of a type in a 'sweet spot' between Sun-analog stars and M stars for the likelihood of evolved life, per analysis of Giada Arney from NASA's Goddard Space Flight Center.^[21]

ε Indi A is a main-sequence star of spectral type K5V. The star has only about three-fourths the mass of the Sun^[22] and 71% of the Sun's radius.^[10] Its surface gravity is slightly higher than the Sun's.^[4] The metallicity of a star is the proportion of elements with higher atomic numbers than helium, being typically represented by the ratio of iron to hydrogen compared to the same ratio for the Sun; ε Indi A is found to have about 87% of the Sun's proportion of iron in its photosphere.^[3]

The corona of ε Indi A is similar to the Sun, with an X-ray luminosity of 2×10²⁷ ergs s⁻¹ (2×10²⁰ W) and an estimated coronal temperature of 2×10⁶ K. The stellar wind of this star expands outward, producing a bow shock at a distance of 63 AU. Downstream of the bow, the termination shock reaches as far as 140 AU from the star.^[23]

This star has the third highest proper motion of any star visible to the unaided eye, after Groombridge 1830 and 61 Cygni,^[24] and the ninth highest overall.^[25] This motion will move the star into the constellation Tucana around 2640 AD.^[26] ε Indi A has a space velocity relative to the Sun of 86 km/s,^{[4][note 1]} which is unusually high for what is considered a young star.^[27] It is thought to be a member of the ε Indi moving group of at least sixteen population I stars.^[28] This is an association of stars that have similar space velocity vectors, and therefore most likely formed at the same time and location.^[29] ε Indi will make its closest approach to the Sun in about 17,500 years when it makes perihelion passage at a distance of around 10.58 light-years (3.245 pc).^[30]

As seen from ε Indi, the Sun is a 2.6-magnitude star in Ursa Major, near the bowl of the Big Dipper.^{[note 2]}

In January 2003, astronomers announced the discovery of a brown dwarf with a mass of 40 to 60 Jupiter masses in orbit around ε Indi A with a projected separation on the sky of about 1,500 AU.^[32][33] In August 2003, astronomers discovered that this brown dwarf was actually a binary brown dwarf, with an apparent separation of 2.1 AU and an orbital period of about 15 years.^[12][34] Both brown dwarfs are of spectral class T; the more massive component, ε Indi Ba, is of spectral type T1–T1.5 and the less massive component, ε Indi Bb, of spectral type T6.^[12] More recent parallax measurements with the Gaia spacecraft place the ε Indi B binary about 11,600 AU (0.183 lightyears) away from ε Indi A, along line of sight from Earth.^[7]

Evolutionary models^[35] have been used to estimate the physical properties of these brown dwarfs from spectroscopic and photometric measurements. These yield masses of 47 ± 10 and 28 ± 7 times the mass of Jupiter, and radii of 0.091 ± 0.005 and 0.096 ± 0.005 solar radii, for ε Indi Ba and ε Indi Bb, respectively.^[36] The effective temperatures are 1300–1340 K and 880–940 K, while the log g (cm s⁻¹) surface gravities are 5.50 and 5.25, and their luminosities are 1.9 × 10⁻⁵ and 4.5 × 10⁻⁶ the luminosity of the Sun. They have an estimated metallicity of [M/H] = –0.2.^[12]

The existence of a planetary companion to Epsilon Indi A was suspected since 2002 based on radial velocity observations.^[38] The planet Epsilon Indi Ab was confirmed in 2018^[39] and formally published in 2019 along with its detection via astrometry.^[11]

A direct imaging attempt of this planet using the James Webb Space Telescope was performed in 2023,^[40] and the image was released in 2024. The detected planet's mass and orbit are different from what was predicted based on radial velocity and astrometry observations.^[41] It has a mass of 6.31 Jupiter masses and an elliptical orbit with a period of about 171.3 years.^[42]

No excess infrared radiation that would indicate a debris disk has been detected around ε Indi.^[43] Such a debris disk could be formed from the collisions of planetesimals that survive from the early period of the star's protoplanetary disk.

• List of nearest stars and brown dwarfs
• List of nearest K-type stars

• Discovery of Nearest Known Brown Dwarf (eso0303 : 13 January 2003)
• "Closest Known Brown Dwarf has a Companion". SpaceRef.ca. 2003-09-19. Archived from the original on February 2, 2013. Retrieved 2008-06-28.
• Kaler, Jim. "EPS IND". STARS. Archived from the original on 2006-12-06. Retrieved 2008-06-28.
• Epsilon Indi Ab at The Extrasolar Planets Encyclopaedia. Retrieved 2018-07-02.