Horse of a Different Mythology

A naming scheme for the star and its planets was developed to make it easier for the participants to refer to them during the project. It was not intended to preclude our sophonts from naming the planet, star and solar system in their own language.

The names are bound with a thread from Celtic and Gallic mythology.

Taranis - god of the thundering sky

Belenos - Sun god, bright as fire

Grannos - Sun god, luminous and salutary

Epona - Celtic goddess connected with horses

Sucellus - god with a hammer, he who strikes well

Rosmerta - Gallic goddess: the great provider

Celtic gods of the bubbling spring




Goddess of stars


Truly Stellar Data

The details of the planetary system were thought up by Martyn Fogg and members of the Space Discussion Society (UK) over e-mail. Our starting point was to choose a star close to the Sun and to run Fogg's Silicon Creation computer model (detailed in Journal of the British Interplanetary Society 45, 3-12 [1992]) a number of times until several habitable worlds were produced. Then we had to choose the system with the most interesting world and start to add 'life' to those data points. Planet III is Epona and we've given it a history, a last moment in time where intelligence has a chance, and a future - for advanced life at least- which seems to be coming to an end. Extinction's on the horizon and you've got just a few million years to evolve your sophont, build a space faring civilization, save the planet and find out what's out there!

The stars parameters were not known with precision in 1982 and the estimates in the table were made for the purpose of guiding development of the habitable conditions of the planet for The Epona Project.

More current astronomical information as well as actual exoplanet data can be found at the following site: 82 Eridani

Star Name Taranis (82 Eridani)
Right Ascension 3 hours 15.9 minutes
Declination -43 degrees 27 minutes
Absolute visual magnitude 5.3
Spectral type G5
Status Single
Distance from the Sun 20.9 light years
Stellar Mass 0.91 M of Sol
Metallicity 0.95 Z of Sol
Main sequence lifetime 14.24 billion years
Age 5 billion years
ZAMS Luminosity 0.45 L of Sol
Present Luminosity 0.58 L of Sol
Earth equivalent distance 0.76 AU

Basic Planetary Data

Number Name Orbital Radius (AU) Eccentricity Mass(relative to Earth) Type
I Belenos 0.37 0.06 0.1 Mercurian
II Grannos 0.53 0.02 0.16 Mercurian
III Epona 0.78 0.08 0.55 Post-Terran
IV Sucellus 1.18 0.01 2 Juvenile Martian
V Rosmerta 2.21 0.01 5.86 Mini Gas Giant
VI Borvo 4.18 0.01 205.9 Gas Giant
VII Bormo 8.49 0.01 14.08 Mini Gas Giant
VIII Bormanus 16.47 0.16 86.83 Gas giant
IX Sirona 29.52 0.01 0.18 Tritonian

For the purposes of the project, nine planets, of a variety of types, were found about Taranis. Their orbital parameters, masses, and basic nature are summarized below:

Fundamental features of the planetary system can be noted from this table. For instance, the largest world is Borvo (Planet VI) which is about 65% the mass of Jupiter; Bormanus (Planet VIII) is a Saturn-mass world with a very eccentric orbit; Epona (Planet III) is in the ecosphere, receiving a fraction less sunlight than does Earth and Sucellus (Planet IV) is a huge equivalent of Mars, double the mass of the Earth. Such details are also shown diagrammatically below.

Detailed Planetary Data


Summary: A planet similar in mass and relative position to Mercury in our own Solar System. Tidal forces from the star have synchronized its orbital and rotational periods. Hence, one hemisphere is in perpetual daylight, the other perpetual darkness.

Planetary Type Mercurian
Distance from Star 0.366 AU
Orbital Eccentricity 0.06
Length of Year 84 Days
Solar Constant 4.3 S (of sun)
Axial Inclination 0 degrees
Equatorial Radius 3281 km
Density 3870 kg/m3
Mass 5.72x1023 kg
Gravity 0.36 g
Escape Velocity 4.83 km/s
Atmospheric Pressure ZERO bars
Principal Gases AIRLESS
Surface Temperature 400 K
Life None


Summary: Another planet, similar to the mass of Mars, which has tidally spin-locked rotation. The sun-facing hemisphere is barren, baked rock. The dark hemisphere is blanketed by the planet's former atmosphere, now frozen as layers of carbon dioxide ice. This might be a very useful inner system locality for a space-faring culture to mine volatiles.

Planetary Type Mercurian
Distance from Star 0.526 AU
Orbital Eccentricity 0.02
Length of Year 145 days
Solar Constant 2.1 S (of Sol)
Axial Inclination 0 degrees
Equatorial Radius 3781 km
Density 4130 kg/m3
Mass 9.36x1023 kg
Gravity 0.45 g
Escape Velocity 6.02 km/s
Atmospheric Pressure ZERO bars
Principal Gases AIRLESS
Surface Temperature ~ 330 K
Life None


Summary: A planet receiving a similar amount of sunlight as the Earth. It is watery and life-bearing. However, it is older and only slightly more than half as massive as the Earth. Thus its carbonate-silicate cycle is running down; planetary homeostasis is intermittent; extensive ice ages and low carbon dioxide partial pressures that stunt photosynthesis are the norm. The land biota is impoverished, moribund, and only rescued from reversion to bacterial level by spasms of terminal volcanism, which temporarily warm and flood the planet, rejuvenating its biosphere and causing a rapid adaptive radiation of life forms. Worldbuilding began just after one of the last such episodes. No moons are present but this world's escape velocity is 2 km/s less than the Earth's, so getting into space will be easier. Seasonal variation is strong, both due to the planet's axial tilt and orbital eccentricity.

Planetary Type Post-Terran
Distance from Star 0.779 AU
Orbital Eccentricity 0.08
Length of Year 262 days
Solar Constant 0.95 S (of Sol)
Axial Inclination 32 degrees
Equatorial Radius 5406 km
Density 4940 kg/m3
Mass 3.27x1024 kg
Gravity 0.76 g
Escape Velocity 9.00 km/s
Rotation Rate 23.96 hours
Atmospheric Pressure 577 millibars
Principal Gases N2 and O2
Surface Temperature 273 - 298 K
Life Present!


Summary: This world is irradiated by a similar level of sunlight as the planet Mars, but is a much more substantial body, weighing in at twice the mass of the Earth. Thus, it is still very geologically active, its carbonate-silicate cycle maintaining a 2.5 bar partial pressure of carbon dioxide in the atmosphere making the planet similar to Mars in its infancy. Most of Sucellus is covered by deep oceans, the only land masses are scattered island chains that represent the tips of numerous submerged volcanoes. No life has evolved here, but anaerobic life forms from Epona could be introduced easily. A satellite similar to the Earth's Moon is present with ice and frozen gases present in small polar caps.

Planetary Type: Juvenile Martian
Distance from Star 1.181 AU
Orbital Eccentricity 0.01
Length of Year 490 days
Solar Constant 0.41 S (of Sol)
Axial Inclination 10 degrees
Equatorial Radius 7656 km
Density 6380 kg/m3
Mass 1.2x1025 kg
Gravity 1.40 g
Escape Velocity 14.49 km/s
Rotation Rate 13.7 hours
Atmospheric Pressure ~ 4 bars
Principal Gases CO2 and N2
Surface Temperature 273 - 283 K
Life Potential for anaerobic life


Summary: This is a tiny gas giant world which never grew to much more than an ice/rock core with a relatively "thin" layer of gases above. Nonetheless, the planet's surface - a world girdling ocean thousands of kilometers deep - exists at temperatures of hundreds of degrees and at pressures of thousands of bars. A number of carbonaceous, asteroidal moons are present.

Planetary Type Mini Gas Giant
Distance from Star 2.21 AU
Orbital Eccentricity 0.01
Length of Year 1250 days
Solar Constant 0.12 S (of Sol)
Axial Inclination 47 degrees
Equatorial Radius 17677 km
Density: 1520 kg/m3
Mass 3.51x1025 kg
Gravity 0.77 g
Escape Velocity 16.34 km/s
Rotation Rate 14.92 hours
Atmospheric Pressure N/A
Principal Gases CH4 and H2
"Surface" Temperature ~ 150 K
Life None


Summary: The principal gas giant of the system - 65% the mass of Jupiter. A retinue of icy, lifeless moons are present.

Planetary Type Gas Giant
Distance from Star 4.18 AU
Orbital Eccentricity 0.01
Length of Year 3260 days
Solar Constant 0.033 S (of Sol)
Axial Inclination 10 degrees
Equatorial Radius 65235 km
Density 1060 kg/m3
Mass 1.24x1027 kg
Gravity 1.98 g
Escape Velocity 50.38 km/s
Rotation Rate 9.29 hours
Atmospheric Pressure N/A
Principal Gases H2 and He
"Surface" Temperature: ~ 110 K
Life None


Summary: A gas giant similar in mass to Uranus and also tilted at a steep angle. Icy moons and rings are present.

Planetary Type Mini Gas Giant
Distance from Star 8.49 AU
Orbital Eccentricity 0.01
Length of Year 9457 days
Solar Constant 0.008 S (of Sol)
Axial Inclination 73 degrees
Equatorial Radius 24613 km
Density 1350 kg/m3
Mass 8.45x1025 kg
Gravity 0.95 g.
Escape Velocity 21.44 km/s
Rotation Rate: 3.4 hours
Atmospheric Pressure N/A
Principal Gases H2 and He
"Surface" Temperature ~ 80 K
Life None


Summary: A gas giant similar in mass to Saturn with all the usual stuff orbiting it. Its orbit is the most eccentric of all the system's planets.

Planetary Type Gas Giant
Distance from Star 16.47 AU.
Orbital Eccentricity 0.16
Length of Year 25538 days
Solar Constant 0.002 S (of Sol)
Axial Inclination 17 degrees
Equatorial Radius 56491 km
Density 0.69 kg/m3
Mass 5.21x1026 kg
Gravity 1.12 g
Escape Velocity 23.64 km/s
Rotation Rate 12.38 hours
Atmospheric Pressure N/A
Principal Gases H2 and He
"Surface" Temperature ~ 50 K
Life None


Summary: An ice/rock body more massive than Mars. It is too cold to have much of an atmosphere. Occasional cryovulcanism keeps a tenuous atmosphere of nitrogen, methane and hydrogen present.

Planetary Type Tritonian
Distance from Star 29.52 AU
Orbital Eccentricity 0.01
Length of Year 61277 days
Solar Constant 0.001 S (of Sol)
Axial Inclination 3 degrees
Equatorial Radius 4453 km
Density 2910 kg/m3
Mass 1.08x1024 kg
Gravity 0.37 g
Escape Velocity 5.69 km/s
Rotation Rate 25.56 hours
Atmospheric Pressure Very low
Principal Gases N2 and CH4
Surface Temperature ~ 40 K
Life None

Ready to find out more?

Visit Epona