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I think though one of the great benifits Gliese 581 g has over Mars is a much higher mass. It's > ~ 30 times the mass of Mars. Earth, at ~10 Mars masses, was able to hold on to its atmosphere much better. Of course having a better magnetosphere helped.
I would say the chances are low that Gliese 581 g's atmosphere has been weathered down like Mars's has, low but of course not impossible. An open question for the next few generations of telescopes.
And here is the thing about the faint young Sun http://en.wikipedia.org/wiki/Faint_young_Sun_paradox if somebody does not believe me. 70 percent of today's insolation (1366 Watts/cm2) means the insolation was only 956.2Watts/cm2. The solar irradiance on today's Mars is 360-700 Watts/cm2, with the average being around 575-590 Watts/cm2. But what's more interesting is that in that age, Mars probably had oceans and maybe even life. All the while it recieved just cca 252-490 Watts/cm2, with the average being 402-473 Watts/cm2! That is why I believe in this case atmosphere will maintain sustainable warm temperatures - Mars had oceans when it recieved 1/3 of the sunlight that Earth recieves now at best, and less than 1/5 at worse. Plus, the planet is massive, meaning long geological activity, and the tides from its star will make it even stronger.
The insolation on Gliese 581 g is around 660 Watts/cm2. Not exactly warm, but better than Mars. It is definitely better than the 2871 Watts/cm2 insolation on Gliese 581 c (more than Venus's 2000-2500 Watts/cm2). When the Sun was young, Venus recieved just 1789-1400 Watts per cm2, so it might have had oceans. Mars, on the other hand, recieves more sunlight now. That means that its current state is not directly caused by the lack of sunlight.
It seems to me that liquid water can exist with insolation as low as 1/3 that of Earth, but the conditions cannot stand a lot more heat. It seems to me that too hot is far more dangerous that a bit of cold. So "outer habitable zone" planets might have a much higher chance than "inner habitable zone" planets that will inevitably slip into a runaway greenhouse as the star ages. For "outer habitable zone" planets things can get better later if they have enough mass, "inner habitable zone" planets are doomed into oblivion.
Still, even our Earth will slip out of the habitable zone eventually, becoming like Venus in circa 3 billion years into the future, long before the Sun is a red giant, and too hot for advanced life in even shorter time, 500 million - 2.3 billion years http://www.wired.com/wiredscience/20...ife-extension/ .
I would certainly consider searching for life on this planet far more fruitful than either on Mars or Europa. Sure, we know just the mass and the orbit. If there would be astronomers on Gliese 581 g they would see just mass and orbit of Earth too, unless they would be lucky enough to be able to see Earth passing in front of the Sun to do some spectroscopy. There is no reason why an alien astronomer with our technology would see Earth as more habitable than Gkliese 581 g, for all he knows, our atmosphere can be gaseous SO3 and the seas liquid hydroflouric acid. He wouldn't even know if we have any seas or atmosphere at all, just as we don't know that about Gliese 581 g today.
Last edited by m1omg; 2010-Oct-04 at 09:15 PM. Reason: Adding content
I've seen people (not on this board) taking his statement in question, apparantly, as a fact. At least initially, on first hearing. That's what really frustrated me, and why I reacted on your post in the first place. Not everyone is in a position or able to readily differentiate for themselves between what is fact, and what mere opinion (not to say wishful thinking), when a scientist's statements are communicated, all the less, of course, when taken out of context entirely.
Last edited by Substantia Innominata; 2010-Oct-04 at 09:19 PM. Reason: typo
He stated his opinion. If you have a problem with opinions, you can always go to North Korea .
Besides, there are people who think Gliese 581 d might be a candidate for habitability too http://arxiv4.library.cornell.edu/abs/1009.5814 http://www.eso.org/public/news/eso0915/ even through personally I find it a bit too far fetched (I wouldn't be too surprised through considering it is 7-14x as massive as Earth, so it can support a very thick atmosphere), with the insolation being just 156-718 Watts/cm2, with the average being around 290 Watts/cm2. So, yeah ... through in its periastron it recieves actually more sunlight than g, perhaps it has a seasonal ice cover as the differences in the amount of sunlight in periastron and apastron are very big. Gliese 581 g, on the other hand, has an even more circular orbit than Earth, meaning stable insolation.
There are atmospheric models that would enable the liquid water on d through, so I think there is definitely a wide range of possible liquid water enabling atmospheres on g.
Last edited by m1omg; 2010-Oct-04 at 09:36 PM. Reason: Adding content.
© European Geosciences Union 2007
A slight nitpick: a tidal lock is assumed, rather than known.
Regarding tectonic activity, what evidence is there that the planet has little or no tectonic activity or internally driven magnetosphere? A super-Earth-massed planet should have more internal heat, boosting the probability for plate tectonics.
Inevitability of Plate Tectonics on Super-Earths
" Coronal Mass Ejection (CME) Activity of Low Mass M Stars as an Important Factor for the Habitability of Terrestrial Exoplanets. I. CME Impact on Expected Magnetospheres of Earth-Like Exoplanets in Close-In Habitable Zones" http://www.liebertonline.com/doi/pdf.../ast.2006.0127
"ATMOSPHERIC LOSS OF EXOPLANETS RESULTING FROM STELLAR X-RAY AND EXTREME-ULTRAVIOLET HEATING" http://iopscience.iop.org/1538-4357/...598_2_L121.pdf
"Coronal Mass Ejection (CME) Activity of Low Mass M Stars as an Important Factor for the Habitability of Terrestrial Exoplanets. II. CME-Induced Ion Pick Up of Earth-Like Exoplanets in Close-In Habitable Zones" http://www.liebertonline.com/doi/pdf.../ast.2006.0128
"Atmospheric Escape and Evolution of Terrestrial Planets and Satellites" http://people.virginia.edu/~rej/pape...ciRev_2008.pdf
"Properties of the short period CoRoT-planet population II: The impact of loss processes on planet masses from Neptunes to Jupiters" - http://arxiv.org/PS_cache/astro-ph/p.../0701565v1.pdf
"Composition and fate of short-period super-Earths The case of CoRoT-7b" http://arxiv.org/PS_cache/arxiv/pdf/...907.3067v2.pdf
I do have a handful more, but these do come through my local university database access, but evidently we are experiencing some differences of accessibility so I hesitate to link the rest.
As for the tidal lock, the entire habitable zone of this star exists well within the short-period tidal lock zone of the star. Sure, there are extraordinary conditions which might let this planet avoid that more general proscription (A large close moon, or a recent massive collision, but there is no reason to assume such).
What is the mechanism through which the system metallicity enhances plate tectonics?
If I'm not mistaken there is some discussion of this in one of the papers I listed earlier, I'll look back through them and make sure that I included it or add it in shortly.
Addendum - the last link given above "Composition and fate of short-period super-Earths" (http://arxiv.org/PS_cache/arxiv/pdf/...907.3067v2.pdf) it does focus more on the outer envelopes of giant planets more than the rocky mantles and cores of more terrestrial planets but it does mention some of the issues I've laid out, I know I've read more detailed studies of this particular area, let me look through my data bases and dig around a bit and I'll see if I can get up a more thorough set of reading.
Last edited by Trakar; 2010-Oct-05 at 05:06 AM.
I never wrote that every planet or moon that lies in the habitable zone is going to be habitable. Moon, however, has obvious barriers to habitability. You can just plug the numbers here http://www.transhuman.talktalk.net/iw/Geosync.htm to see that obviously Moon is not going to retain an atmosphere. Of course mass matters when it comes to planetary habitability. However, this planet is more massive than Earth, so it is probable it has retained a thick atmosphere, thick enough to maintain water in its liquid phase. And the composition of interstellar medium is mostly the same everywhere, so I think it is almost impossible for it to be a carbon planet, a block of iron etc. in my opinion it is more implausible than that it is like Earth.Yes, they are... or not. Depends if we take in account only mass and distance, or any possible variation of atmosphere. This can significantly change situation. If, for example, Mars was significantly more massive and with different atmosphere, life could flourish on surface without problem. Other example: Earth's Moon. It is in middle of habitable zone of our star. I assume you speak of habitable zone only in terms of solar insolation. This is not enough.
And while Mars might be in the habitable zone, it is too low mass to retain a sensible atmosphere, and Venus definitely isn't in the habitable zone. It is true that we don't know almost anything about the Gliese 581 system. However, the laws of the physics are the same, so we can assume Gliese 581 g is not likely to have underwent the runaway greenhouse effect required for a Venuslike atmosphere. And contrary to the common misconception, Venu's atmosphere is not the consequence of extreme volcanism, the planet is actually geologically dead (because of no liquid water = no plate tectonics) except for short periods of remelting that occur maybe once per 500 million years. It is the consequence of having all of its CO2 baked out of rocks and boiled out of past seas. If you increased the insolation, the same thing would happen to Earth (and it will eventually happen if there is no artificial intervention around 3 billion years into the future as I already mentioned earlier).
By the way, I am glad you responded with an intelligent, thought out post, and not an insult calling me a pathetic fool for speculating.
EDIT - Also, Gliese 581 is supposed to be too high mass to be a flare star. Aren't flare stars mostly just young and very low mass red dwarfs? Gliese 581 is fairly heavy and bright for an M dwarf.
I found something about the lower metallicity too http://kencroswell.com/RedDwarfPlanets.html , it appears that it is only a problem when you want Jupiter sized planets.
EDIT 2 - I found a bit prophetic blog post ... http://oklo.org/2009/04/29/bodes-law/
How would a reduced load of radioactives keep the core hot? And why would a a lower percentage of heavier elements make the core smaller?Through a smaller core and a reduced load of radioactives to keep the core hot and the mantle plumes flowing. Both of these are directly relatable to a lower percentage of heavier elements.
The core is smaller because there are less heavy elements to form the core.How would a reduced load of radioactives keep the core hot? And why would a a lower percentage of heavier elements make the core smaller?
Reduced radioactives allow the core to cool quicker.
The radioactives generate a significant portion of a large planet's internal heat, driving mantle convection and stimulating tectonic activity.
"... a reduced load of radioactives to keep the core hot and the mantle plumes flowing..."
perhaps worded a bit awkwardly. but a reduced load of radioactives means that the core doesn't stay as hot over time, and the planet's heat engine cools down, you lose mantle convection and large-scale tectonics cease.
Last edited by Trakar; 2010-Oct-06 at 03:29 AM.
If you have a more specific question with regards to any of the issues or papers, I will happily endeavor to address your queries. These papers are actually fairly easy to read and understand with even a minimal amount of field specific knowledge and expertise and most of them are only a handful of pages long, but if you are having difficulties please let me know how I can help you and I will be glad to do so.
The composition of the interstellar medium isn't known well enough to rule out local variations. In particular the space near a recent supernova will be enriched with iron and heavy elements. Some sort of differentiation in the protoplanetary disk might also lead to the formation of iron-rich planets.Originally Posted by m1omg
After the disruption of a white dwarf in a Class 1a supernova, or after the outgassing of a carbon-rich star, local star-forming nebulae could be enriched with carbon. Some theories suggest that Jupiter has a carbon core. So carbon planets remain a distinct possibility. However the chance that Gliese 581 g and other planets in that system may be carbon-rich doesn't necessarily diminish the chance of biospheres on those planets; in fact it might increase the chances.
But you're right that a carbon or iron planet is a possibility, perhaps a better example would be something with very uncommon elements such as a planet having free fluorine or hydrogen flouride on it's surface, through I guess that's possible too, but probably extremely uncommon.