Epsilon Eridani

The "sub-millimeter" image, above, shows emissions from tiny dust particles (that are only a fraction of a millimeter in size) in orbit around Epsilon Eridani. Yellow to red areas of the image indicate the highest concentrations of cold dust, while blue to black areas suggest very little dust. Most of the dust lies in a ring between 35 and 75 times the Earth to Sun distance (AU) around the star, peaking at a radius of about 60 AUs, with an estimated mass of about 0.014 to 0.4 Sol's mass.

The ring structure may be a young analog to the E-K Belt of the Solar System, where the central region has been partially cleared by the aggregation of dust grains into planetesimals. There is much less dust in an apparent hole around the star at a distance within the radius of Neptune's orbit, and the peak emissions found at 65 AU lies within a dust disk or ring of between 30 to 105 AUs radius, resulting from colliding bodies of five to nine Earth-masses. The dust disk appears to be inclined about 25° from Earth's line of sight. Epsilon Eridani itself is not seen because its small, hot surface emits very little in sub-millimeter wavelengths. (For further information, see Greaves et al, 2005; and Greaves et al, 1998).

The prominent bright peak within the ring -- at the lower left -- may represent a concentration of dust particles trapped in mean-motion resonances by an orbiting planet (Liou and Zook, 1999), or (less likely) the remnants of a major cometary collision. Although there has been as yet no confirmation of a substellar companion around Epsilon Eridani, this image suggests that at least one such object may exist. In any case, it is likely that the tiny dust particles around the star will gradually accumulate into icy bodies like those in the Solar System's E-K Belt.

© Lynette Cook (Artwork from Extrasolar Planets - Collection III, used with permission)
View from an icy moon of a ringed, planetary candidate "b," an inner volcanic moon, and
zodiacal light and part of the dust ring around Epsilson Eridani, as imagined by Cook

In 2000, astronomers announced the discovery (or a possible confirmation of earlier detections) of a Jupiter-like planet around this Sun-like star (press release). A team of astronomers (including Artie P. Hatzes, Barbara McArthur, and Diane B. Paulson) led by William D. Cochran of the University of Texas McDonald Observatory announced the discovery of this nearby extra-Solar planet around Epsilon Eridani on August 7, 2000, at the 24th General Assembly of the International Astronomical Union in Manchester, England (United Kingdom). The planet was initially estimated to have about 0.8 and 1.6 times the mass of Jupiter (or 256 times the mass of Earth) and an orbital distance from Epsilon Eridani of about 3.3 AUs -- within the outer region of the Main Asteroid Belt in the Solar System. (Subsequent astrometric measurements -- which were superceded in 2006 -- suggested an actual mass of 1.2 (+/- 0.3) times Jupiter's -- see George Gatewood, 2000). The planet was detected using six independent data sets from competing planet search groups, taken with four telescopes and using three different measurement techniques over more than two decades. Work by Sallie L. Baliunas, an astronomer at the Harvard-Smithsonian Center for Astrophysics, helped to rule out alternative explanations for the observed patterns in the data, such as violent fluctuations in the young star. Past indications of a planetary companion around Epsilon Eridani have been difficult to verify because the young star is "jittery" with a very active chromosphere. However, the high chromospheric activity of the star makes this planet detection less certain (see summary of debate at U.C. Berkeley).

Greg Bacon,
Benedict et al,
STScI, ESA, NASA





Larger illustration.





A Jupiter-class
planetary around
Epsilon Eridani
was confirmed
in 2006 (more).

On October 9, 2006, a team of astronomers (led by G. Fritz Benedict and Barbara E. McArthur) working with the Hubble Space Telescope announced "definitive evidence" for the existence of a Jupiter-class planet around Epsilon Eridani using astrometic measurements combined with those made at the University of Pittsburgh's Allegheny Observatory (NASA press release). Coming astrometric with radial-velocity measurements to determine the tilt of the planet's orbit, the astronomers were able to estimate the "true mass" of planet b as about 1.55 +/- 0.24 times the mass of Jupiter. Located at an average distance (semi-major axis) of 3.39 +/- 0.36 AUs from Epsilon Eridani, the planet takes about 6.9 (6.85 +/- 0.03) years to complete its highly eccentric orbit (e= 0.702 +/- 0.039). Approaching as close as 2.4 and as far as 5.8 AUs from its host star, the planet's orbit is also tilted about 30.1 +/- 3.8 degrees from Earth's line of sight (Benedict et al, 2006). Although the planet's orbit takes it so far from Epsilon Eridani that oceans on any moons would freeze, life could potentially survive on a moon if it is massive enough to hold onto a dense heat-trapping atmosphere like Saturn's moon, Titan, according to astronomer G. Fritz Benedict. Astronomers hope to image the planet in 2007, when its orbit is closest to Epsilon Eridani and may reflect sufficient starlight for an image.

Greg Bacon,
Benedict et al,
STScI, ESA, NASA





Larger illustration.




Planet "b"
moves around
Epsilon Eridani
within its dusk
disk at about
the same orbital
plane (more).

The presence of planets around Epsilon Eridani is the most likely explanation for the depletion of dust within a radius of 35 AUs of the star because planets absorb such dust when they form. Moreover, past astrometric, doppler-shift, and radial-velocity analysis of perturbations ("wobbles") in the position of Epsilon Eridani -- by the late Peter van de Kamp (1901-1995) from 860 photographic plates made at Sproul Observatory in 1973 and by Bruce Campbell and others in 1988 -- suggested that a large planetary companion orbited this star (Lawton and Wright, 1990; and Campbell et al, 1988). That planetary candidate appeared to have less than five (perhaps three) times Jupiter's mass and to be orbiting at about 7.7 AU (between the orbits of Jupiter and Saturn in the Solar System) out from the star in an elliptical orbit (e= 0.5) with a period of about 25 years.

Further reinforcing the indirect evidence of a planetary system around this star is the observed substructure within the dust ring and other asymmetries in the ring itself, all of which could be due to perturbations by more than one substellar companion, including one just inside the dust ring at an orbital distance of 30 AU. Moreover, recent modelling of the asymmetric circumstellar disk around the star suggests that there may be another planet with a fifth of Jupiter's mass at an orbital distance of about 55 to 65 AU from the star -- more than one and a half times the "average" orbital distance of Pluto in the Solar System (Gorkavyi et al, 2000; and (Liou and Zook, 1999).

© Alice C. Quillen and
Stephen Thorndike
(Used with permission)




Simulated image of clumping
within the dust disk around
Epsilon Eridani (star) and
hypothesized planet "c"
(black dot) -- more.

On October 24, 2002, astronomers (Alice C. Quillen and Stephen Thorndike) announced that computer modelling of dust ring clumping patterns (i.e., where the planet orbited a star three times for every two times the dust orbited, or five times for every three dust orbits) suggested the presence of a relatively smaller planet with around a tenth of a Jupiter-mass -- around 30 Earth-masses. The planet moves around Epsilon Eridani at a semi-major axis around 40 AUs. Tentatively named "c", the planet has an eccentric orbit (e~ 0.3) that takes around 280 years to complete (Quillen and Thorndike, 2002). (See an animation of the suspected planetary orbits of this system, with a table of basic orbital and physical characteristics. John Whatmough also has illustrated web pages on this system in Extrasolar Visions.)

The apparent hole in Epsilon Eridani's inner region contains about 1,000 times more dust than is found today in our Solar System's own inner region -- which would encompass the orbits of the planets from Mercury to Neptune. This implies that Epsilon Eridani's inner region may have about 1,000 times more cometary bodies than the Solar System's planetary region today. This interpretation is consistent with the early history of the Solar System, where heavy bombardment of the planets occurred during the System's first 600 million years although major impacts gradually tapered after about 3.5 billion years.

In September of 2002, a team of astronomers (including Cristiano Cosmovici of the Institute for Cosmic and Planetary Science) announced at the Second European Workshop on Exo/Astrobiology that they had detected water "maser" emissions from three of 17 star systems suspected of hosting planets, including Epsilon Eridani, using the 32-meter Medicina radio telescope near Bologna. These microwave emissions could be generated from water molecules in a planet's atmosphere when they are excited by the infrared light of its host star. In an interview with New Scientist magazine, Astronomer Hugh Jones (Liverpool John Moores University) noted that the water signals could be coming from the host star rather than from a planetary atmosphere, but that additional telescopic observation should be able to pinpoint the exact source of the signal. Astronomer Geoff Marcy (University of California at Berkeley) added that he would not expect water maser emissions from the planets to be strong enough to be detected from Earth, but noted: "It wouldn't be the first time a surprising result came from extra-Solar planets." On September 21, 27, and 28, 2002, however, P. Kondratko and J. Lovell were unable to confirm the detection of water-maser emissions with the 70-meter NASA Deep Space Network antenna (+ 400-MHz Smithsonian and 16-MHz Australia Telescope correlation spectrometers) near Canberra (More discussion at Extrasolar Planets Encyclopaedia).

The distance from Epsilon Eridani where an Earth-type rocky planet may have liquid water on its surface may have to be between 0.47 and 0.91 AU (Jones and Sleep, 2003) -- between the orbital distances of Mercury and Earth in the Solar System. In that distance range from the star, such a planet would have an orbital period shorter an Earth year. Given the apparent youth of this star system, however, it is likely that only primitive, single-celled organisms like bacteria that can survive heavy meteorite or cometary bombardment would be likely to survive on any Earth-type planet that has cooled sufficiently to allow carbon-based lifeforms to develop. (For an illustrated discussion, see Christoph Kulmann's web page on the potential habitable zone around Epsilon Eridani. See an orbital depiction of the center of the habitable zone of this system, with a table of basic orbital and physical characteristics.)