Epsilon Indi

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 K5 Ve. The star has about 77 percent of Sol's mass (RECONS), 76 percent of its diameter (Johnson and Wright, 1983, page 701), and about 14.7 percent of its luminosity. It appears to be about 59 to 110 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 310).

© 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). New analysis, however, suggests that it may be between 0.8 and two billion years old -- possibly 1.3 billion years based on its 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 identified its spectral type as 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 would find it very difficult to detect using present methods.

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.