Epsilon Indi

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Jon Lomberg, Gemini Observatory

Larger illustration.

Epsilon Indi is an orange-red dwarf
star, with two methane brown dwarf
companions in orbit around each
other (more). See a 2MASS Survey
image of Epsilon Indi from the NASA
Star and Exoplanet Database.

System Summary

This star system is located about 11.8 light-years (ly) away from our Sun, Sol, at the northwestern edge (22:03:21.66-56:47:09.51, ICRS 2000.0) of Constellation Indus, the Indian -- southeast of Delta Indi and northwest of Alpha Tucanae. The fifth brightest star in Indus, this star is the title member of the Epsilon Indi stellar moving group. Although smaller and dimmer than Sol, it is clearly visible with the naked eye. Epsilon Indi has such a high proper motion that, within a few thousand years, it will have moved out of Constellation Indus and into neighboring Constellation Tucana, the Toucan. As Epsilon Indi has become one of the top 100 target stars for NASA's planned Terrestrial Planet Finder (TPF).

JPL, CalTech, NASA

Larger illustration

Astronomers have identified Epsilon
Indi as a prime target for the
Terrestrial Planet Finder (TPF),
now planned for launch between
2014 and 2020.

On January 13, 2003, astronomers announced the discovery of a methane brown dwarf companion to this nearby star (ESO and AIP joint press release and API press release in German -- more below). In August 2003, the same team (as well as another team) of astronomers discovered that the brown dwarf had its own brown dwarf companion (Gemini press release -- more below). As of late 2003, the two substellar objects are the closest known brown dwarfs to the Solar System.

Jon Lomberg,
Gemini Observatory

Larger map.

The Epsilon Indi system is located
less than 12 light-years away in
southern Constellation Indus.

The Star

Epsilon Indi is a orange-red main sequence dwarf star of spectral and luminosity type K4-5 Ve. The star has about 70 to 77 percent of Sol's mass (RECONS; and NASA Star and Exoplanet Database, from David F. Gray, 1992), 68 to 76 percent of its diameter (Johnson and Wright, 1983, page 701; and NASA Star and Exoplanet Database, from Kenneth R. Lang, 1980), and about 14.7 percent of its visual luminosity and 20.4 percent of its theoretical bolometric luminosity, correcting for infrared output (NASA Star and Exoplanet Database, from Kenneth R. Lang, 1980). According to a 2006 study, it appears to be around 87 percent as enriched as Sol with elements heavier than hydrogen ("metallicity") based on its abundance of iron, although earlier studies found a range of 59 to 110 percent of Sol's metallicity (Santos et al, 2006, page 7 for HD 209100; and Cayrel de Strobel et al, 1991, page 310). The star has a rotational period of 22 days (NASA Star and Exoplanet Database).

© Torben Krogh & Mogens Winther,

(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)

Epsilon Indi is an orange-red dwarf
star, like Epsilon Eridani at left
center of meteor. (See a Digitized
Sky Survey field image of Epsilon Indi
from NASA's NStars Database.

The star was once suspected of being older than Sol, because it does not rotate rapidly and exhibits moderately high U,V,W velocities in its galactic orbital motion (Ken Croswell, 1995, pp. 253-254). In 2009, a new study argued that mass and evolutionary models for the two brown dwarf companions Ba and Bb indicated a system age of 3.7 to 4.3 billion years old (King et al, 2009), which is much older than an age estimate of between 0.8 and two billion years based on the star's rotational speed (press release; Scholz et al, 2003; and Lachaume et al, 1999). Epsilon Indi has a stellar wind similiar to the Sun's "Solar wind" (Wood et al, 1995). Useful star catalogue numbers for Epsilon Indi include: Eps Ind, HR 8387, Gl 845, Hip 108870, HD 209100, CP(D)-57 10015, SAO 247287, FK5 825, LHS 67, LTT 8813, and LFT 1677.

Brown Dwarf "ba" or "Ba"

SuperCOSMOS, API, ESO

Comparison of optical with
2MASS infrared image.

A methane brown dwarf
companion "b", encircled
at left, has been detected
at a wide separation of
around 1,500 AUs from
Epsilon Indi, at right
(press release).

On January 13, 2003, a team of astronomers (including Ralf-Dieter Scholz, Mark McCaughrean, Nicolas Lodieu, and Bjoern Kuhlbrodt) announced the discovery of a brown dwarf companion "b" -- now re-designated "ba" -- to this nearby star with a total (bolometric) luminosity of just 0.002 percent that of the Sun (ESO and AIP joint press release and API press release in German -- more below). The extremely dim companion object was observed to share the same high proper motion as Epsilon Indi -- around 4.7 arcseconds per year -- from the perspective of an observer in the Solar System. According to two model estimates based on its presumed age and measurements of its temperature, brightness, and distance, the substellar companion was estimated to have between 40 and 60 times Jupiter's mass, with a most likely estimate around 43 Jupiter-masses. By September 2003, however, the astronomers had refined the object's estimated mass to be closer to 47 +/- 10 Jupiter-masses (assuming an age of 1.3 billion years) (McCaughrean et al, 2003). Its diameter is estimated to be around that of Jupiter's (Scholz et al, 2003).

2MASS, U. of Massachusetts,
IPAC (JPL/Caltech),
NASA, NSF

Comparison of infrared with
optical image.

The brown dwarf is much
brighter in infrared
wavelengths (press release).

The brown dwarf has a surface temperature of only around 987 +/- 60 °C (1,810 °F or 1,260 °K). (By comparison, its host star, Epsilon Indi, has a surface temperature of around 4,000 °C, or 7,200 °F, which is somewhat cooler than the Sun.) Epsilon Indi ba is so cool that methane has been detected in its atmosphere and so it has been classified as the earliest T-type (T1 V), methane brown dwarf (McCaughrean et al, 2003) -- a "T-dwarf"), like the brown dwarf companion to Gliese 229. It is separated from its host star by an estimated 1,459 AUs (around seven arcminutes) -- into the reaches of the Oort Cloud in the Solar System (Scholz et al, 2003).

NTT + SOFI, ESO

Larger illustration of near-infrared spectrum
of the brown dwarf's atmosphere.

Methane, carbon monoxide, and water
molecules were detected in the relatively
cool atmosphere of the brown dwarf
companion (more).

Brown Dwarf "bb" or "Bb"

In August 2003, the same team of astronomers as well as another team of astronomers (including Gordon Walker, Suzie Ramsay Howat, Kevin Volk, Robert Blum, David Balam, and Verne Smith) found that the brown dwarf had its own brown dwarf companion designated "bb" (Gemini press release and IAUC 8188). The new object is separated from ba by 2.65 AUs and has an estimated, nominal orbital period of around 15 years (McCaughrean et al, 2003). Its estimated mass is around 28 +/- 7 Jupiter-masses (assuming an age of 1.3 billion years), and its spectral type is T6. Cooler and less massive than ba, the object is also much fainter in both optical and infrared telescopes.

Phoenix, Gemini Observatory

Larger infrared image.

Brown dwarf bb, left lower center,
is a less massive, cooler, and
fainter companion of Epsilon Indi
ba, at center (more).

Hunt for Planetary Companions

Since Epsilon Indi is sort of like a distant cousin to Sol, some speculate whether it might just be bright enough to support Earth-type life on a planet lucky enough to orbit in its water zone. The distance from Epsilon Indi where an Earth-type planet could possibly have liquid water on its surface is centered around only 0.38 AU -- around Mercury's orbital distance in the Solar System. At that distance from the star, such a planet would have an orbital period of around 99 days -- just over a quarter of an Earth year. Astronomers are hoping to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around Epsilon Indi. As currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph to launch around 2014; and a "formation-flying" infrared interferometer to launch before 2020, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub beginning in 2015.

Previously, a search for faint companions using the Hubble Space Telescope in 1996 found no supporting evidence for a large Jupiter or brown dwarf sized object close to the star, possibly due to guide star acquisition problems (Scholz et al, 2003; and Schroeder et al, 2000). The failure to find large substellar objects like brown dwarfs or a Jupiter- or Saturn-class planet in a "torch" orbit (closer than the Mercury to Sun distance) around Epsilon Indi -- with even the highly effective radial-velocity methods of Geoff Marcy and Paul Butler -- bodes well for the possibility of Earth-type terrestrial planets around this star (Scholz et al, 2003; and Endl et al, 2002). On the other hand, the discovery of a brown dwarf companion in a wide orbit that could perturb dormant comets in an Oort Cloud around Epsilon Indi inwards towards the star's inner planetary regions may periodically shower an Earth-type, inner planet with catastrophic impacts.

Brown Dwarfs or Planets?

When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.

© American Scientist
(Artwork by Linda Huff for Martin et al, 1997; used with permission)

Although brown dwarfs lack sufficient mass (at least 75 Jupiters) to
ignite core hydrogen fusion, the smallest true stars (red dwarfs) can
have such cool atmospheric temperatures (below 4,000° K) that it is
difficult to distinguish them from brown dwarfs. While Jupiter-class planets
may be much less massive than brown dwarfs, they are about the same
diameter and may contain many of the same atmospheric molecules.

University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 75 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.

Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.

Cool Methane Brown Dwarfs

While brown dwarfs have too little mass to fuse "regular" hydrogen (which has a single proton nucleus), virtually all of the ones discovered until 1999 were too hot -- that is "young" -- to show evidence of methane which is destroyed by stellar temperatures. In fact, while methane is a atmospheric characteristic of giant gas planets like Jupiter, the only brown dwarf found to even have a trace of methane was Gliese 229 b.

© John Whatmough (Artwork from Extrasolar Visions, used with permission)
The two brown dwarf companions are similar to Gliese 229 b -- shown here
with its own dark satellite, as imagined by Whatmough

In Spring 1999, however, two very dim and reddish brown dwarfs were found as solitary objects (one 30 light-years away in Ophiuchus and another also relatively nearby in Virgo). Analysis of their spectra indicated that both have atmospheres that are rich in methane. In addition, four similar objects that are too cool to be observed in visible light were found using near-infrared telescopes also to have the methane fingerprint of extremely cool (that is "old") brown dwarfs. These discoveries represent strong evidence that, although hard for astronomers to detect, faint brown dwarfs which have had billions of years to cool may represent a significant population of the universe. Some astronomers speculate that these objects may well be as numerous as the stars, reviving theories of stellar formation that suggest the existence of uncountably numerous brown dwarfs, rather than the relatively few easy-to-detect, bright ones found thus far.

Closest Neighbors

The following star systems are located within 10 light-years of Epsilon Indi.

Star System	Spectra & Luminosity	Distance (light-years)
Lacaille 8760	K7-M2 Ve	4.3
Lacaille 9352	M0.5 Ve	4.7
CD-49 13515 / Gl 832	M1.5 V	4.8
Cincinnati	M2-4 V	7.1
EZ Aquarii 3	M5.0-5.5 Ve ? ?	8.2
Ross 154	M3.5 Ve	8.9
Delta Pavonis	G5-8 V-IV	9.2
Alpha Centauri 3	G2 V K0 V M5.5 Ve	9.7
Hip 82725	?	9.7
CD-46 11540	M2.5-3 V	9.9
L 347-14	M4.5 V	9.9
Luyten 726-8	M5.6 Ve	10.0

Other Information

Note: Special thanks to Ken Croswell on a new estimate of the system's age based on analyses of its brown dwarf companions and to Mike Stevens for notifying us of the discovery of Epsilon Indi bb.