Orbital data

[darue]

Taking a K6 V star to be 0.64 solar masses and to have a luminosity of 0.137 (very middle-of-the-road values):

The orbital distance at which a planet receives the same insolation as Earth is 0.37 AU.

The orbital period of the planet will be 0.308 years. It's proportional to the 3/2 power of the semi-major axis divided by the square root of the star's mass.

Realistically, the planet's rotation period is almost certainly going to equal to the length of its year (i.e., it will be tidally locked to its star if it's older than a few hundred million years).

As a rule of thumb, planets in habitable-zone orbits are expected to be tidally locked to any star of 0.7 solar masses or less, because of the necessary proximity to the star. You can thwart the tidal lock with an eccentric orbit and the right rotation-to-revolution ratio (see Mercury for an example), but this is only going to make conditions on the planet less conductive to complex life than a simple tidal lock.

some questions, hope they're easy...

If it was a planet with significant liquid oceans, like 100% coverage and fairly deep, would that cause it to become tidally locked faster, slower or have no effect?

Does a large moon have any effect toward resisting tidal locking?

 

[shaunhilburn]

some questions, hope they're easy...

If it was a planet with significant liquid oceans, like 100% coverage and fairly deep, would that cause it to become tidally locked faster, slower or have no effect?

Oceans are the mechanism by which the sun and moon are slowing the Earth's rotation. A planet can be tidelocked without oceans, but the friction of the liquid tidal bulge against the seabed creates increases the braking effect. More coverage gives rise to more bulge and more surface to act on, hence more braking. Depth probably isn't a factor. If the planet were entirely fluid, like a gas giant, the locking timescale is much longer.

Remember,though, that planets in the tidelocking zone accrete in synchronous rotation. They are in a tidelocked state from their birth. The only free-spinning planets that go though a spin-down process are those wandering into the tidelocking zone.

Does a large moon have any effect toward resisting tidal locking?

Three-body perturbations prevent stable orbits around a tidelocked body. In this case, retrograde orbits are somewhat more stable than prograde orbits. Most of the moons in our solar system are tidelocked, and this is the reason we don't see them with moons of their own.



I wouldn't expect to find moons in the tidelocking zone. Tides will either drive a moon closer to its planet to eventually merge with it, or draw it outward and strip it way.
From there, a variety of fates are possible. The interactions with other planets could cause the rogue moon to be shepherded into a planetary orbit of its own; it could collide with one of the other planets or its original parent; it could be ejected from the system or collide with the star.

 

[darue]

thanks much!

 

[aramis]

The one possible exception to a 3-body perturbation problem is Body B orbits body A, and body C falls into a stable L1, L4 or L5 orbit, itself becomeing tidelocked to A (and thus apparently to B). (L2 and L3 are inherently less stable than L1, L4, or L5, and L1 is less so than either L4 or L5.)

 

[shaunhilburn]

The one possible exception to a 3-body perturbation problem is Body B orbits body A, and body C falls into a stable L1, L4 or L5 orbit

The L1 position is not stable. L4 and L5 are stable (the object orbits the L4 or L5 point). It's possible a moon-sized body would stay there for geologic timescales, but I doubt it.

Theia, the hypothetical moon-forming impactor, is thought to have originated at Earth's L5 position. It is orbit de-stabilized as it accreted more mass, and it began oscillating in a increasingly wider orbit, approaching proto-Earth closer with each pass until its ultimate collision - so goes the theory.

 

[aramis]

The L1 position is not stable. L4 and L5 are stable (the object orbits the L4 or L5 point). It's possible a moon-sized body would stay there for geologic timescales, but I doubt it.

Theia, the hypothetical moon-forming impactor, is thought to have originated at Earth's L5 position. It is orbit de-stabilized as it accreted more mass, and it began oscillating in a increasingly wider orbit, approaching proto-Earth closer with each pass until its ultimate collision - so goes the theory.

L1 is stable enough for high ratios.

The Theia-as-L4/L5 hypothesis is dubious - L4 and L5 have been stable long enough despite perterbations from other bodies - Earth has at least one, Mars 4, Jupiter thousands, Neptune 9+, Uranus a couple, and two of saturn's moons have L4/L5 companion moons.

L1 would only be stable IF there are no other significant bodies besides A, B, and C... or it B is hugely massive.

Still, due to the needed insertion, it's much more likely than the L2... and the L3 is the least stable of the lot.

Which reminds me - trojan bodies are NOT covered in Traveller generation.

 

[shaunhilburn]

Which reminds me - trojan bodies are NOT covered in Traveller generation.

In double adventure 2, "Across the Bright Face", Dinom was originally a satellite of the system's gas giant, now trapped in the trailing trojan position of the gas giant.

It seems MMW took for granted a certain level of sci-literacy among his readers. There was probably an assumption that refs/players would naturally incorporate rules like these without the need for supplements.