Epsilon Eridani

Also known as 18 Eridani, BD -09º697, GCTP 742.00, GJ 144, HD 22049, HIP 16537, HR 1084, LHS 1557, SAO 130564, WDS 03330-0928

     The constellation Eridanus resembles (in one respect at least) the more northerly Draco, in that it twists and meanders about, threading its way between the eight neighboring constellations that hem it in from every side. Beginning with Beta Eridani (a star I’ve always felt should really be included in Orion) at the northeast of Rigel, and wandering far south of what can be seen from anywhere in Maryland to the fabled Achernar (an Arabic word which appropriately means “End of the River”), Eridanus spans an astounding 57º of sky, from the celestial equator more than half way to the south pole. Fortunately from our point of view, the river starts out on a roughly due east course until it bumps up against the main body of Cetus, then taking a sharp turn southward before doubling back to the west. Why fortunately? Because this means that by the time we have floated along to Epsilon Eridani, we are only a little more than one degree further south than Rigel.

     The last of the “Water Constellations” (Aquarius the Water Bearer, Pisces the Fishes, Cetus the Whale, and Eridanus the River), Eridanus resembles its aquatic neighbors to the east in that it has no easily discernable shape, no instantly obvious asterisms than jump out at the observer to announce the constellation’s presence. In fact, I’ve often wondered whether it was the very anonymity of this particular region of the sky which led the Ancients to think of water – of its formless, flowing substance, spreading out in all directions with no particular structure. But whatever the case, despite its naked eye visibility, Epsilon Eridani is not the easiest thing in the world to locate. My advice at this point is to ignore the lines drawn in most star charts (such as the one at the top of this posting) depicting the celestial river Eridanus, and star hop straight from Beta Eridani (the constellation’s 2nd brightest star) over to Gamma Eridani (the fourth brightest). From there it is a short hop to the scarcely dimmer Delta Eridani, and finally on to Epsilon Eridani. All the other stars along this path are 4th magnitude or dimmer, and should be ignored.

     Now that we’ve taken a look, let’s think about what it is that we’ve found.

     As a long-time fan of science fiction, I’ve always had a soft spot in my heart for this particular star. Its proximity to our solar system, along with its naked eye visibility, has long made it a favorite of science fiction writers as a destination for interstellar journeys, as a convenient place to locate a colony or star base, or as the home world for visitors from another planet. And at first glance, there is good reason for such speculation. After all, we have come upon the star most like our own of all the stars we will be observing in our tour of the Solar Neighborhood. Epsilon Eridani is a main sequence star of spectral class K2, which means that is slightly smaller, dimmer, cooler, and more orange than the Sun, but not by much. It weighs in at approximately 80 percent of Sun’s mass, with slightly less than three quarters of its diameter, one third its luminosity, and a roughly equivalent surface temperature (5,085º as opposed to 5,778º for the Sun). It does have some significant differences, however. Its magnetic field is 40 times stronger than the Sun’s, and its surface activity correspondingly higher. Gigantic sunspots dot its surface, dwarfing those seen on the Sun. The spots are so huge that they affect the luminosity of the star to an extent that it is classed as a BY Draconis Variable (that is, the apparent magnitude of the star dips as one of these huge magnetic storms rotates into view). Epsilon Eridani’s stellar wind is more than 30 times as strong as our Sun’s, and extends out to an amazing 8,000 AU. Were it visible to our eyes, this “astrosphere” would appear half again as wide as the full moon! Astronomers estimate the star to be no more than a billion years old (and perhaps as young as 200 million years), but strangely enough for a star of such relative youth, it has a lower metallicity than the Sun (about 15 percent less). 

     Most excitingly, Epsilon Eridani is the closest star to the Earth with a 100% confirmed planet (not surprisingly named Epsilon Eridani b), as well as no less than two asteroid belts, and an enormous outer disk possibly similar to this solar system’s Kuiper Belt of icy Plutoids. Epsilon Eridani b is an object of perhaps 1.5 Jupiter masses and orbits its parent sun at a probable distance of 3.5 AU. Its orbit defines the outer edge of an asteroid belt that is centered at 3 AU from the star. A second asteroid belt lies about 20 AU out, and beyond that is a gigantic disk of dust and ice of between 5 and 9 times the mass of the Earth (compare to the Kuiper Belt’s mass of 10 Earths), extending out to 100 AU. Chemical analysis of this belt suggests a much lower level of volatiles than our own outer solar system, indicating a probable correspondingly higher degree of rockiness for the constituent bodies. Computer modeling of the outer asteroid belt suggests the presence of a second, unobserved planet (Epsilon Eridani c) of approximately one tenth Jupiter mass, orbiting at a distance of 40 AU. No Earth-sized worlds have been observed. The space making up the star’s Habitable Zone appears clear of debris, giving at least a chance that were there to be such a world, it would not be subjected to the life-killing bombardment that would be the fate of any planet circling Tau Ceti.