Welcome to the star system of Taranis and to its third planet!

The details below 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!

Read the origin of the naming convention.

THE STAR

82 Eridani is the chosen star and it was named Taranis; a sun nearly 21 light years away from Earth but barely visible in our night skies. It's a single, yellow, G-type star like the Sun, and is about 91% the mass. Since the luminosity of stars is very sensitive to mass, Taranis is only a little over half as bright as the Sun and burns its fuel more slowly. It's quite an old star - at 5 billion years being half the age of the galaxy - yet it is just over a third the way through its stable, main sequence, lifetime.

Taranis itself may have plenty of life left; however, 5 billion years is a long time in terms of planetary evolution, especially if the planet in question is small. If senescence hasn't already paralyzed such worlds, then it's just around the corner.

Taranis Stellar Data	The following parameters are not known with precision and are estimated for the purposes of The Epona Project:
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

Nine planets, of a variety of types, were found about Taranis. Their orbital parameters, masses, and basic nature are summarized below:

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

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

Silicon Creation is capable of producing detailed parameters of all the planets in its randomized solar system. Numerical data describing the planets about Taranis (82 Eridani) are listed below along with brief comments. Epona (Planet III) is to be described in greater detail in the next sections.

BELENOS (PLANET I)

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

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.

GRANNOS (PLANET II)

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

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.

EPONA (PLANET III)

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: 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.

SUCELLUS (PLANET IV)

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 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.

ROSMERTA (PLANET V)

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: 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.

BORVO (PLANET VI)

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

BORMO (PLANET VII)

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 Uranus and also tilted at a steep angle. Icy moons and rings are present.

BORMANUS (PLANET VIII)

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: 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.

SIRONA (PLANET IX)

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

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.

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