40 (Omicron2) Eridani 3 - Keid

JPL, CalTech, NASA


Larger illustration


Astronomers have identified 40
(Omicron2) Eridani A as a prime
target for the Terrestrial Planet
Finder (TPF), now delayed
indefinitely.

That Star A was not simply a single star was discovered in 1783 by Sir William Friedrich Wilhelm Herschel (1738-1822, portrait), who was born Friedrich Wilhelm Herschel and who subsequently discovered the planet Uranus in 1781 -- which led to his appointment in 1782 as private astronomer to the King of England. Star A's companion itself was discovered to be a binary pair BC in 1851 by Otto Wilhelm von Struve (1819-1905), who succeeded his father (Friedrich Georg Wilhelm) as director of Russia's Pulkova Observatory, made the first accurate determination of the constant of precession, and discovered some 500 binary stars. Finally, Star B became the first "white dwarf" to be discovered in 1910 when astronomers -- Henry Norris Russell (1877-1957), Edward Charles Pickering (1846-1919), and Williamina ["Mina"] Paton Stevens Fleming (1857-1911) -- realized that this dim star was of spectral type A rather than M (Ken Croswell, 1995, page 55).

A-BC Star System

Star A and the binary pair BC have a wide separation of about 418 AUs and an orbital period of some 8,000 years (Wulff Dieter Heintz, 1974). The B and C pair of stars have an "average" separation of about 35 AUs (of a semi-major axis) in a highly elliptical orbit (e= 0.410) that lasts about 252 years and swings between 21 and 49 AUs, and their orbital inclination is around 108.9 degrees from the Earth's line of sight (Heintz, 1974).

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	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass Estimate (Solar)	Diameter (Solar)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
A-BC Mass Center	0.0	...	...	...	...	...	...	...	...
Keid A	184	~8,000	?	?	0.89	0.85	...	...	0.46-1.02
Inner H.Z. Edge A?	0.556	0.439	0	?	...	...	...	...	...
Outer H.Z. Edge A?	1.103	1.228	0	?	...	...	...	...	...
BC Mass Center	234	~8,000	?	?	...	...	...	...	...
Keid B	9.5	252.1	0.410	108.9	0.50	0.02	...	...	...
Keid C	25.5	252.1	0.410	108.9	0.20	0.28	...	...	...
Inner H.Z. Edge C?	0.06	0.031	0	108.9	...	...	...	...	...
Outer H.Z. Edge C?	0.11	0.083	0	108.9	...	...	...	...	...

40 Eridani A

This main sequence, orange-red dwarf (K1 Ve) may have about 89 percent of Sol's mass (RECONS), about 85 percent of its diameter (Johnson and Wright, 1983, page 655), and 36 percent of its luminosity. The star appears to be 46 percent to 102 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 282). It appears to be around 5.2 +/-1.2 billion years old (Ballouz et al, 2010). The system shows a radial velocity of about 25 miles per second (mps) -- about 40 km per second or kps -- in recession, but the true space velocity is about 62 mps (or 100 kps). Some alternative useful star catalogue numbers for the star are: Omi2 Eri A, 40 Eri A, HR 1325, Gl 166 A, Hip 19849, HD 26965, BD-07 780, SAO 131063, LHS 23, and LTT 1907.

© Torben Krogh & Mogens Winther,



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




Star A is an orange-red dwarf
star, like Epsilon Eridani
at left center of meteor.

Habitable Zone around Star A

According to calculations performed for the NASA Star and Exoplanet Database, the inner edge of Star A's habitable zone could be located at around 0.556 AUs from the star, while the outer edge lies farther out at around 1.103 AUs. Near the inner H.Z. edge at 0.6 AU from Star A -- between the orbital distance of Mercury and Venus in the Solar System -- a planet would have an orbital period of almost 203 days, or more than half an Earth year.

40 Eridani B

Star B is a white dwarf, stellar remnant of spectral and luminosity type DA4. It has a mass estimated from about 50.1 percent of Sol's (Provencal et al, 1998), only two percent of its diameter, and 33/10,000th of its brightness. It appears to be about 5.0 +/-1 billion years old, in cooling a "cooling time" of around 100 million years since it left the main sequence as a white dwarf (Ballouz et al, 2010). Some alternative useful star catalogue numbers for the star are: Omi2 Eri B, 40 Eri B, Gl 166 B, HD 26976, BD-07 781, G 160-60, LHS 24, ADS 3093, W 33, and Struve 518.

H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA

40 Eridani B is a young white dwarf (a remnant stellar core which enriched its closer companion, Eridani C,
with elements heavier than hydrogen when it cast off its outer gas layers) like planetary nebula NGC 2440.

Water Zone around Star B

40 Eridani C

A very dim red dwarf (M4.5 Ve) with only about 19.5 percent of Sol's mass (RECONS), 28 percent of its diameter (Johnson and Wright, 1983, page 655), and 7/10,000th of its visual luminosity. It appears to be around 5.2 +/-1.2 billion years old (Ballouz et al, 2010). Star C is also a UV Ceti type, flare star, which has the variable star designation of DY Eridani. Some alternative useful star catalogue numbers for the star are: DY Eri, Omi2 C, 40 Eri C, Gl 166 C, LTT 1909, and LHS 25.

NASA -- larger image

40 Eridani C is a dim red dwarf star, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.

Habitable Zone around Star C

With a spectral type of M4, Ross 128 can be used as a rough proxy for GJ 1214 (M4.5). According to calculations performed for the NASA Star and Exoplanet Database, the distance from Ross 128 where an Earth-type rocky planet may have liquid water on its surface has been estimated to be between 0.06 and 0.11 AU -- well within the orbital distance of Mercury in the Solar System. In that distance range from the star, such a planet would have a "year" of only 13 to 34 days. For Star C, its regular flares would sterilize a planet in its water zone.

Hunt for Substellar Companions

Using a highly sensitive, radial velocity method, the Lick Planet Search has not detected a brown dwarf or large Jupiter or Saturn class planet in the triple star system thus far (Cumming et al, 1999). A subsequent study ruled out the presence of planets larger than four Jupiter-masses within 5.2 AUs of the stars (Wittenmyer et al, 2006). As astronomers would find it very difficult to detect planets around all three stars using present methods, they had been hoping to use NASA's proposed 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 40 Eridani A. As currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph; and a "formation-flying" infrared interferometer, of which both are indefinitely postponed, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub but has been similarly postponed.

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