The neighbourhood, system by system

Every system within 25 parsecs that has a measured star, generated from that star’s real mass and seeded from its name. These are the systems the map shows: the seed is FNV-1a over the name, so Tau Ceti here is the Tau Ceti you fly into, on any machine, forever.

Nothing in this document was written by hand. Every number and every description is derived by epos.physics from the star’s mass and the seed, and this file is regenerated by tools/guide.py whenever the physics changes. If something here is wrong, the model is wrong — which is the point of writing it down this way.

There are no people in it, and no aliens either. The generator stops at large animals on purpose: everything below that rung can be argued from energy, time and gravity, and tool use cannot.

Where to look first

Ranked by how unusual the physics makes them, not by preference:

Census

9 systems, 46 planets and 17 moons — 63 worlds in 12 kinds.

Kind Count Share
desert 16 25.4%
volcanic 11 17.5%
hazy 10 15.9%
airless rock 7 11.1%
methane frost 5 7.9%
greenhouse 4 6.3%
sulphur 3 4.8%
glacial 2 3.2%
snowball 2 3.2%
eyeball 1 1.6%
ice giant 1 1.6%
ammonia-cloud giant 1 1.6%
Biosphere Worlds
sterile 45
chemosynthetic life, buried 12
vegetation and animals 3
microbial mats 2
microbial, oxygenating 1

Proxima Centauri

M4, 0.122 M☉ and 0.00182 L☉, 4.25 light years out. System age 12.76 Gyr against a main-sequence life of 6.7e+02 Gyr.

Nearest star. Flare star with a habitable-zone planet at 0.0485 AU – tidally locked within ~100,000 years of formation.

Habitable zone 0.0459–0.0884 AU. Snow line 0.115 AU. 4 planets, 0 moons. Seed 33615998.

Planet Orbit Mass Class Surface Moons Life
b 0.00957 AU 0.07 M⊕ dwarf planet desert — —
c 0.0156 AU 0.18 M⊕ terrestrial desert — —
d 0.0319 AU 0.14 M⊕ terrestrial desert — —
e 0.0665 AU 0.50 M⊕ terrestrial glacial — vegetation and animals

b — desert · tidally locked

dwarf planet, 0.07 M⊕ and 0.48 R⊕ at 0.00957 AU, going round in 0.00 years. Surface gravity 0.29 g, mean temperature 549 K (+275 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

c — desert · tidally locked

terrestrial, 0.18 M⊕ and 0.62 R⊕ at 0.0156 AU, going round in 0.01 years. Surface gravity 0.45 g, mean temperature 431 K (+158 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

d — desert · tidally locked

terrestrial, 0.14 M⊕ and 0.59 R⊕ at 0.0319 AU, going round in 0.02 years. Surface gravity 0.41 g, mean temperature 304 K (+31 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

e — glacial · habitable zone, tidally locked

terrestrial, 0.50 M⊕ and 0.83 R⊕ at 0.0665 AU, going round in 0.05 years. Surface gravity 0.73 g, mean temperature 263 K (-10 °C).

Ice sheets reach well into the mid-latitudes, leaving a belt of open water around the equator.

Life. Old enough, wet enough and bright enough for the whole sequence: oxygen, multicellular life, colonisation of the land, and large mobile animals to eat it. The land is vegetated wherever water and temperature allow, and the vegetation is the colour the star makes it.

Local physics sets the limits. Gravity here is 0.73 g, so the heaviest a land animal can get before its own legs fail is about 182 tonnes, against seventy on Earth, and a tree can stand 175 m tall before it can no longer pull water to its own crown. Powered flight is workable, on roughly Earth’s terms.

Vegetation covers 32% of it, coloured #432b13. This star’s photons peak at 1098 nm, outside the range an eye can see. A pigment tuned there works in the infrared, and the visible light that does arrive is far too scarce to be worth reflecting – so it absorbs that as well. Vegetation is black.

Minerals

Alpha Centauri A

F9, 1.08 M☉ and 1.35 L☉, 4.37 light years out. System age 2.74 Gyr against a main-sequence life of 8 Gyr.

The nearest solar twin, and the clearest demonstration that the mass-radius relation needs an age term: at 1.08 M_sun it should be 1.04 R_sun, but it is old enough to have swollen to 1.22.

Habitable zone 1.12–1.91 AU. Snow line 3.12 AU. 3 planets, 0 moons. Seed 1888288469.

Companion — Alpha Centauri B (main sequence)

0.909 M☉ and 0.498 L☉ at 5,260 K, radius 0.863 R☉. The pair close to 11.3 AU and open to 35.7 over 80 years (e = 0.52).

Planets are stable only inside 2.78 AU (Holman & Wiegert 1999). The cut also falls inside the snow line (3.12 AU), so this system could never build a giant and has no outer reservoir of ice to deliver inward. It is dry by dynamics, not by heat.

A K dwarf on an eccentric 80 year orbit, closing to 11.3 AU and opening to 35.7. Stability truncates the disc at 2.8 AU, just inside the snow line, so this system can neither build a giant nor be delivered ice from outside.

Planet Orbit Mass Class Surface Moons Life
b 0.281 AU 0.40 M⊕ terrestrial volcanic — —
c 0.339 AU 0.38 M⊕ terrestrial volcanic — —
d 0.412 AU 0.36 M⊕ terrestrial volcanic — —

b — volcanic · tidally locked

terrestrial, 0.40 M⊕ and 0.78 R⊕ at 0.281 AU, going round in 0.14 years. Surface gravity 0.65 g, mean temperature 529 K (+256 °C).

Dry, dark and geologically alive: flood basalts resurface it faster than impacts scar it, so the crust is young and almost featureless between the fissures.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

c — volcanic · tidally locked

terrestrial, 0.38 M⊕ and 0.77 R⊕ at 0.339 AU, going round in 0.19 years. Surface gravity 0.64 g, mean temperature 482 K (+209 °C).

Dry, dark and geologically alive: flood basalts resurface it faster than impacts scar it, so the crust is young and almost featureless between the fissures.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

d — volcanic · tidally locked

terrestrial, 0.36 M⊕ and 0.76 R⊕ at 0.412 AU, going round in 0.25 years. Surface gravity 0.63 g, mean temperature 438 K (+165 °C).

Dry, dark and geologically alive: flood basalts resurface it faster than impacts scar it, so the crust is young and almost featureless between the fissures.

Sterile. At 438 K nothing stays folded. The hottest organism ever recovered lives at 395 K, and it needs the pressure of a hydrothermal vent to do it.

Minerals

Alpha Centauri B

G6, 0.909 M☉ and 0.683 L☉, 4.37 light years out. System age 1.76 Gyr against a main-sequence life of 13 Gyr.

Habitable zone 0.826–1.43 AU. Snow line 2.22 AU. 7 planets, 1 moons. Seed 1871510850.

Planet Orbit Mass Class Surface Moons Life
b 0.275 AU 0.15 M⊕ terrestrial volcanic — —
c 0.372 AU 0.22 M⊕ terrestrial volcanic — microbial mats
d 0.582 AU 0.40 M⊕ terrestrial greenhouse — —
e 0.894 AU 0.18 M⊕ terrestrial desert — —
f 1.25 AU 0.54 M⊕ terrestrial hazy 1 chemosynthetic life, buried
g 1.94 AU 0.60 M⊕ terrestrial hazy — chemosynthetic life, buried
h 4.81 AU 4.41 M⊕ super-Earth hazy — chemosynthetic life, buried

b — volcanic · tidally locked

terrestrial, 0.15 M⊕ and 0.60 R⊕ at 0.275 AU, going round in 0.15 years. Surface gravity 0.42 g, mean temperature 451 K (+178 °C).

Dry, dark and geologically alive: flood basalts resurface it faster than impacts scar it, so the crust is young and almost featureless between the fissures.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

c — volcanic · tidally locked

terrestrial, 0.22 M⊕ and 0.66 R⊕ at 0.372 AU, going round in 0.24 years. Surface gravity 0.50 g, mean temperature 384 K (+111 °C).

Dry, dark and geologically alive: flood basalts resurface it faster than impacts scar it, so the crust is young and almost featureless between the fissures.

Life. Photosynthesis has been running for 0.8 billion years, which is not yet long enough to have changed the air. Everything it produces is being consumed the moment it appears – by dissolved iron, by volcanic gases, by rock. The oceans carry mats and blooms and the atmosphere carries no free oxygen at all.

This is what Earth looked like for its first billion years of life.

Vegetation covers 0% of it, coloured #6f764b. This star’s photons peak at 671 nm. A pigment takes that band and the blue below 490 nm, where the photons are worth the most each, and reflects the window between them.

Minerals

d — greenhouse · tidally locked

terrestrial, 0.40 M⊕ and 0.78 R⊕ at 0.582 AU, going round in 0.47 years. Surface gravity 0.66 g, mean temperature 257 K (-16 °C).

Every drop of water is in the atmosphere, where it is a powerful greenhouse gas, which keeps it there. The surface is hidden under unbroken cloud and hot enough to glow in the infrared.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

e — desert · habitable zone

terrestrial, 0.18 M⊕ and 0.62 R⊕ at 0.894 AU, going round in 0.89 years. Surface gravity 0.45 g, mean temperature 253 K (-20 °C).

What water it has is locked in polar ice and subsurface reservoirs. Iron in the crust has oxidised, so the dust is red and the sky with it.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

f — hazy · habitable zone

terrestrial, 0.54 M⊕ and 0.85 R⊕ at 1.25 AU, going round in 1.47 years. Surface gravity 0.75 g, mean temperature 265 K (-8 °C).

Methane and nitrogen in the upper air are being taken apart by ultraviolet light and put back together as heavy organics, which rain out as a red-brown tar. There is enough of it to hide the surface completely. Cold, orange, and chemically busy – this is Titan.

Life. An ocean sealed under ice, with hot rock at the bottom of it. Water reacting with fresh silicate releases hydrogen, and hydrogen plus dissolved carbon dioxide is a complete energy budget that never once refers to the star. Earth’s own hydrothermal vent communities run on exactly this, and are among the strongest candidates for where life here began.

It does not lead anywhere. The chemical energy available is a tiny fraction of what sunlight delivers to a surface, and there is no route from it to an oxygen atmosphere or to anything large. Expect mats, films and plumes, and expect them to have stayed that way for as long as the ocean has existed.

Minerals

Moons (1)

# Radius Orbit Period Surface Notable
1 1,408 km 12 planetary radii 2.64 d airless rock —

g — hazy

terrestrial, 0.60 M⊕ and 0.87 R⊕ at 1.94 AU, going round in 2.84 years. Surface gravity 0.79 g, mean temperature 208 K (-65 °C).

Methane and nitrogen in the upper air are being taken apart by ultraviolet light and put back together as heavy organics, which rain out as a red-brown tar. There is enough of it to hide the surface completely. Cold, orange, and chemically busy – this is Titan.

Life. An ocean sealed under ice, with hot rock at the bottom of it. Water reacting with fresh silicate releases hydrogen, and hydrogen plus dissolved carbon dioxide is a complete energy budget that never once refers to the star. Earth’s own hydrothermal vent communities run on exactly this, and are among the strongest candidates for where life here began.

It does not lead anywhere. The chemical energy available is a tiny fraction of what sunlight delivers to a surface, and there is no route from it to an oxygen atmosphere or to anything large. Expect mats, films and plumes, and expect them to have stayed that way for as long as the ocean has existed.

Minerals

h — hazy · beyond the snow line

super-Earth, 4.41 M⊕ and 1.86 R⊕ at 4.81 AU, going round in 11.05 years. Surface gravity 1.27 g, mean temperature 117 K (-156 °C).

Methane and nitrogen in the upper air are being taken apart by ultraviolet light and put back together as heavy organics, which rain out as a red-brown tar. There is enough of it to hide the surface completely. Cold, orange, and chemically busy – this is Titan.

Life. An ocean sealed under ice, with hot rock at the bottom of it. Water reacting with fresh silicate releases hydrogen, and hydrogen plus dissolved carbon dioxide is a complete energy budget that never once refers to the star. Earth’s own hydrothermal vent communities run on exactly this, and are among the strongest candidates for where life here began.

It does not lead anywhere. The chemical energy available is a tiny fraction of what sunlight delivers to a surface, and there is no route from it to an oxygen atmosphere or to anything large. Expect mats, films and plumes, and expect them to have stayed that way for as long as the ocean has existed.

Minerals

Barnard’s Star

M4, 0.144 M☉ and 0.00267 L☉, 5.96 light years out. System age 3.36 Gyr against a main-sequence life of 5.4e+02 Gyr.

Largest known proper motion: 10.3 arcsec/yr.

Habitable zone 0.0555–0.107 AU. Snow line 0.138 AU. 6 planets, 0 moons. Seed 121549997.

Planet Orbit Mass Class Surface Moons Life
b 0.0185 AU 0.08 M⊕ dwarf planet desert — —
c 0.0228 AU 0.06 M⊕ dwarf planet desert — —
d 0.028 AU 0.12 M⊕ terrestrial desert — —
e 0.0348 AU 0.13 M⊕ terrestrial volcanic — microbial, oxygenating
f 0.234 AU 2.20 M⊕ terrestrial hazy — chemosynthetic life, buried
g 0.417 AU 3.08 M⊕ super-Earth hazy — chemosynthetic life, buried

b — desert · tidally locked

dwarf planet, 0.08 M⊕ and 0.51 R⊕ at 0.0185 AU, going round in 0.01 years. Surface gravity 0.32 g, mean temperature 435 K (+162 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

c — desert · tidally locked

dwarf planet, 0.06 M⊕ and 0.46 R⊕ at 0.0228 AU, going round in 0.01 years. Surface gravity 0.27 g, mean temperature 391 K (+118 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

d — desert · tidally locked

terrestrial, 0.12 M⊕ and 0.57 R⊕ at 0.028 AU, going round in 0.01 years. Surface gravity 0.38 g, mean temperature 355 K (+81 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

e — volcanic · tidally locked

terrestrial, 0.13 M⊕ and 0.58 R⊕ at 0.0348 AU, going round in 0.02 years. Surface gravity 0.39 g, mean temperature 311 K (+38 °C).

Dry, dark and geologically alive: flood basalts resurface it faster than impacts scar it, so the crust is young and almost featureless between the fissures.

Life. The sinks are saturated and oxygen has begun to accumulate. This is the single largest change a biosphere ever makes to its planet: it rewrites the atmosphere, strips the methane haze, and – because oxygen and its ultraviolet-made ozone are both strong absorbers – changes what the world looks like from outside.

Life is still simple and still confined to water. There is not yet enough oxygen in the air to pay for an animal.

Vegetation covers 0% of it, coloured #432b13. This star’s photons peak at 1098 nm, outside the range an eye can see. A pigment tuned there works in the infrared, and the visible light that does arrive is far too scarce to be worth reflecting – so it absorbs that as well. Vegetation is black.

Minerals

f — hazy · tidally locked, beyond the snow line

terrestrial, 2.20 M⊕ and 1.27 R⊕ at 0.234 AU, going round in 0.30 years. Surface gravity 1.36 g, mean temperature 146 K (-128 °C).

Methane and nitrogen in the upper air are being taken apart by ultraviolet light and put back together as heavy organics, which rain out as a red-brown tar. There is enough of it to hide the surface completely. Cold, orange, and chemically busy – this is Titan.

Life. An ocean sealed under ice, with hot rock at the bottom of it. Water reacting with fresh silicate releases hydrogen, and hydrogen plus dissolved carbon dioxide is a complete energy budget that never once refers to the star. Earth’s own hydrothermal vent communities run on exactly this, and are among the strongest candidates for where life here began.

It does not lead anywhere. The chemical energy available is a tiny fraction of what sunlight delivers to a surface, and there is no route from it to an oxygen atmosphere or to anything large. Expect mats, films and plumes, and expect them to have stayed that way for as long as the ocean has existed.

Minerals

g — hazy · tidally locked, beyond the snow line

super-Earth, 3.08 M⊕ and 1.53 R⊕ at 0.417 AU, going round in 0.71 years. Surface gravity 1.32 g, mean temperature 100 K (-173 °C).

Methane and nitrogen in the upper air are being taken apart by ultraviolet light and put back together as heavy organics, which rain out as a red-brown tar. There is enough of it to hide the surface completely. Cold, orange, and chemically busy – this is Titan.

Life. An ocean sealed under ice, with hot rock at the bottom of it. Water reacting with fresh silicate releases hydrogen, and hydrogen plus dissolved carbon dioxide is a complete energy budget that never once refers to the star. Earth’s own hydrothermal vent communities run on exactly this, and are among the strongest candidates for where life here began.

It does not lead anywhere. The chemical energy available is a tiny fraction of what sunlight delivers to a surface, and there is no route from it to an oxygen atmosphere or to anything large. Expect mats, films and plumes, and expect them to have stayed that way for as long as the ocean has existed.

Minerals

Wolf 359

M5, 0.11 M☉ and 0.00144 L☉, 7.86 light years out. System age 7.55 Gyr against a main-sequence life of 7.7e+02 Gyr.

Habitable zone 0.0407–0.0786 AU. Snow line 0.102 AU. 5 planets, 2 moons. Seed 101325210.

Planet Orbit Mass Class Surface Moons Life
b 0.00981 AU 0.04 M⊕ dwarf planet airless rock — —
c 0.0149 AU 0.05 M⊕ dwarf planet desert — —
d 0.0269 AU 0.12 M⊕ terrestrial desert 1 —
e 0.047 AU 0.18 M⊕ terrestrial desert 1 —
f 0.272 AU 3.04 M⊕ super-Earth hazy — chemosynthetic life, buried

b — airless rock · tidally locked

dwarf planet, 0.04 M⊕ and 0.43 R⊕ at 0.00981 AU, going round in 0.00 years. Surface gravity 0.24 g, mean temperature 533 K (+260 °C).

No atmosphere, so no erosion. Every impact of the last few billion years is still there, and solar wind has darkened the regolith to charcoal.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

c — desert · tidally locked

dwarf planet, 0.05 M⊕ and 0.44 R⊕ at 0.0149 AU, going round in 0.01 years. Surface gravity 0.25 g, mean temperature 415 K (+141 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

d — desert · tidally locked

terrestrial, 0.12 M⊕ and 0.57 R⊕ at 0.0269 AU, going round in 0.01 years. Surface gravity 0.38 g, mean temperature 309 K (+35 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

Moons (1)

# Radius Orbit Period Surface Notable
1 859 km 4 planetary radii 0.57 d airless rock solar-wind volatiles (rich)

e — desert · habitable zone, tidally locked

terrestrial, 0.18 M⊕ and 0.63 R⊕ at 0.047 AU, going round in 0.03 years. Surface gravity 0.46 g, mean temperature 243 K (-30 °C).

What water it has is locked in polar ice and subsurface reservoirs. Iron in the crust has oxidised, so the dust is red and the sky with it.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

Moons (1)

# Radius Orbit Period Surface Notable
1 979 km 4 planetary radii 0.53 d airless rock solar-wind volatiles (rich)

f — hazy · tidally locked, beyond the snow line

super-Earth, 3.04 M⊕ and 1.52 R⊕ at 0.272 AU, going round in 0.43 years. Surface gravity 1.32 g, mean temperature 108 K (-166 °C).

Methane and nitrogen in the upper air are being taken apart by ultraviolet light and put back together as heavy organics, which rain out as a red-brown tar. There is enough of it to hide the surface completely. Cold, orange, and chemically busy – this is Titan.

Life. An ocean sealed under ice, with hot rock at the bottom of it. Water reacting with fresh silicate releases hydrogen, and hydrogen plus dissolved carbon dioxide is a complete energy budget that never once refers to the star. Earth’s own hydrothermal vent communities run on exactly this, and are among the strongest candidates for where life here began.

It does not lead anywhere. The chemical energy available is a tiny fraction of what sunlight delivers to a surface, and there is no route from it to an oxygen atmosphere or to anything large. Expect mats, films and plumes, and expect them to have stayed that way for as long as the ocean has existed.

Minerals

Lalande 21185

M1, 0.389 M☉ and 0.0262 L☉, 8.31 light years out. System age 9.13 Gyr against a main-sequence life of 1.5e+02 Gyr.

Habitable zone 0.173–0.326 AU. Snow line 0.434 AU. 5 planets, 0 moons. Seed 387115451.

Planet Orbit Mass Class Surface Moons Life
b 0.0444 AU 0.35 M⊕ terrestrial desert — —
c 0.0721 AU 0.29 M⊕ terrestrial desert — —
d 0.114 AU 0.62 M⊕ terrestrial greenhouse — —
e 0.186 AU 0.35 M⊕ terrestrial hazy — —
f 0.351 AU 1.20 M⊕ terrestrial hazy — chemosynthetic life, buried

b — desert · tidally locked

terrestrial, 0.35 M⊕ and 0.75 R⊕ at 0.0444 AU, going round in 0.01 years. Surface gravity 0.62 g, mean temperature 498 K (+225 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

c — desert · tidally locked

terrestrial, 0.29 M⊕ and 0.72 R⊕ at 0.0721 AU, going round in 0.03 years. Surface gravity 0.57 g, mean temperature 394 K (+120 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

d — greenhouse · tidally locked

terrestrial, 0.62 M⊕ and 0.88 R⊕ at 0.114 AU, going round in 0.06 years. Surface gravity 0.80 g, mean temperature 275 K (+2 °C).

Every drop of water is in the atmosphere, where it is a powerful greenhouse gas, which keeps it there. The surface is hidden under unbroken cloud and hot enough to glow in the infrared.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

e — hazy · habitable zone, tidally locked

terrestrial, 0.35 M⊕ and 0.76 R⊕ at 0.186 AU, going round in 0.13 years. Surface gravity 0.62 g, mean temperature 270 K (-3 °C).

Methane and nitrogen in the upper air are being taken apart by ultraviolet light and put back together as heavy organics, which rain out as a red-brown tar. There is enough of it to hide the surface completely. Cold, orange, and chemically busy – this is Titan.

Sterile. There is an ocean under the ice, but nothing to power a chemistry with: no tidal flexing and no volcanism, so the rock below it is cold and unreactive.

Minerals

f — hazy · tidally locked

terrestrial, 1.20 M⊕ and 1.05 R⊕ at 0.351 AU, going round in 0.33 years. Surface gravity 1.09 g, mean temperature 256 K (-17 °C).

Methane and nitrogen in the upper air are being taken apart by ultraviolet light and put back together as heavy organics, which rain out as a red-brown tar. There is enough of it to hide the surface completely. Cold, orange, and chemically busy – this is Titan.

Life. An ocean sealed under ice, with hot rock at the bottom of it. Water reacting with fresh silicate releases hydrogen, and hydrogen plus dissolved carbon dioxide is a complete energy budget that never once refers to the star. Earth’s own hydrothermal vent communities run on exactly this, and are among the strongest candidates for where life here began.

It does not lead anywhere. The chemical energy available is a tiny fraction of what sunlight delivers to a surface, and there is no route from it to an oxygen atmosphere or to anything large. Expect mats, films and plumes, and expect them to have stayed that way for as long as the ocean has existed.

Minerals

Sirius A

A1, 2.06 M☉ and 17.7 L☉, 8.60 light years out. System age 0.19 Gyr against a main-sequence life of 1.2 Gyr.

Brightest star in the sky, and only 2 M_sun – a reminder that luminosity goes as roughly the fourth power of mass. Its main sequence lifetime is about 1.2 Gyr.

Habitable zone 3.81–6.42 AU. Snow line 11.3 AU. 4 planets, 1 moons. Seed 959139127.

Companion — Sirius B (white dwarf)

1.02 M☉ and 0.0556 L☉ at 25,200 K, radius 5,849 km. The pair close to 8.07 AU and open to 31.5 over 50 years (e = 0.59).

Planets are stable only inside 2.16 AU (Holman & Wiegert 1999). The habitable zone begins at 3.81 AU, so this star has a habitable zone that nothing can stay in. Not a gap in the generator — there is nowhere stable to put a world. The cut also falls inside the snow line (11.3 AU), so this system could never build a giant and has no outer reservoir of ice to deliver inward. It is dry by dynamics, not by heat. The system age above (191 Myr) is dated by the dwarf, not sampled: its progenitor’s main-sequence life plus how far it has cooled since.

A solar mass of degenerate carbon and oxygen in a body smaller than Earth, still at 25,200 K. It was once the larger of the pair – about 5 M_sun – and it burned out first, shedding four fifths of itself. Everything in this system was inside that.

Planet Orbit Mass Class Surface Moons Life
b 0.524 AU 0.81 M⊕ terrestrial volcanic — —
c 0.622 AU 0.45 M⊕ terrestrial volcanic 1 —
d 0.71 AU 0.89 M⊕ terrestrial desert — —
e 0.863 AU 0.78 M⊕ terrestrial greenhouse — —

b — volcanic · tidally locked

terrestrial, 0.81 M⊕ and 0.94 R⊕ at 0.524 AU, going round in 0.26 years. Surface gravity 0.91 g, mean temperature 736 K (+463 °C).

Dry, dark and geologically alive: flood basalts resurface it faster than impacts scar it, so the crust is young and almost featureless between the fissures.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

c — volcanic · tidally locked

terrestrial, 0.45 M⊕ and 0.81 R⊕ at 0.622 AU, going round in 0.34 years. Surface gravity 0.69 g, mean temperature 675 K (+402 °C).

Dry, dark and geologically alive: flood basalts resurface it faster than impacts scar it, so the crust is young and almost featureless between the fissures.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

Moons (1)

# Radius Orbit Period Surface Notable
1 1,324 km 6 planetary radii 1.03 d volcanic —

d — desert

terrestrial, 0.89 M⊕ and 0.97 R⊕ at 0.71 AU, going round in 0.42 years. Surface gravity 0.95 g, mean temperature 633 K (+360 °C).

What water it has is locked in polar ice and subsurface reservoirs. Iron in the crust has oxidised, so the dust is red and the sky with it.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

e — greenhouse

terrestrial, 0.78 M⊕ and 0.93 R⊕ at 0.863 AU, going round in 0.56 years. Surface gravity 0.89 g, mean temperature 428 K (+155 °C).

Every drop of water is in the atmosphere, where it is a powerful greenhouse gas, which keeps it there. The surface is hidden under unbroken cloud and hot enough to glow in the infrared.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

Epsilon Eridani

K0, 0.82 M☉ and 0.452 L☉, 10.48 light years out. System age 5.45 Gyr against a main-sequence life of 18 Gyr.

Young (~600 Myr), with a debris disc and a confirmed giant planet.

Habitable zone 0.683–1.2 AU. Snow line 1.8 AU. 8 planets, 7 moons. Seed 953137737.

Planet Orbit Mass Class Surface Moons Life
b 0.188 AU 0.25 M⊕ terrestrial desert — —
c 0.247 AU 0.47 M⊕ terrestrial volcanic — —
d 0.355 AU 0.45 M⊕ terrestrial volcanic — microbial mats
e 0.514 AU 0.62 M⊕ terrestrial greenhouse — —
f 0.684 AU 0.75 M⊕ terrestrial eyeball — vegetation and animals
g 1.06 AU 1.05 M⊕ terrestrial glacial — vegetation and animals
h 1.57 AU 0.62 M⊕ terrestrial hazy — chemosynthetic life, buried
i 6.01 AU 35.04 M⊕ ice giant ice giant 7 —

b — desert · tidally locked

terrestrial, 0.25 M⊕ and 0.69 R⊕ at 0.188 AU, going round in 0.09 years. Surface gravity 0.53 g, mean temperature 492 K (+219 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

c — volcanic · tidally locked

terrestrial, 0.47 M⊕ and 0.82 R⊕ at 0.247 AU, going round in 0.14 years. Surface gravity 0.71 g, mean temperature 432 K (+159 °C).

Dry, dark and geologically alive: flood basalts resurface it faster than impacts scar it, so the crust is young and almost featureless between the fissures.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

d — volcanic · tidally locked

terrestrial, 0.45 M⊕ and 0.81 R⊕ at 0.355 AU, going round in 0.23 years. Surface gravity 0.70 g, mean temperature 354 K (+81 °C).

Dry, dark and geologically alive: flood basalts resurface it faster than impacts scar it, so the crust is young and almost featureless between the fissures.

Life. Old enough, wet enough and bright enough for the whole sequence: oxygen, multicellular life, colonisation of the land, and large mobile animals to eat it. The land is vegetated wherever water and temperature allow, and the vegetation is the colour the star makes it.

At 354 K this is above the limit for any cell with a nucleus, so the biosphere stays bacterial however long it has had and however much light it gets.

Vegetation covers 13% of it, coloured #7c6c43. This star’s photons peak at 710 nm. A pigment takes that band and the blue below 490 nm, where the photons are worth the most each, and reflects the window between them.

Minerals

e — greenhouse · tidally locked

terrestrial, 0.62 M⊕ and 0.88 R⊕ at 0.514 AU, going round in 0.41 years. Surface gravity 0.80 g, mean temperature 263 K (-10 °C).

Every drop of water is in the atmosphere, where it is a powerful greenhouse gas, which keeps it there. The surface is hidden under unbroken cloud and hot enough to glow in the infrared.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

f — eyeball · habitable zone, tidally locked

terrestrial, 0.75 M⊕ and 0.92 R⊕ at 0.684 AU, going round in 0.62 years. Surface gravity 0.87 g, mean temperature 282 K (+9 °C).

Tidally locked, so one face never leaves the light. Liquid water survives as a disc of ocean under the star with ice everywhere beyond the terminator — and the night side is a cold trap that slowly drags the rest of the water into itself.

Life. Old enough, wet enough and bright enough for the whole sequence: oxygen, multicellular life, colonisation of the land, and large mobile animals to eat it. The land is vegetated wherever water and temperature allow, and the vegetation is the colour the star makes it.

Local physics sets the limits. Gravity here is 0.87 g, so the heaviest a land animal can get before its own legs fail is about 105 tonnes, against seventy on Earth, and a tree can stand 145 m tall before it can no longer pull water to its own crown. Powered flight is workable, on roughly Earth’s terms.

Vegetation covers 37% of it, coloured #7c6c43. This star’s photons peak at 710 nm. A pigment takes that band and the blue below 490 nm, where the photons are worth the most each, and reflects the window between them.

Minerals

g — glacial · habitable zone

terrestrial, 1.05 M⊕ and 1.01 R⊕ at 1.06 AU, going round in 1.20 years. Surface gravity 1.02 g, mean temperature 270 K (-3 °C).

Ice sheets reach well into the mid-latitudes, leaving a belt of open water around the equator.

Life. Old enough, wet enough and bright enough for the whole sequence: oxygen, multicellular life, colonisation of the land, and large mobile animals to eat it. The land is vegetated wherever water and temperature allow, and the vegetation is the colour the star makes it.

Local physics sets the limits. Gravity here is 1.02 g, so the heaviest a land animal can get before its own legs fail is about 66 tonnes, against seventy on Earth, and a tree can stand 124 m tall before it can no longer pull water to its own crown. Powered flight is marginal – gliding at best, and only for very small bodies.

Vegetation covers 60% of it, coloured #7c6c43. This star’s photons peak at 710 nm. A pigment takes that band and the blue below 490 nm, where the photons are worth the most each, and reflects the window between them.

Minerals

h — hazy

terrestrial, 0.62 M⊕ and 0.88 R⊕ at 1.57 AU, going round in 2.17 years. Surface gravity 0.80 g, mean temperature 214 K (-60 °C).

Methane and nitrogen in the upper air are being taken apart by ultraviolet light and put back together as heavy organics, which rain out as a red-brown tar. There is enough of it to hide the surface completely. Cold, orange, and chemically busy – this is Titan.

Life. An ocean sealed under ice, with hot rock at the bottom of it. Water reacting with fresh silicate releases hydrogen, and hydrogen plus dissolved carbon dioxide is a complete energy budget that never once refers to the star. Earth’s own hydrothermal vent communities run on exactly this, and are among the strongest candidates for where life here began.

It does not lead anywhere. The chemical energy available is a tiny fraction of what sunlight delivers to a surface, and there is no route from it to an oxygen atmosphere or to anything large. Expect mats, films and plumes, and expect them to have stayed that way for as long as the ocean has existed.

Minerals

i — ice giant · beyond the snow line

ice giant, 35.04 M⊕ and 5.82 R⊕ at 6.01 AU, going round in 16.26 years. Surface gravity 1.03 g, mean temperature 85 K (-188 °C).

Methane in the upper atmosphere absorbs red light and lets blue through, which is the whole explanation for the colour. Below the cloud tops it is water, ammonia and methane ices under enough pressure to conduct.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Moons (7)

# Radius Orbit Period Surface Notable
1 811 km 3 planetary radii 0.83 d sulphur held eccentric by a resonance; tidal heat 8.30 W/m²; ocean under 30 m of ice; chemosynthetic life, buried; hydrothermal sulphides (rich)
2 1,252 km 3 planetary radii 0.90 d sulphur held eccentric by a resonance; tidal heat 20.05 W/m²; ocean under 10 m of ice; chemosynthetic life, buried; hydrothermal sulphides (rich)
3 510 km 10 planetary radii 4.71 d airless rock held eccentric by a resonance; ocean under 124 km of ice; volatile clathrates (workable)
4 717 km 15 planetary radii 8.21 d airless rock held eccentric by a resonance; volatile clathrates (workable)
5 510 km 25 planetary radii 17.08 d methane frost held eccentric by a resonance; ocean under 140 km of ice; volatile clathrates (rich)
6 657 km 41 planetary radii 36.46 d methane frost volatile clathrates (rich)
7 342 km 63 planetary radii 69.95 d methane frost ocean under 63 km of ice; platinum-group metals (rich)

Tau Ceti

K1, 0.783 M☉ and 0.376 L☉, 11.75 light years out. System age 8.08 Gyr against a main-sequence life of 21 Gyr.

Metal-poor, stable, and the classic destination in hard SF.

Habitable zone 0.627–1.1 AU. Snow line 1.64 AU. 4 planets, 6 moons. Seed 48167712.

Planet Orbit Mass Class Surface Moons Life
b 0.0667 AU 0.15 M⊕ terrestrial desert — —
c 0.0753 AU 0.25 M⊕ terrestrial desert — —
d 1.91 AU 4.11 M⊕ super-Earth hazy 1 chemosynthetic life, buried
e 6.92 AU 335.02 M⊕ gas giant ammonia-cloud giant 5 —

b — desert · tidally locked

terrestrial, 0.15 M⊕ and 0.60 R⊕ at 0.0667 AU, going round in 0.02 years. Surface gravity 0.42 g, mean temperature 787 K (+514 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

c — desert · tidally locked

terrestrial, 0.25 M⊕ and 0.69 R⊕ at 0.0753 AU, going round in 0.02 years. Surface gravity 0.53 g, mean temperature 741 K (+468 °C).

Bare rock under thin air, with no water left to speak of.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Minerals

d — hazy · beyond the snow line

super-Earth, 4.11 M⊕ and 1.79 R⊕ at 1.91 AU, going round in 2.99 years. Surface gravity 1.28 g, mean temperature 177 K (-96 °C).

Methane and nitrogen in the upper air are being taken apart by ultraviolet light and put back together as heavy organics, which rain out as a red-brown tar. There is enough of it to hide the surface completely. Cold, orange, and chemically busy – this is Titan.

Life. An ocean sealed under ice, with hot rock at the bottom of it. Water reacting with fresh silicate releases hydrogen, and hydrogen plus dissolved carbon dioxide is a complete energy budget that never once refers to the star. Earth’s own hydrothermal vent communities run on exactly this, and are among the strongest candidates for where life here began.

It does not lead anywhere. The chemical energy available is a tiny fraction of what sunlight delivers to a surface, and there is no route from it to an oxygen atmosphere or to anything large. Expect mats, films and plumes, and expect them to have stayed that way for as long as the ocean has existed.

Minerals

Moons (1)

# Radius Orbit Period Surface Notable
1 3,389 km 22 planetary radii 7.26 d snowball ocean under 99 km of ice; volatile clathrates (workable)

e — ammonia-cloud giant · beyond the snow line

gas giant, 335.02 M⊕ and 11.53 R⊕ at 6.92 AU, going round in 20.56 years. Surface gravity 2.52 g, mean temperature 70 K (-203 °C).

Cold enough for ammonia to condense into a bright cloud deck, banded by its own rotation. This is a Jupiter.

Sterile. Liquid water has never been stable here for the half billion years that even the fastest reading of Earth’s record requires.

Moons (5)

# Radius Orbit Period Surface Notable
1 1,161 km 4 planetary radii 0.85 d sulphur held eccentric by a resonance; tidal heat 22.06 W/m²; ocean under 10 m of ice; chemosynthetic life, buried; hydrothermal sulphides (rich)
2 773 km 17 planetary radii 8.95 d airless rock held eccentric by a resonance; volatile clathrates (workable)
3 2,974 km 18 planetary radii 9.52 d snowball held eccentric by a resonance; ocean under 184 km of ice; volatile clathrates (workable)
4 2,362 km 26 planetary radii 16.87 d methane frost ocean under 292 km of ice; volatile clathrates (rich)
5 2,159 km 36 planetary radii 27.63 d methane frost volatile clathrates (rich)
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