~76% Main Sequence Stars are Red Dwarfs

[jcrocker]

I think the "Ancients wot dunnit" defence is more viable here. After all, they transplanted humans in many systems and wide-scale terraforming in favour of human-habitable worlds makes sense. But motive? Unless human conditions are favoured by Ancients/Droyne as well.

Motive? That could be anything - pure science to 'hold my beer'.

It's a bit of a cop-out, I admit, but since these Marches were rolled from tables when we knew of exactly 8 planets we didn't live on...

 

[Wol]

A formula I have read in a world building book is 0.0161 x ((number of years to test for tidal locking) x (star mass in Solar Masses)^2 / (density in g / cm^3))^1/6 will give you the distance in AU from the star mass where a world will be tidally locked given the number of years entered. Generally use 5,000,000,000 years, but you could adjust this up or down depending on the lifetime of the star which can be derived from its mass.

For example, using 4.1 billion years as our age, a stellar mass of 1.46 Sol, and a planet density of 0.94 Earths (x 5.515 g / cm^3) yields a tidal locking limit of 0.56 AU for that star - i.e. Orbits 0 and 1 are tidally locked and Orbits 2 and further are not. I've repeated this calculation ad nauseum seeing if I can find a sweet spot where the planet is not too cold but is not tidally locked, and it comes out as "somewhere between orbit 1 and 2" in nearly all cases.

The formula was derived from this Wikipedia entry (not by me).

So you can get some relief by having denser planetary bodies. Hmmm

 

[OjnoTheRed]

So you can get some relief by having denser planetary bodies. Hmmm

A tiny amount, yes; but all worlds in a rocky / metallic density range will still end up tidally locked in side about 0.5 AU for most main sequence stars. The rule in Traveller5 is reasonably accurate (Mercury and 3:2 resonance notwidthstanding).

Sometimes even tidally locked in Orbit 0 the planet is still fairly cold!

 

[Wol]

Yes indeed. I could get about 5-10% with densities of 7-10g/cm^3. Maybe just enough in a few marginal cases. Lower density or younger stars also help a bit.

It would be interesting to know the currently estimated average age of red dwarfs. Presumably < 13 billion years but does anyone know?

regards

 

[Wol]

Looking into this further I note that the formula appears to assume an initial 12 hour rotation period, on the basis that most asteroids rotate once between 2 hours and 2 days. A guess but seems as good as any. I have not played yet with the full formula, but considering time elapsed you could crudely interpolate based on what the current tidal locking radius is and the actual planetary orbit, whether it is reasonable for it to be rotating slowly (and estimate what that period is), based on a faster initial rotation rate.

After all, there is not much real world data to go on yet.

This might, as a hand wave, explain why the habitable planets are at least somewhat habitable, and those not so are tidally locked

regards

 

[whulorigan]

It would be interesting to know the currently estimated average age of red dwarfs. Presumably < 13 billion years but does anyone know?

Red dwarf star age could be anywhere from recently formed high-metallicity Population I stars, to over 10-12 billion year old low-metallicity Population II stars, AFAIK, since star-formation is an ongoing process in the galaxy. But not so old as to be primordial, as such stars would be Population III and would be observed to have virtually NO metallicity.

 

[Wol]

Interestingly, trying two similar but different formulas for a world at .4 Au from a a K9V (just outside the approx .39 Tidal locking at 5 billion years) gives values of 5.3 billion and 7.6 billion years respectively. Hardly an exact science with so little real data on these UWPs. Again if they are rotating faster at start they may well still be rotating now.

regards