Tau Ceti

Tau Ceti

Fact

Tau Ceti is a Sun-like (G type) star about 11.9 light years from here; it's the closest single such star to us (the Alpha Centauri system is closer and has a sun-like star, but is a multiple). As such it's been a popular place in science fiction, authors from Asimov to Zsoldos have set novels or parts of novels there.

It's a much older star than its near neighbor, Epsilon Eridani (about 5.5 ly away from τ Ceti), and like that star we believe it has planets, as well as a dust/asteroid disk, bigger than ours but smaller than ε Eridani's. This disk seems to be towards the outer part of the system, whereas the so-far discovered planets are fairly close in. The star itself is a bit smaller, cooler and dimmer than our own Sun, which moves the habitable zone in closer, but it is still quite a bit brighter and warmer than K-type stars like ε Eridani or α Centauri B.

Planets

Note: Earlier reports suggested five planets, b through f. More recently (August, 2017) this has been refined to just four, with b, c and d being superceded by g and h. Tau Ceti is also believed to have approximately 10x the mass of asteroids and dust as our own solar system.

Name	Mass M⊕	Orbit AU	Period (x24h)
I (g)	≥1.7 ± 0.3	0.133 ± 0.002	20.00 ± 0.01
II (h)	≥1.9 ± 0.4	0.243 ± 0.003	49.41 ± 0.09
Inner H.Z. Edge?		~0.55	~165
III(e) (binary)	≥3.99 ± 0.7	0.538 ± 0.006	162.7 ± 0.5
III-1 (e-1) (Skead)	3.1	as above
III-2 (e-2) supermoon	0.3	as above
IV (f)	~3.77 ± 1.21	~1.37 ± 0.08	616 ± 31
Outer H.Z. Edge?		1.26 to 1.32	~600
Others TBD
Dust disk		6.2 ( >4.6, <9.8) to ≥36 AU

(Note: For fiction, III is assumed to be binary (or with a very large moon) with a total mass of ~3.4 M⊕)

Fiction

So far in T-Space I haven't written much about Tau Ceti. In The Chara Talisman (aka The Venaticorum Archive), a world circling it is the location of Port Skead, the main spaceport in the system and home base for Jackie Roberts and her ship Sophie. It's a bit over 13.5 light years from α Centauri, so it takes about ten days in warp to get there from Sawyers World or Kakuloa. Once there, it's only a four day trip to ε Eridani.

As the above chart shows, there is some question as to whether my hypothetical Tau Ceti III, Skead, is really habitable. It appears to lie just to the warm side of the habitable zone (and IV just to the zone's cold side). Further, both III and IV seem to mass anywhere from 3 to 5 times Earth. That would give them a surface gravity of about 1.5 (depending on density, etc), which might be tolerable, but would also mean a very high escape velocity. This would be beyond anything but multiple stage chemical rockets, but is probably within the capability of the fusion/plasma engines of later T-Space, especially with some aerodynamic lift. However, the above data is very preliminary (and noisy), and contradicts earlier (circa 2015) conclusions about planets in the system.

Of course, the give mass (3 to 5 Earths) isn't necessarily in a single planet. If we take the lower bound (3 M⊕), we can split that into a double planet, say one at 2.5 M⊕ and the other at 0.5 M⊕, or 2 M⊕ and 1 M⊕. They wouldn't necessarily both be habitable, but if the smaller one orbited within the eclipse region of the larger (depending on orbital inclination relative to the primary star), it would be shaded for a portion of every orbit, making it cooler (thus more habitable) than otherwise. Currently the data is too noisy to disprove the double-planet conjecture.

I dunno what that would do for the coffee crop, though.

More calculation needed.

Update: Assume III-1 (Skead) mass 3.1 Me, III-2 mass 0.3 Me, III-1 is the terraformed one, III-2 is vaguely terraform? would it tidal lock in 65 my? Surface gravs ~1.46 and ~0.57 respectively. Skead is heavy but do-able (take-off barely possible with chemical, okay with plasma).

Note 12/15/19: For mass ~3.1, radius 1.46e, density 5.52, sfc grav is 1.46g. For radius 1.45e, density 5.6, sfc grav is 1.47g - more realistically, per Seager2007, radius would be ~1.35, density ~6.9, sft grav 1.69g (due to compression of rocks)