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
Borvo
Bormo
Bormanus
Goddess of stars
Sirona
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:
| 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
BELENOS (PLANET I)
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 |
| Rotation Rate | SYNCHRONOUS |
| Atmospheric Pressure | ZERO bars |
| Principal Gases | AIRLESS |
| Surface Temperature | 400 K |
| Life | None |
GRANNOS (PLANET II)
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 |
| Rotation Rate | SYNCHRONOUS |
| Atmospheric Pressure | ZERO bars |
| Principal Gases | AIRLESS |
| Surface Temperature | ~ 330 K |
| Life | None |
EPONA (PLANET III)
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! |
SUCELLUS (PLANET IV)
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 |
ROSMERTA (PLANET V)
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 |
BORVO (PLANET VI)
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 |
BORMO (PLANET VII)
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 |
BORMANUS (PLANET VIII)
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 |
SIRONA (PLANET IX)
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 |
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