Can a world be tidally locked to it's primary sun, but still have a moon that orbits?

Would the moon necessarily need to be tidally locked?

Would the tidal stresses be too much or just enough to keep the moon very geologically active?
(And also the parent planet)

-Mike

We don't know the true mass - only a minimum value. Also we don't know the orbital eccentricity. It's probably not very high but still the range of possible values provides a huge spectrum of tidal heating scenarios, both for the planet and any moons (even moons in circular orbits about the planet).

It's a fair bet that we haven't found all the planets yet so there could be all kinds of resonance-locking configurations operating. It's an old system with small orbits, so presumably very highly evolved.

It's pretty darn cool that a likely target is so close to our solar system.

Mike: Without trying to work it out exactly, I think the short answer is "no." The moon would (best case) end up at L4 or L5. In fact, I'll bet it won't have any stable orbits around it at all -- much as our own moon has no stable orbits.

tuvas: I'm still trying to figure out why you think there would be any distinction between the north and south poles vs. any other points on the terminator. (Or am I misunderstanding you?) The rotation is so slow, it seems we could ignore coriolis force. Then there are no trade winds; just a single low-level flow from antisolar to subsolar and a single high-altitude flow the other way.

Did anyone have a look at this paper about reappraising the liklihood of habitable planets around red dwarves?

http://www.liebertonline.com/doi/pdf/10.1089/ast.2006.0124

(Someone posted it earlier.) It seems quite optimistic about such a planet keeping its ocean and air liquid.

--Greg

It's wonderful, but only the first of many hundreds (I'm sure) as the minimum detectable mass goes on decreasing. We will need an OWL to look at them.

This is SO wonderful!!!!! I can hardly contain myself.

I have two grandchildren...... 8 months old. When I was that young (1953), the planets in our own system were vague blurry mysteries. The imagination could still well up visions of prehistoric swamps on Venus and the vegetarian wave of darkening in the spring on Mars. The worlds that were eventually revealed to us are stranger by far.

Just 16 years ago, there were NO confirmed extrasolar planets. Than came the report of the pulsar planets.
Not long after (1995) came 51 Pegasi a. Which turned the staid theories of how solar systems should look on their heads.

Now, as the instrumentation gets better and better, we have the detection of atmopsheric properties from Spitzer on transiting hot jupiters, the beginnings of terestrial planet detection (from the GROUND and soon from COROT and KEPLER).

What will my grandsons know when they are my age (54)? I fully expect to see a catalog of terrestrial worlds unfold before us in the next 10 years. Gliese 581c is just the tip of the iceberg.

Glorious indeed.

Craig

Detailed models of super-Earths: How well can we infer bulk properties?

The field of extrasolar planets has rapidly expanded to include the detection of planets with masses smaller than that of Uranus. Many of these are expected to have little or no hydrogen and helium gas and we might find Earth analogs among them. In this paper we describe our detailed interior models for a rich variety of such massive terrestrial and ocean planets in the 1-to-10 earth-mass range (super-Earths). The grid presented here allows the characterization of the bulk composition of super-Earths detected in transit and with a measured mass. We show that, on average, planet radius measurements to better than 5%, combined with mass measurements to better than 10% would permit us to distinguish between an icy or rocky composition. This is due to the fact that there is a maximum radius a rocky terrestrial planet may achieve for a given mass. Any value of the radius above this maximum terrestrial radius implies that the planet contains a large (> 10%) amount of water (ocean planet).

This just arrived on the arXiv queue, and may be of interest to those following this discovery.

Bill

Thank you, Mongo.

What a gripping thought: huge bluish/white (well...perhaps a bit dim and reddish/yellow in some cases from the host star) worlds covered completely by a vast ocean.

Thanks for that link. In attempt to connect dots...

As the paper notes, super-Earths with a large core mass fraction, large ice mass fraction, but low mantle mass fraction are unlikely to exist:



So the idea that Gliese 581c is either a super-Mercury (large iron core, no more surface water than Earth)...OR an ocean planet with lighter core but extensive covering of water...is correct. But the two don't intertwine.

Some estimates of the radius yield a planet with a density approaching that of iron, leading to the idea that its an oversized terrestrial planet with a dense iron core. However, I suspect that this may not be the case. It is important to realize that Gliese 581 is a relatively metal-deficient star, and quite a bit more so than the Sun. Not only does this preclude the development of large-mass planets, but it should also hinder super-dense terrestrial planets, given the accretion disk likewise had low metal content.

With a mass at least 5 times Earth, the radius has to be larger than popular estimates of 1.5 times Earth...otherwise it is dense iron ball. Indeed, the aforementioned paper sets upper constraints on radius for terrestrial planets based on mass:



The upper constraint for a Gliese 581c-size object is 10400km. I do not have a calculator at hand, but I'm pretty sure this would still yield a very high densnity as it is only 1.6-1.7 times Earth's radius. Again, I imagine such a high-density rocky object is unlikely orbiting a metal-poor star. This further indicates that Gliese 581c is not a terrestrial planet, but an ocean planet. The presence of a hot Neptune inside its orbit also favors, although does not require, Gliese 581c to have migrated inward earlier in the stellar system history.

This adds up to a frozen water planet that formed outside the snow line and migrated inward to its present orbit. The result is a ocean planet, probably sporting a thick greenhouse atmosphere and high surface temperatures. While this is hostile by human standards, I wonder if aquatic life could develop on such a world.

Still many assumptions so this hypothesis could be wrong.

I find it strange that every time a planet is discovered via doppler and miminum mass is inferred, it's assumed that's the actual mass. Isn't there a statistically more probable mass in case orbit inclination seen from earth is somewhere around 45 degrees or so? Furthermore, all modelling is done with that low mass and radii deduced, surface gravities, etc.
Regarding eccentricity, IIRC the paper in question states the best fit to observed data is achieved if the two inner planets are on circular orbits so we can eliminate appreciably elliptic orbits.

Actually, I'm sure the probabilities are skewed towards lesser inclination (WRT Earth) in a system detected via the radial acceleration method. And the cos(30-deg) is still >0.85. The mimimum is probably not far from the actual.

I still don't get it where that probability skew comes from. Sure, for a given mass planet and given instrument sensitivity we'd have a greater probability of finding a lower inclination planet, but really, who's to say we're not actually looking at Jupiters in almost bullseye orbits? Who's saying we have a "given" planet mass?
That obviously doesn't apply here, but some other star systems I can imagine it would. I'm probably missing something here.

On another note, is there a way to constrain actual system inclination by observing the host star's rotation? The paper analyzes effects of stellar rotation and "sunspots" on spectral lines, would it be possible to find out the inclination knowing only star type (assuming radius) and period of rotation and matching the doppler amplitude on the starlight itself? Afterwards assuming the star's rotational axis is coincident with the entire planetary system (to say within 20 degrees)?

So why does the Extrasolar Planet Encyclopedia give e = 0.16 plus or minus 0.07?
I think it depends on what you mean by appreciable. My very rough calculations suggest tidal effects would be significant for a planet with e = 0.16 at a distance of 11 million km. (but I'd like someone else to check that).

I don't know about that value ngunn, but the paper clearly states in a table that both GL 581a and GL 581b are fixed in the team's fit at 0.0 eccentricity, while the 3rd planet has a modest eccentricity of 0.20+/-0.09.

Furthermore, they say this in the text:

Table 1 is the table I pulled those numbers out. It is possible that the ecc. figure you're referring to in fact belongs to the (tentative) 3rd planet, GL 581c.

I'm using this information:
http://vo.obspm.fr/exoplanetes/encyclo/sta...l%20581#a_publi

Edited - I see the table 1 lists both values as 2 separate scenarios.

But yes, we are talking about planet 'c'.

Ahhh, messed up my naming, the figures I refer to are planets b,c,d, not a,b,c. Apparently the paper belleraphon1 linked to is from April 4 and the referenced version from your link is an almost identical paper from April 27 and it indeed has the 2nd planet on a eccentric orbit in one of the fits. It appears to be an updated version and even has the slight eccentricity included in the same paragraph I originally quoted.

I think the link is going to 'preprint' versions that are being updated as we speak! Anyhow, my understanding of the 'circular' case is that it's a computational convenience to reduce the number of free parameters, whereas the free eccentricity case is the truest fit to the real data.

True, but since error bars are pretty substantial in the data, practically both scenarios are equally plausible. That's just the limitation of current data.

Of course all these details could change when we find ourselves pondering a six or seven planet solution in a few years' time. Still, it looks as if it would be unwise to completely dismiss tidal heating as a significant possibility for habitable zone planets of M dwarfs, including this one.

Tidal heating would be a good thing.

Tectonic activity could potentially do two really good things for exosolar-life:

1) Greenhouse gas release from volcanic activity
[Thicker atmosphere, warmer climate, and superrotating winds may do a really good job of distributing heat around the planet (bye-bye anitsolar ice cap?)]

2) Black smokers.
Nice rich volcanic vents outgassing rich sulfides in hot water. Life goes "yum". What a great stable enivoronment to have when the host star might throw off the occasional hyperviolent CMB. [the red dwarf seems quiescent now - but who's to say it doesn't periodically go through really long-term temper tantrums?]


I'm really starting to like the idea of a tropical wet super-Venus for Gleise 581. Add swamps and it's just like in the old 1950's textbooks of Venus!


-Mike

What would be a good calculated magnetic field strength of such a metal poor super-Venus, assuming the current models of formation? How would it compare to Earth's?

What about the atmospheric stripping potential of the "solar" wind from the red dwarf primary?

Could 581c hold on to a CO2 and H2O atmosphere for billions of years? Or would it dessicate out like Mars?

-Mike

The Systemic Group website has an animated weather simulation of GL581c up. The model, by Jonathan Langton, assumes that the planet is spin-synchronous, with a radius of 1.7 times that of Earth, and a planetary Bond albedo of 55%.

Bill

Interesting speculation by all.

One cautionary note, though. 581c doesn't necessarily have to be some sort of analog to any planetary environment we've seen or contemplated before.

Remember what a challenge Titan has been to our collective understanding. The more we discover the more I am personally convinced that potential planetary diversity is far beyond what we can postulate...and super-Earths are really a new class of planet in many significant respects.

I kinda agree its very early to speculate specifics about GL581c, since we don't know the composition and extent of the atmosphere, nor of the extent of an ocean... both of which make huge changes in potential heat transfer.

However, it is great fun to think about. My uneducated-guess paints a much more stagnant flow pattern than systemic's model. The reasons why I imagine a double-lock on stagnating global currents:

1. It seems on earth, a major force that establishes major heat transfer between poles is the Coriolus effect. With 581c begin tidally locked, this ability to coax rotation into large fluids is missing. No circulation with the poles sets up large polar ice-caps.

2. The "dark side" of 581c also seems a very vast area for any global current to make much of a dent. I'd see this side of the planet as a massive Greenland of super-huge glaciers; which slowly keep growing as fronts from the warm side trap moisture. Over the eons, this dark-side Thule would become so massive it could impact the precession of the planet, especially if there's no moon. So it's possible that over the scale of thousands of rotations, the dark side is pulled slowly back into the light. This planet may not experience continental drift, but without a big moon, I'd expect it to experience serious axis-drift.

So when's the Planetary Society going to host a challenge for designing a mission to explore this system? ;-) 2000 year time-limit to reach orbit?

Why would there not be a Coriolis effect? Even if tidally locked (at 1:1) the planet still rotates every 312 h (13 days). Wouldn't this be enough to induce a healthy spin and atmospheric banding?

Venus also has a very long rotation period as well (longer than the orbit), but has definite banding in the atmosphere and quite impressive cloud top speeds resulting from superrotation. Couldn't Gliese 581c do the same?

Check out: http://ams.allenpress.com/archive/1520-046...-47-17-2053.pdf


OK, so here's my wild guesses:

Topography like Ganymede/Venus/Earth (ocean basins, but lower relative topography)
Elemental makeup like Earth (atmosphere, surface rocks, oceans)
Weather structure like Venus (cloud patterns and banding)/Earth (temperature)
Geological structure like Earth (crust, core etc.)

And probably as wierd as Titan...

-Mike

See also this press release by Astrobiology.

Thanks for the link, Alex!

I just read the 3 page abstract, which brought up the fact that tidal-locking probably means a weak magnetosphere... oh, and the potential for even the atmosphere to freeze out on the dark side! Eeek!

Is it possible that a planet's core could spin at a different rate of the rest of the planet, or would tidal locking work equally on both the outer and inner components of a planet, even seperated by a molten boundary? Also, according to some statements I've seen, the dynamo that creates the magnetosphere is more related to motions in the core than the rotation of the planet:

For instance:
http://www-ssc.igpp.ucla.edu/personnel/rus...pers/venus_mag/
"It is important to note that, contrary to popular belief, dynamo theory does not credit the smallness of the magnetic moment to the slow rotation of Venus."

And speaking of Venus, why is it that it's amosphere is not undergoing serious erosion despite being closer the sun and having a weak field? The solar wind induces a field for Venus, so why not for 581c?

I'm surprized that red dwarfs experience frequent CMEs. I thought these were as quiescent and long-lived as a star can be... so who needs a magnetosphere?

There is one significant difference. The libration zone has zero width at the poles.

tty: True, but if one assumes there is no significant libration, then what?

If there IS significant libration (e.g. if the orbit is eccentric), then we have a different discussion, but I thought we were discussing the completely locked, circular scenario. In that case, why are the poles distinct from the rest of the terminator?

--Greg

Also:
http://exoplanets.org/index_gl.html

Magnetic field, if any, might have an effect.

What you're saying is that if the planets orbital mechanics were such that it would have full-circle-polar region, then the planet would have a full circle polar region

Doug

Hey, most people don't argue with tautologies. :-)

Seriously, I don't think Earth's magnetic field has a huge effect on the weather at the poles; it's the weak/angled sunlight that does it (or lack of sunlight). In the case of a tidally-locked planet, though, the planet's sun just appears to hang in the same place in the sky all the time, so all places at the same distance from the substellar point should get pretty much the same light from it. Including the poles, which are indistinguisable from any other points on the terminator.

To tell the truth, I doubt the differential libration at the poles vs. the equator would have much effect either, unless the libration were colossal.

I'm still fascinated by that paper that concluded such worlds really could be habitable.

--Greg

You wouldn't want any greenhouse effect on Gliese 581 c. The planet's inferred surface temperature without greenhouse effect is 0-40°C which is way more than on Venus.

The planet is most likely a giant analog of Venus (if it has no water) or an ocean planet with atmosphere full of super-critical steam.

Just out of interest, if you went there and looked for Ol' Mother Earth, you'd find her shining close to two familiar star clusters, the Pleiades and Hyades...

Click to view attachment

... and although Orion would look pretty much the same...

Click to view attachment

... it would take some getting used to seeing M31 shining so close to a familiar, bright star we're used to seeing far away from it...

Click to view attachment

Well, 20 ly is not a long distance compared to most naked-eye stars. All the main Orion stars are very luminous and distant so that constellation looks similar from stars in our local neighborhood.

Stu: This is a place where you may have to travel a long way if you want to see the stars at all. And dress warmly. :-)

--Greg

You're right Greg, and Jyril, I just thought it was kind of cool to imagine the view... you wouldn't have to be on the surface of GL581c (BORING!!! About time we had a working name, don't you think? Suggestions on a postcard please! ) to see those stars after all, just "in the area" or in orbit around it.

Ignore the practicalities, the science, guys... just think about it... It makes the hairs on the back of my neck stand up when I think of the first explorers to land on 'C, standing there on their first post-landing night, looking up at the stars, seeing all those familiar specks of light and then realising that the patterns above their heads are new, and unique to them... realising that they'll have the responsibility of joining the dots to make new constellations, drawn to represent and named after... what? Figures from the exploration and colonisation program? Events from their history that are in our future? Their own family?

Am I the only unashamed romantic here who thinks about things like this?

BTW, new poem about this here...

Hmm. Well, to steal from Niven, how about "Jinx"? That was a habitable (just barely) planet with heavy gravity...watch out for the bandersnatches, though...

Re naming this planet, I propose Giordano - after Frau Bruno, who was burned at the stake in Rome, February 17, 1600.


His heresies included: Claiming the existence of a plurality of worlds and their eternity.

Bruno saw a solar system of a sun/star with planets as the fundamental unit of the universe.

[http://en.wikipedia.org/wiki/Giordano_Bruno#The_cosmology_of_Bruno.27s_time]

"I cleave the heavens, and soar to the infinite. What others see from afar, I leave far behind me." - Giordano Bruno

Nice poem, Stu, although technically speaking, it's not an old star at all -- it's very young. And it will still be young when our own sun burns out.

For a name, might I suggest something to honor Wilhelm Gliese? (Not that I can come up with anything better than "Wilhelm" at the moment.) :-)

I like to envision Gliese C as a sleepy kind of place where nothing ever changes. An eternal afternoon beach party waiting for the guests to arrive.

--Greg

I propose that this new planet be named after the founder of our own UMSF forum.

Planet Doug.

If it has a moon, we can always name that The Other Doug...

-the other Doug

Thinking about looking at Earth from another star reminds me of Robert Heinlein's "Time for the Stars," a novel in which Earth sends out a dozen starships to look for nearby planets. At one point, the teenage hero lies awake at night on a planet of Tau Ceti, and finally gets out of his sleeping bag and goes to look at the stars. It has been almost 40 years since I read it, but I still remember the gist very well.

"I found Orion easily enough, but Sirius wasn't there, of course; it's closer to Earth than Tau Ceti is. I tried to find it for a bit, but I got a headache trying to work the spherical triangles backwards in my head. From there I found Ursa Major, even though the dipper was somewhat squashed, and from that I found Bootes, which was what I'd really been seeking. The distance had spoiled the club shape, but all I wanted to see was the medium-bright yellow star in the middle of it."

Never mind how silly it was to think we'd send people before we sent probes -- and we probably wouldn't send teenagers on the first trip regardless -- this really, really grabbed me, and I so badly wanted to go someplace like that. Somewhere you could see the Sun as just another star.

It still gets me, if I think about it hard enough.

--Greg

Actually, no. The Tarter et al paper (http://www.liebertonline.com/doi/pdf/10.1089/ast.2006.0124) states:

"The critical ingredients for planetary dynamo action are a sufficient volume of electrically conducting fluid, an energy source to drive motions in the fluid, and net organization of the flow field (Roberts and Glatzmaier, 200; Aurnou, 2004). Typically, Coriolis forces produced by planetary rotaion act to organize core flow. Planetary rotation periods within the M dwarf habitability zone are estimated to be between 10 and 100 days. These rotation rates, though slower than of Earth, will still produce strong Coriolis foreces in ~100-1,000-km-deep core fluid layers. Thus, in terms of dynamo theory (Stevenson, 2003), tidally locked planets orbiting M dwarfs are viable candidates for core dynamos."

If tidally locked, Gliese 581c's rotation period is the same as it's orbital period - 13 days. This is towards the faster (better) end of the rotation given above. So it is even more likely to induce a core dynamo and thus a protective magnetic field.

According to this analysis, as long as there is enough of a molten iron core, Gliese 581c should have a protective magenetic field.

-Mike

Thick atmosphere, global ocean, magnetic field, tidal heating - it's a pressure-cooker! Naming is going to be a big problem once these things proliferate. 'Gliese' is hardly an option since hundreds of the nearby stars in the Gliese catalogue probably have a number of planets each.

How 'bout we just refer to it as a larger, greenhouse-warmed (by CO2 and also CH4), tectonically active, humid version of our nearest neighbor? ("massive, hot, throbbing, Venus")

Are you saying there are cows out there?

Photodissociation of methane in the stratosphere by the much redder M dwarf stars is gonna be a whole lot slower than in the Sun's inner solar system. (Less light in the UV band).

While CO2 might exhange with oceans and eventaully get locked away by plate tectonics on Gliese581c, methane will hang out in the upper atmosphere and be a greenhouse gas player for a much longer time than here on Earth.

CH4 could play a much bigger role as a greenhouse gas for planets orbiting M dwarf stars.

[Does this mean Al Gore gets to do another movie?]

-mike

Naming? That's my department...
I would go for...
Planet Kur...
and in case it has a moon...: Sin
Besides sounding, to me, awfuly good, it makes reference to a new Earth a new Moon by adopting Assyrian-Babylonian gods of the same bodies...

Wow, ustrax, there you are! I thought you'd been abducted by a flying saucer full of Gliesians...!

Am I alone in not buying into this "New Earth" stuff? Yes, it's a fascinating discovery, but, well, I mean, from all I've read - in fact, just from what I've read here - "c" is about as much like Earth as a cow is like a sheep: they've both got 4 legs, but that's about it. If the 'pressure cooker' model is correct then "New Venus" might be more appropriate. If it's an iceball, then "New Hoth" might be more fitting.

Just for fun tho, here's an imaginary scene... Sol setting into the Great Ocean of Gliese C, as seen from the Rose Cliffs of the Northern Continent...

Click to view attachment

I strongly suspect that any part of Jinx\Doug\Kur that is locally dark will also be locally very very cold all the time, if any ocean is visible it would be solid and covered in quite a lot of ice. Mad place.