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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.
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.
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.
Astronomers have identified Eta Cas A
as a prime target for the Terrestrial Planet
Finder (TPF), and the Space Interferometry
Mission (SIM), now both indefinitely postponed.
Due to Epsilon Indi's relative proximity and similarity of spectral type to Sol, the star has been an object of intense interest among astronomers. Epsilon Indi became one of the top 100 target stars for NASA's proposed Terrestrial Planet Finder (TPF), but the project has been postponed indefinitely. It was also selected as a "Tier 1" target star for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as three Earth-masses within two AUs of its host star (and so some summary system information and images of Epsilon Indi may still be available from the SIM Teams), but the SIM project manager announced on November 8, 2010 that the mission was indefinitely postponed due to withdrawal of NASA funding.
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, interpolated from David F. Gray, 1992), 68 to 76 percent of its diameter (Johnson and Wright, 1983, page 701; and NASA Star and Exoplanet Database, derived using the power law formula 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, derived using exponential formula 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, in line with earlier studies that found a range from 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, from Pizzolato et al, 2003).
© 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 2MASS Survey image of Epsilon Indi
from the NASA Star and Exoplanet 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 2010, a new study (which included better model atmosphere fitting) argued that the system is around six billion years old, even older than estimates of 3.7 to 4.3 billion years derived from a 2009 study based on mass and evolutionary models for the two brown dwarf companions Ba and Bb (Liu et al, 2010; and King et al, 2009), which are much older than an earlier age estimate of between 0.8 and 2.0 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.
Estimates provided by the NASA Star and Exoplanet Database indicate that the inner edge of Epsilon Indi's habitable zone could be located around 0.411 AU from the star, while the outer edge edge lies around 0.810 AUs. The distance from the star where an Earth-type planet could have liquid water on its surface is centered around 0.611 AU -- between the orbital distances of Mercury and Venus in the Solar System. At that distance from Epsilon Indi and assuming that it has 0.77 Solar-mass, such a planet would have an orbital period of around 199 days (or a bit over half an Earth year).
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
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).
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).
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
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.
Hunt for Planetary Companions
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.
© Estate of John Whatmough
(Artwork from Extrasolar Visions, used with permission from Whatmough)
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.
The following star systems are located within 10 light-years of Epsilon Indi.
|Star System||Spectra &|
|Lacaille 8760||K7-M2 Ve||4.3|
|Lacaille 9352||M0.5 Ve||4.7|
|CD-49 13515 / Gl 832||M1.5 V||4.8|
|EZ Aquarii 3||M5.0-5.5 Ve |
|Ross 154||M3.5 Ve||8.9|
|Delta Pavonis||G5-8 V-IV||9.2|
|Alpha Centauri 3||G2 V |
|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|
Up-to-date technical summaries on these stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS, the NASA Star and Exoplanet Database, and the Research Consortium on Nearby Stars (RECONS) list of the 100 Nearest Star Systems. Additional information may be available at Roger Wilcox's Internet Stellar Database.
Constellation Indus was created by Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman, who charted the southern skies from 1595 to 1597. Since many Europeans were exploring North America at the time, Johann Bayer decided to honor the new constellation by naming it for the American Indian in a collection of new constellations for his 1603 book Uranometria. For more information about the stars and other objects in this constellation, go to Christine Kronberg's Indus. For an illustration, see David Haworth's Indus.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
Note: Special thanks to Ken Croswell on studies revising previous estimates 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.
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