Celestia Forums

In one of my systems, I have a planet tidally locked to its host star. Now obviously this means one side is completely frozen, the other one is in perpetual daylight.

There are three things I'm not very sure on, which is why I'm posting:

1) Wouldn't there be a massive storm at the point of most sunlight?
2) If there were a storm, would it be similar to a hurricane/typhoon with an "eye" in the middle?
3) Would the planet tilt on its axis like Earth, or not?

I'll probably have more questions depending on the answers you guys give me.

Thanks in advance.

Well, Angry Space Goat,
I certainly don't have the qualifications to answer your questions but, even those who do know the answers may need a little more information about your planet. For example; the orbital distance, geophysical features (gaseous or rocky) and planet size. These are only a few variables off the top of my head that I'm guessing others will need to know in order to provide satisfactory answers to your questions.

The planet is terrestrial, weighs in with a radius of 8,097 km, 4.5 million kilometers from its host star which is class M with a radius of 0.11 Rsun.

Angry Space Goat wrote:In one of my systems, I have a planet tidally locked to its host star. Now obviously this means one side is completely frozen, the other one is in perpetual daylight.

No, it means that one side is in perpetual darkness, the other is in perpetual daylight. Doesn't necessarily mean the darkside is frozen though, depends on how thick the atmosphere is (thicker atmosphere will be able to transport heat better to the darkside). It wouldn't be warm on the nightside, but it may be not too far below freezing on the antisolar point.

1) Wouldn't there be a massive storm at the point of most sunlight?
2) If there were a storm, would it be similar to a hurricane/typhoon with an "eye" in the middle?

Why would there be a storm? The subsolar point is basically the hottest place on the planet, any moisture would be driven away from there by the relentless heat toward the nightside. If any part is going to have significant clouds, it'll be the twilight zone where the hot and cold fronts meet from the two sides.

3) Would the planet tilt on its axis like Earth, or not?

No, it can't be tilted if it's tidally locked. And if you think about it, even if it did tilt, it'd make no difference - axial tilt is only relevant if the planet is rotating relative to the star.

Understandable. I saw something similar to this on something aired by the National Georgraphic called Extraterrestrial - however, I didn't completely agree with the science behind it. That's why I asked.

However...

Malenfant wrote:Why would there be a storm? The subsolar point is basically the hottest place on the planet, any moisture would be driven away from there by the relentless heat toward the nightside. If any part is going to have significant clouds, it'll be the twilight zone where the hot and cold fronts meet from the two sides.

What of a very moist planet with a thick atmosphere? With these changes in variables, would it change the outcome?

You're probably going to have a lot of convection at the subsolar point. If the region is oceanic, there would be a lot of moisture going into the atmosphere there. Not sure if this would be enough to set up a storm though.

If you have a relatively thick atmosphere and an oceanic environment in both hemispheres, both of these would serve to effectively transport heat about the planet, keeping the planet at a somewhat clement temperature... at least as far as liquid water goes.

Also, I'm assuming the star being orbited is a red dwarf. If that's the case, here are some interesting articles:

http://www.space.com/searchforlife/seti ... 50728.html

http://www.spacedaily.com/news/life-01c.html

http://www.spacedaily.com/news/seti-05f.html

I used to have links to some good pdf papers on the subject, but I'll be buggered if I can find them **sighs**

...John...

I made a couple of tidally locked worlds for the Orion's arm package;
here are the pages describing them
http://www.orionsarm.com/worlds/Twilight.html
http://www.orionsarm.com/worlds/Dante.html

on these pages I have assumed that the only habitable regions would be along the terminator, and that the substellar point would be too hot to support life or to retain liquid water or clouds.
This might not necessarily be true, as the temperature there would probably only be in the region of 50 Celsius on some such planets, according to the links provided by John Dollan.

Because they orbit the local star in a few days or so, there will be a certain coriolis effect, which might cause the sort of permanent cyclone over the substellar point described by the National Geographic programme. But I am not sure that all such planets will have such a feature.

For a tidally locked terrestrial (rocky) planet - the only arguement really against there being an Earthlike environment on it does NOT have anything to do with it being tidally locked - but the fact that your theoretical planet orbits an M class red dwarf, and NOT a main sequence solar dwarf (Population V, Spectrums F6 through K5 - Sun is a G class but there is currently a debate raging as to whether our sun [SOL] is a G2, a G4, a G5, or a G6).

Your planet is to cool if your orbital length (distance) for your planet is not adjusted so that it is to scale with the M stars LZ (Life Zone) - which for our sun lies about between 0.86 AU and 1.60 AU (My approximation - an educated guesstimate). The LZ is a planet's distance from a star where it is at the scale distance when it get's an equivalent amount of stellar thermal radiation as our Earth does from our sun.

M class stars have an extremely short life cycle for their current phase - you might call them "cosmic farts" - Advanced life beyond simple opportunistic organisms (theoretically) would not have time to develope.

HOWEVER - a tidally locked Earth twin CAN theoretically exist if it orbits a yellow white, yellow, orange, or reddish orange star, provided that the conditions for the maintanences of it's atmospheric canopy were superbly optimal. Here's how I see it:

The Planet would be very stormy, and storm fronts would constantly pound and pelt it's surface all over the planet - not just at its terminal meridian (juncture of "light side" to "dark side") - if it orbits the sun on it's axis the way our Earth does - then the whole planet may get gradual sun cover anyway - and there wouldn't be a "dark side" in the truest sense - but planetary sunlight distribution across the whole of it's sphere would take an entire planetary year to complete - and not in the ~24 hour period as it does on Earth.

These massive planetary storms would be caused by super sized high and low pressure fronts - especially if your world is an ocean planet like our's where at the very least it's surface is 50% water cover to land mass.
You would have steamy,humid ocean streams jutting through cold water pockets - producing whopper storm fronts. The planet's jet stream would be a mess. If your planet is a desert world with minimal or no significant precipitation - then expect massive planet wide dust and/or sand storms powerful enough to strip the skin off of you in less than a minute.

There would be hurricanes that would NOT break up over land as they do on Earth - but would form "super twisters" possibly as wide as Texas, that would scrape across land masses - wearing down even mountain ranges.
The rocky areas on this planet would have a lot of smoothness due to constant wind and rain erosion.

The planet would NOT have polar caps - because it's surface would not have a stable enough climatology - the formation of polar caps is a gradual process - and from what we know of - from our own planet's history - only occurrs during periods of glaciation (Ice Ages).
For most of our own planet's history - our poles have generally played host to either warm , mild, or cool temperate zones and forests - NOT ice and glaciers! We have had polar caps rarely in our world's geological history - we are naturally a planet that has been a mixture of tropical rain forest and arid deserts - so don't be too shocked about all the "global warming" chatter - it's just our planet returning to it's proper tropical climatology.

M class stars have an extremely short life cycle for their current phase - you might call them "cosmic farts" - Advanced life beyond simple opportunistic organisms (theoretically) would not have time to develop

No, I think that is exactly opposite to the true situation. M class red dwarfs have a period of flareing activity when they are young, which might prevent the development of complex life; but then I understand that they settle down, and continue to shine at a roughly constant rate for tens, or even hundreds of millions of years.
(edit - ach. I mean billions).
If a planet is located within the habitable zone it will be
close to the star
probably tidally locked
not exposed to exceptional UV light levels
and sustained by constant energy levels for a very, very long time.

It might be that the red dwarfs will eventually be the home of most of the life in the universe.

Since solar tides in the habitable zone are going to be stronger than those Earth receives, presumably they could stabilise a planet's obliquity rather like lunar tides do for our planet, without which we could be spending a few million years with the poles pointed at the Sun every once in a while. That could have an interesting though not particularly pleasant effect on the climate.

So maybe we are located around a more luminous star than is typical for a world with advanced life forms.

eburacum45 wrote:

M class stars have an extremely short life cycle for their current phase - you might call them "cosmic farts" - Advanced life beyond simple opportunistic organisms (theoretically) would not have time to develop

No, I think that is exactly opposite to the true situation. M class red dwarfs have a period of flareing activity when they are young, which might prevent the development of complex life; but then I understand that they settle down, and continue to shine at a roughly constant rate for tens, or even hundreds of millions of years.
If a planet is located within the habitable zone it will be
close to the star
probably tidally locked
not exposed to exceptional UV light levels
and sustained by constant energy levels for a very, very long time.

It might be that the red dwarfs will eventually be the home of most of the life in the universe.

Right. M Dwarfs are the most long-lived stars around - everything ELSE is a 'cosmic fart' by comparison. An M V star is going to be in its main sequence not for hundreds of millions of years, but for hundreds of BILLIONS of years. The least massive last for TRILLIONS of years.

A planet in an M dwarf's habitable zone will be very close to the star, it will definitely be tidally locked, it'll only be exposed to high UV during the period when it is a Flare Star, and it'll be a fairly constant stellar environment for billions and billions of years.

This was one of the premises explored in National Geographic Channel's EXTRATERRESTRIAL last year. (Sadly, I doubt this will be released on video/DVD. )

d.m.f.