Epsilon Indi

Epsilon Indi (ε Ind) is a star system located only 11.867 light-years from the Sun in the southern constellation Indus. With an apparent magnitude of 4.8310, it is visible to the unaided eye.

Epsilon Indi is one of the nearest visible stars to Earth. The primary component in the system hosts an exoplanet, Epsilon Indi Ab, whose presence was confirmed in 2018. The planet was imaged by the James Webb Space Telescope (JWST) in 2024.

Star system

The Epsilon Indi system is composed of Epsilon Indi A, an orange main sequence star of the spectral type K5V, and a pair of brown dwarfs with the spectral classes T1 and T6, designated Epsilon Indi Ba and Epsilon Indi Bb. The brown dwarfs are separated from the primary component by 1,460 astronomical units (0.183 light-years).

Epsilon Indi A has a mass of 0.782 solar masses and a radius of 0.711 solar radii. With an effective temperature of 4,649 K, it shines with 21% of the Sun’s luminosity. The star spins at 2.00 km/s, taking 35.732 days to complete a rotation. It has an estimated age of around 3.5 billion years.

Epsilon Indi, image: Wikisky (DSS2)

Epsilon Indi Ba and Bb were discovered in 2003. The substellar objects have an orbital period of 11.0197 years and are separated by 2.4058 astronomical units, corresponding to an angular separation of 661.58 milliarcseconds.

Both brown dwarfs have the stellar classification T. Class T brown dwarfs are cool substellar objects with prominent lines of methane in their spectra, whose emission peaks in the infrared.

Epsilon Indi Ba has 66.92 times the mass of Jupiter and a radius 8% that of the Sun, and Epsilon Indi Bb has a mass of 53.25 Jupiter masses and a radius of 0.082 solar radii. Epsilon Indi Ba is the hotter of the two, with a surface temperature of 1,352 – 1,385 K, while Epsilon Indi Bb has an effective temperature of 976 – 1,011 K.

Observations with the Spitzer Space Telescope in 2007 did not find any excess infrared emission from the Epsilon Indi system, which would indicate the presence of a debris disk.

This image shows the Epsilon Indi system. Epsilon Indi A is in the center and the brown dwarf binary Epsilon Indi Bab orbiting each other is on the left side. An insert shows the resolved system with the NICMOS instrument on the Hubble Space Telescope. Image credit: SkyMapper (Wolf et al. 2018) and Hubble Space Telescope NICMOS; Wikimedia Commons/Meli thev (CC BY-SA 4.0)

Planet

Epsilon Indi A has a confirmed orbiting planet, Epsilon Indi Ab. The gas giant planet has a mass 6.31 times that of Jupiter. It orbits the parent star at a distance of around 28.4 astronomical units with a period of 173.6 years and an eccentricity of 0.4.

Epsilon Indi Ab is the second nearest known Jovian exoplanet to the Sun, after Epsilon Eridani b in the Epsilon Eridani system. Located almost as far from its host star as Neptune is from the Sun, Epsilon Indi Ab is one of the coldest extrasolar planets ever detected. It has an estimated temperature of only 35 degrees Fahrenheit, or 2 degrees Celsius. The planet is about 100° C (180° F) warmer than the gas giants in the solar system.

The existence of Epsilon Indi Ab was confirmed in 2018, but it had been suspected since 2002, when radial velocity measurements indicated the presence of a companion orbiting Eps Indi A with a period of over 20 years. Observations with the High Accuracy Radial Velocity Planet Searcher (HARPS) in 2013 supported the presence of an orbiting planet.

In 2018, a team led by F. Feng of the Centre for Astrophysics Research, University of Hertfordshire, UK, confirmed the presence of a cold Jupiter with a mass of at least 2.71 Jupiter masses and an orbital period of 52.62 years.

In 2023, a study led by F. Philipot of the Laboratory of Space Studies and Instrumentation in Astrophysics (LESIA) at the Paris Observatory found a mass of 3 Jupiter masses, a semi-major axis of 8.8 AU, and an eccentricity of 0.48.

The extrasolar planet was directly imaged by an international team of astronomers led by Elisabeth Matthews of the Max Planck Institute for Astronomy in Germany using the James Webb Space Telescope (JWST) in 2023. The image, which was released in 2024, shows the planet as a point source. Eps Indi Ab is the closest exoplanet to Earth to be directly imaged.

Observations with the James Webb telescope found a different mass and orbital elements for Epsilon Indi Ab than the studies based on radial velocity and astrometric measurements. The astronomers found a mass of 6.31 Jupiter masses, a semi-major axis of 28.4 AU, and an eccentricity of 0.4. The mid-infrared imaging with Webb’s MIRI instrument shows a separation of 4.1 arcseconds between the planet and the star, while observations with the VLT imager on the Very Large Telescope (VLT) show a 4.8 arcseconds separation.

Astronomers found that Epsilon Indi Ab appeared fainter than expected at shorter wavelengths of light. This may indicate significant amounts of methane, carbon dioxide, and carbon monoxide in the planet’s atmosphere.

This image of the gas-giant exoplanet Epsilon Indi Ab was taken with the coronagraph on NASA’s James Webb Space Telescope’s MIRI (Mid-Infrared Instrument). A star symbol marks the location of the host star Epsilon Indi A, whose light has been blocked by the coronagraph, resulting in the dark circle marked with a dashed white line. Epsilon Indi Ab is one of the coldest exoplanets ever directly imaged. Light at 10.6 microns was assigned the color blue, while light at 15.5 microns was assigned the color orange. MIRI did not resolve the planet, which is a point source. Credit: NASA, ESA, CSA, STScI, Elisabeth Matthews (MPIA) (CC0)

Facts

Located 11.8670 light years away, Epsilon Indi is the 26th nearest individual star to the Sun. It is only a little closer to us than the Sun-like star Tau Ceti (11.918 ly).

Epsilon Indi Ba and Bb are the sixth and seventh nearest known brown dwarfs, after Luhman 16 A and B in the constellation Vela, WISE 0855−0714 in Hydra, Sirius B in Canis Major, and Procyon B in Canis Minor.

The planet Epsilon Indi Ab is the twelfth closest exoplanet to the Sun discovered to date, and the closest planet that is more massive than Jupiter.

Epsilon Indi has the ninth highest proper motion of all known stars and the third highest proper motion of visible stars, after 61 Cygni in the constellation Cygnus and Groombridge 1830 (Argelander’s Star) in Ursa Major. As a result, ε Indi will cross the border into the neighbouring constellation Tucana (the Toucan) around the year 2640 CE.

The stellar wind of Eps Indi A produces a bow shock around 63 astronomical units from the star. The termination shock has been detected as far as 140 AU from the star.

The brown dwarfs in the Epsilon Indi system were discovered in 2003. Astronomers initially detected a single brown dwarf, Epsilon Indi B, with a mass 40 to 60 times that of Jupiter orbiting ε Indi A at a distance of around 1,500 astronomical units. Several months later, the object was discovered to be a binary brown dwarf with an orbital period of around 15 years and a separation of 2.1 AU.

Original Gemini South detection image obtained on August 18, 2003 with PHOENIX using a narrow-band filter within the J-band. This image is 4×4 arc-seconds across and was obtained without adaptive optics. Image credit: International Gemini Observatory/NOIRLab/NSF/AURA/PHOENIX Image (CC BY 4.0)

Artist’s conception of the Epsilon Indi system showing Epsilon Indi and the brown-dwarf binary companions. Due to the perspective of the brown dwarf companions, the relative sizes are not represented in this illustration. Brown dwarfs have a radius close to Jupiter. The more massive brown dwarf would be denser and brighter in the Near-IR. Image credit: International Gemini Observatory/NOIRLab/NSF/AURA/Lomberg J. (CC BY 4.0)

Epsilon Indi will make its closest approach to the solar system in approximately 17,500 years, when it comes within 10.58 light-years.

The nearest neighbours of Epsilon Indi are the red dwarfs Lacaille 8760 (AX Microscopii, 4.3 light-years) in the constellation Microscopium, Lacaille 9352 in Piscis Austrinus (4.7 ly), and Gliese 832 (GJ 832, 4.8 ly) in Grus.

Epsilon Indi was discovered to be a high proper motion star by Heinrich Louis d’Arrest in 1847. The German astronomer compared the star’s position in the catalogues of Lacaille, Brisbane and Taylor over the course of a century and found that it had changed position.

The star’s parallax was measured by the Scottish astronomer David Gill and American astronomer William L. Elkin from the Cape of Good Hope in South Africa in 1882-83. The astronomers found a parallax of 0.22 ± 0.03 arcseconds.

American astronomer Harlow Shapley of the Harvard Observatory found a parallax of 0.45 arcseconds and a distance of 7 light-years with a 15% uncertainty in 1923. At the time, Epsilon Indi was believed to be the second nearest visible star to the Sun, after Alpha Centauri.

In 1972, Epsilon Indi, Epsilon Eridani and Tau Ceti were scanned by the Copernicus satellite (Orbiting Astronomical Observatory 3) for signs of laser emission, but none were detected.

In 2007, American astronomers Margaret Turnbull and Jill Tarter of the Carnegie Institution for Science in Washington named Epsilon Indi the star most likely to have planets that could support complex life. The astronomers compiled a list of 17,129 nearby low-mass stars at least 3 billion years old with high levels of iron, required to form rocky planets.

Turnbull listed Epsilon Indi A as one of the top five targets for the proposed Terrestrial Planet Finder (TPF), along with Epsilon Eridani (Ran), Omicron² Eridani (Keid, 40 Eridani), Toliman (Alpha Centauri B), and Tau Ceti. The TPF project was cancelled in 2011 after several postponements.

In 2019, Giada N. Arney of the Planetary Systems Laboratory at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, named Epsilon Indi as one of the top targets for biosignature searches and signs of evolved life, along with 61 Cygni (Bessel’s Star) in the constellation Cygnus, Groombridge 1618 in Ursa Major, and Guniibuu (36 Ophiuchi) in Ophiuchus.

These K-type stars (orange dwarfs) are thought to be in a “sweet spot” between Sun-like stars and red dwarfs. Arney’s analysis found that K-type stars were more likely to host planets with significant amounts of oxygen and methane in their atmospheres, which is a strong biosignature. Unlike M-type stars (red dwarfs), they are not prone to dramatic stellar flares.

Seen from a hypothetical planet in the Epsilon Indi system, the Sun would appear as a magnitude 2.6 star in the constellation Ursa Major. It would appear near the subgiant star Upsilon Ursae Majoris. The Sun’s nearest neighbour, Alpha Centauri, would also appear in the Great Bear constellation, in the region of the Three Leaps of the Gazelle asterism.

The position of the Sun and Rigil Kentaurus (Alpha Centauri) from Epsilon Indi, image credit: Wikimedia Commons/Tomruen (CC BY-SA 4.0)

Epsilon Indi was notably used in several episodes of the Star Trek series: “And the Children Shall Lead” (Star Trek: The Original Series), “Terra Firma, Part 1” (Star Trek: Deep Space Nine), and “Eye of the Beholder” (Star Trek: The Next Generation). In Star Trek lore, Epsilon Indi is the parent star of Andoria, the homeworld of the Andorians.

Name

Epsilon Indi is a faint star located in the far southern sky and it does not have a proper name approved by the International Astronomical Union (IAU). It does not have any known traditional names.

Location

Epsilon Indi lies in the region between the Southern Birds constellations Grus (the Crane), Tucana (the Toucan), and Pavo (the Peacock). It appears about a third of the way from Alpha Tucanae in Tucana to Alnair, the brightest star in Grus.

With an apparent magnitude of 4.83, the star is visible from areas without too much light pollution. At declination -56° 47’, it never rises for observers north of the latitude 32-33° N.

The location of Epsilon Indi, imag: Stellarium

Constellation

Epsilon Indi is located in the constellation of Indus. It marks one of the arrows in the left hand of the celestial Indian.

Indus is one of the small, faint constellations charted by Dutch explorers in the late 16th century. Largely invisible to northern observers, it was not known to Europeans before the Dutch-Flemish astronomer Petrus Plancius included it on his celestial globe in 1597 or 1598. The constellation appeared in the German astronomer Johann Bayer’s celestial atlas Uranometria in 1603.

Stretching across 294 square degrees of the southern sky, Indus is the 49th largest of the 88 constellations. It does not contain any stars brighter than magnitude 3.0. The constellation’s lucida, the orange giant Alpha Indi (the Persian), shines at magnitude 3.11 from a distance of 99.1 light-years.

Other notable stars in Indus include the variable orange giant Beta Indi, the binary system Delta Indi, the multiple star system Theta Indi, the semiregular variable red giant star T Indi, and the K-type giants Zeta Indi, Mu Indi, and HD 207229. HD 207229 hosts an orbiting gas giant exoplanet.

Indus constellation map by IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

Indus does not contain any bright nebulae or clusters, but it hosts a great number of galaxies. The constellation contains a part of the Pavo-Indus Supercluster of galaxies, which stretches into the neighbouring constellations Pavo and Telescopium. The brighter galaxies in Indus include the spiral galaxies NGC 7090, NGC 7038, NGC 7124 and NGC 7083, the elliptical galaxies NGC 7196 and NGC 7029, and the lenticular galaxies NGC 7049 and NGC 7041.

The best time of the year to see the stars and deep sky objects in Indus is during the month of September, when the constellation climbs higher in the sky around 9 pm. The entire constellation is visible from locations south of the latitude 15° N.

The 10 brightest stars in Indus are Alpha Indi (mag. 3.11), Beta Indi (mag. 3.67), Theta Indi (mag. 4.40), Delta Indi (mag. 4.40), Eta Indi (mag. 4.52), Epsilon Indi (mag. 4.8310), Zeta Indi (mag. 4.90), Iota Indi (mag. 5.06), Mu Indi (mag. 5.17), and Nu Indi (mag. 5.28).

Epsilon Indi

Epsilon Indi A

Epsilon Indi Ba

Epsilon Indi Bb