Tidally locked worlds

[Gruffty]

I'm trying to find out how to tell if a world is tidally locked to its star.

A prolonged search on the net seems to indicate that planets orbiting stars with a mass of less than 0.7 sol will be tidally locked.

Has anyone one got any further information or ideas on this? I'm (hopefully

) looking for a formula I can plug into excel to do the maths for me....

 

[Merxiless]

Sent Response Via COTI PM System

 

[TempMal]

Malenfant here. Hey, Gruff

Your statement is correct, with the extra caveat that planets orbiting within the lifezone of those stars will be tidally locked.

The actual formulae to calculate this are very complex and rely on a lot of factors, including the rigidity of the planet, the distance from the star, and initial rotation period.

Also remember that tidally-locked worlds that close to the star will have no moons.

 

[Pickles]

Welcome back, Mal.

 

[vutpakdi]

A tidally locked world shows up in Jack McDevitt's latest novel Seeker near the very end. Seeker is another one of McDevitt's well written sci-fi archeological / historical mysteries. The mystery starts when a woman tries to sell a single cup, emblazoned with the emblem of a space mission that disappeared thousands of years ago, to a seller/finder of historical artifacts.

Ron

 

[TempMal]

Originally posted by Bromgrev:
Welcome back, Mal.

I should point out that I'm not really "back", I just returned to chip into an ongoing discussion about the status of the TNE:1248 pre-orders. I don't intend to be helping people out with worldbuilding, I've got other things on my plate at the moment. That and my interests have moved onto other things somewhat.

So don't get your hopes up .

 

[Border Reiver]

Oh well, bye then.

 

[archhealer]

Ok, not to sound like a complete rube, but what's tidally locked mean?

 

[jfetters]

Where one side of a planet is always facing another astronomical body, like Luna to Earth:

http://en.wikipedia.org/wiki/Tidal_locking

 

[Merxiless]

I think Across the Bright Face used this concept.

When mapping worlds using Scouts/Grand Survey, you map it such that one pole is lit, the other dark, with the lit side facing the local star.

Figuring out lighting and seasons for a tidally locked moon is a fun TI-Graphing Calculator exercise.

 

[TheDS]

First In gives you an idea how to compute it as well. Useful for most of your world-building needs.

 

[Border Reiver]

Not wanting to upset any planetary scientists/geologists but aren't tidally locked planets still only theoretical. To date only moons (of varying sizes) have been found to be tidally locked. Don't know if the new exosolar planets will prove the all the maths correct but Gliese 436 is sizing up to be a possible candidate for tidal locking.

Hell I don't know what defines a planet, star and whatever now. It's all becoming blurred.

 

[parmasson]

Kurega Gikur’s guide to the stuff in space.

Planet: Has monsters and you can land on it.
Star: bright thing in the sky
Moon: Smaller thing orbiting something else. You can land on these.
Gas Giant: good for refueling and hiding from Imperial patrols.
Asteroid: You build your secret base in one of these.
Brown Dwarf: Useless
Black Hole: Dangerous
Pulsar: Good for dramatic effects and navigation if you misjump but other than that?
Cutlass: Thing you use to start a war about the effectiveness of swords in space.
Space Station: Spaceship with no engines
Arcology: Really big space station

 

[Border Reiver]

Thanks Kurega, can you add some more. I need definitions for Nebulae, Galaxies, Dark Matter, Comets, Lasers, White hole?

 

[Gruffty]

Originally posted by TempMal:
Malenfant here. Hey, Gruff

Hi Mal, good to see you back, even if only temporarily

Originally posted by The DS:
First In gives you an idea how to compute it as well. Useful for most of your world-building needs.

Thanks for that, DS, indeed it does. However, GT:FI, whilst an excellent book, doesn't actually answer my question in the way I want, i.e. GT:FI gives us DMs to throws, etc, that modify the world building outcomes. I'm looking for a formula, the sort of thing for orbits, length of year, etc, like GT:FI provides.

Originally posted by Archhealer:
Ok, not to sound like a complete rube, but what's tidally locked mean?

Like Jim Fetters says. Teh. Don't sweat it, Archhealer - despite mucho input from multiple sources, including a real astro-planetary-scientist-person with a PhD it took me a while to grasp the idea of a world with "infinite rotation" and the other concepts involved. Heck, the only resonance I had heard about up til then was musical resonance...."vvvmmmmmmmmmmmmmm......"

Originally posted by Border Reiver:
aren't tidally locked planets still only theoretical

I dunno. There seems to be a lot of scientists talking about tidally locked worlds on the web, and I had no problems finding references to it on numerous sites. I also found some fantastic looking formulae (?sp) on the 'net for luminosity, mass, lifespan, absolute magnitude, bolometric luminosity, etc etc. But only vague mentions of tidally locked worlds, i.e. "M V stars may only have small, tidally locked worlds orbiting them..."yaddayaddayadda (that's not a direct quote from a website, BTW, but it is the sort of thing I kept finding on the 'net).

Oringially posted by Merxiless:
I think Across the Bright Face used this concept

It does indeed

I suppose I could actually be looking for a formula that doesn't translate across easily to world building in Traveller

Oringially posted by TempMal:
Your statement is correct, with the extra caveat that planets orbiting within the lifezone of those stars will be tidally locked. The actual formulae to calculate this are very complex and rely on a lot of factors, including the rigidity of the planet, the distance from the star, and initial rotation period.

Also remember that tidally-locked worlds that close to the star will have no moons.

So, given the above (i.e. the huge complexity of the maths involved), could I construct a "general rule" about tidally locked planets and M V stars? Such as:

"A planet orbiting an M V star (with a mass of less than 0.7 sols) will be tidally locked to its star if:

The planet orbits the M V star less at less than xx.xxxx AUs;

or:

The planet's orbit is within the M V star's habital zone."

 

[Gruffty]

Originally posted by Border Reiver:
Thanks Kurega, can you add some more. I need definitions for Nebulae, Galaxies, Dark Matter, Comets, Lasers, White hole?

I can give you a definition of a brown hole, but you'd probably have a better one, being a paramedic

 

[Border Reiver]

Seems fair enough to me Gruffty. All my reading indicated that many scientists stated as fact that "such & such a" planet will be tidally locked, these staments based on minimal observations and math with too many assumptions thrown in.

I think the rule for tidal locking you came up with is more than adequate and potentially models reality quite well. My own caveat for tidal locking is that recent models indicate that tidally locked worlds can retain a habitable atmoshere. Could make things interesting.

 

[TempMal]

Originally posted by Border Reiver:
Not wanting to upset any planetary scientists/geologists but aren't tidally locked planets still only theoretical. To date only moons (of varying sizes) have been found to be tidally locked. Don't know if the new exosolar planets will prove the all the maths correct but Gliese 436 is sizing up to be a possible candidate for tidal locking.

Hell I don't know what defines a planet, star and whatever now. It's all becoming blurred.

Tidally locked planets are theoretical in the sense that we haven't seen any in a 1:1 lock. Mercury is in tidally locked in the sense that it's in a 3:2 spin-orbit resonance though. In english, that means that solar tides have forced its rotation rate to slow to the extent that it completes three rotations for every two orbits around the sun. And we know moons can lock to planets.

There is no reason whatsoever to believe that 1:1 tidal-locking - where the planet has the same rotation rate as its orbital period around the star and so keeps the same hemisphere pointing to the star all the tiem - is not possible. The maths and physics behind this are rock-solid: if moons can lock to planets then planets in the right situation can lock to stars too.

All my reading indicated that many scientists stated as fact that "such & such a" planet will be tidally locked, these staments based on minimal observations and math with too many assumptions thrown in.

I suspect many of the gas giants that orbit close to the stars are probably NOT tide-locked - gas giants are mostly fluid and require a lot more tidal interaction to slow them down. But the 'assumptions' stem from not knowing what they're made of - as I said, the maths and physics are sound. There may be doubts as to whether specific planets are tidelocked to their stars, but there is no doubt whatsoever that generally planets CAN be tidelocked to stars.

 

[Gruffty]

Gruffty's Tidally Locked Worlds Rule
(a.k.a. the "Gruffty is opting out of doing lots of really hard maths" rule ):

"A planet orbiting an M V star (with a mass of less than 0.7 sol) will be tidally locked to its star if the planet's orbit is within the M V star's habital zone. Additionally, such a tidally locked world will not have any moons in orbit."

I agree, Border Reiver. A generic rule seems like a good idea (it can be used for all Traveller rules sets without wrecking the dice throwing parts).

So, what about the types of world that are tidally locked?

1) What size (or sizes) can/should they be?
2) What sort (or sorts) of atmosphere(s) can these worlds have?
3) How much (if any) would/should the Hydrographics % be?

My feelings (and this is only what they are, feelings) are:

1) World sizes 1, 2, 3, 4 or 5;
2) Atmospheres 0, 8, 9, A, B or C;
3) hydrographics 0 or 1.

Thoughts?

 

[TempMal]

Originally posted by Gruffty:
Hi Mal, good to see you back, even if only temporarily

I suspect I'll be around to correct any serious misunderstandings/bloopers about astronomy that I see. I just don't have the inclination to be figuring things out for people anymore.

it took me a while to grasp the idea of a world with "infinite rotation" and the other concepts involved. Heck, the only resonance I had heard about up til then was musical resonance...."vvvmmmmmmmmmmmmmm......"

"Infinite rotation" is actually confusing. Forget that

.

A tidally locked planet (or moon) is simply one where the rotation period is the same as the body's orbital period around the thing that it's orbiting. Thus, it keeps the same face pointing toward the primary all the time.

Technically, that's a "1:1 spin-orbit resonance" because one rotation = one orbital period. Mercury's 3:2 spin-orbit resonance means that 3 rotations = 2 orbital periods. You can get "orbit-orbit resonances" too - the Galilean satellites of Jupiter are in a 4:2:1 orbit-orbit resonance involving Io, Europa and Ganymede. That means that Io completes 4 orbits in the same time as Europa completes 2 orbits and Ganymede completes 1 orbit in the same period too. However (just to confuse you), Io, Europa and Ganymede are in 1:1 spin-orbit resonances around Jupiter: in other words, they're tide-locked to Jupiter as described above, so they always show the same face to Jupiter. Clear as mud now?

I suppose I could actually be looking for a formula that doesn't translate across easily to world building in Traveller

It really doesn't. It involves a lot of obscure terms and constants, and it's a bitch to calculate.

So, given the above (i.e. the huge complexity of the maths involved), could I construct a "general rule" about tidally locked planets and M V stars? Such as:

"A planet orbiting an M V star (with a mass of less than 0.7 sols) will be tidally locked to its star if:

The planet orbits the M V star less at less than xx.xxxx AUs;[/QB]

Basically yes - but only if you're talking about a terrestrial world (remember gas giants take a LOT longer to slow down their rotation because of their fluid nature). Solid bodies tidelock so fast when they're close to their star that pretty much the only time you'll see freely-rotating bodies close to the star is less than a million years just after the system forms.

Certainly though, the life zones of M V stars are so close to the primary that anything there HAS to be tidelocked. But it's complicated by the presence of other planets nearby, and the eccentricity of the orbit (which is why Mercury's isn't a 1:1 lock)