Gliese 581g -- in the habitable zone (barely)

[PetersCreek]

The rude tone and wording of this post was totally unnecessary. Let's have no more of it.

This should have been adequate warning to all that rude behavior should cease. Apparently, it wasn't. So, let me make it pointedly clear to MaDeR, m1omg, and anyone else who feels like resorting to this kind of behavior: the next instance will almost certainly result in suspension(s). Enough already.

If anyone has a problem with a post, including this one, report it.

[Hungry4info]

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.

[m1omg]

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/ .

[m1omg]

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.

I agree. I don't understand the passion of some of the people here to argue against any possibility of life there. I am not claiming there definitely is life, just that it is the most probable body that has been discovered to this date to have a possibility of it. As you can see I have some numbers to support it.

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.

[Substantia Innominata]

When Robert Goddard said in 1920 that in the future we will have manned flights to the Moon did that mean that that had to be true? No. The majority of even scientists probably thought this to be farfetched at the time. But his opinion on the matter carried more weight than that of just any ordinary person on the street.

Whether Vogt's prediction will turn out to be true, time will tell. I think it will be in rather near term since over time our telescopes are growing ever more powerful, far earlier for example than when we will be able to send probes there.

By stating that this is his personal feeling he is saying it should not be taking as proven scientific fact. Do you think Robert Goddard was wrong to state as fact that we will have manned fights to the Moon?

No, I don't think so. It only shows what a fantastically keen, far-seeing, and visionary scientist Robert Goddard was. But, you see, he was a physicist and even back then, and despite the fact of his vision at the time being practically unheard-of, yes even utopian, he still was talking plain physics, wasn't he? He was a physicist; he must have known what he was talking about, at least in principle. Steven Vogt, on the other hand, is an astronomer, having, as far as I can tell, no professional training in biology (or any related subject) whatsoever. Yet he's giving his opinion about the probability of life on some other planet. This is what makes the difference to me. That's of course not to say that anyone shouldn't be at liberty to form an opinion about whatever they like and communicate it, if they like. What I mean is just that it should be clear and clearly stated, at all times, if and when someone is leaving his or her respective scientific subject area when giving an estimate, especially one of such momentum. I've certainly got no problem with what he said, or even with the fact that he said it, even though personally I'm far from sympathizing with his views on this matter. That's okay. However, it's been disseminated (maybe not even his fault) in a fashion that gave the impression, at least to some, he would be talking science here -- which, evidently, is not the case.

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.

[m1omg]

He stated his opinion. If you have a problem with opinions, you can always go to North Korea .

[m1omg]

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.

[MaDeR]

It's not wishful thinking to say that. He did not say that such things exist, he said they might be there. Within what we know, that's certainly possible. There may well be all sorts of life on that planet.

"May" or "not may" do not even enter picture. Inssuficent data. While this discovery is good and I wait for more (2011 feburary will be interesting), this is not game-changer. We already know that earth-sized planets should exist in habitable zones, just by virtue of chance and statistic. Game changing will be assessing atmospheres of these small bodies. This will be real validation (and almost certainly modifications) of planetary models, mentioned by m1omg below. We already started it with hot jupiters. Low hanging fruits first and all of that.

It would have been more accurate to say that the planet receives slightly less energy from its star than Earth does, but I think that was rather understood.

I meant the "it is colder than Earth", as in, the insolation is lower the Earth.

Yes, received energy is less. But we cannot know if planet will be colder in any way. I intepreted sentence "This planet is a bit colder than Earth" as something that can be known only with knowledge about atmosphere. We do not know anything about atmosphere, so this statement do not have any basis. Distance. Mass. Inssuficient data.

And neither Venus nor Mars are in the habitable zone.

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.

They are in the more wishful thinking habitable zones, but there is no reason they should be. As far as I know current planetary models predict a hot greenhouse for Venus and a frigid desert for Mars.

Predictions are easy if you know result already. There are also fact that we know about Mars and Venus (and in our solar system in general) about zilion times more thant about, say, Gliese 581g.

Yes, it can have a Venuslike atmosphere for all what we know. But in my opinion this is more unlikely than anything Earthlike as the planet simply doesn't have enough heat to start the runaway greenhouse effect.

How you got this out from distance from star and minimum mass? I do not see any basis to your claim. Let me repeat it: inssuficient data.

But it is the most hopeful candidate.

Yes, it is. Because we currently do not know anything better. Hardly assuring.

[Luckmeister]

I would certainly consider searching for life on this planet far more fruitful than either on Mars or Europa.

Perhaps so, if we had the capability to search for life on Gliese 581g, but since our technology will not yet allow that, other than perhaps very basic spectroscopy, I vote for Mars or Europa.

[whimsyfree]

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.

Most things seem to point that way: higher mass and lower irradiance than Earth. Thermal escape doesn't seem likely. M dwarfs can have high episodic emissions of EUV and x-rays though, which would contribute to atmospheric loss. I've also read that terrestrial planets much larger than Earth would have internal dynamics not conducive to a strong magnetic field.

[whimsyfree]

"The impact of nonthermal loss processes on planet masses from Neptunes to Jupiters" -

I don't find that title in Google scholar or my uni database. I don't believe Gl 581g would fall into the class of close-in or highly irradiated exoplanets to which the authors refer.

[Trakar]

I don't find that title in Google scholar or my uni database. I don't believe Gl 581g would fall into the class of close-in or highly irradiated exoplanets to which the authors refer.

Geophysical Research Abstracts, Vol. 9, 07850, 2007
SRef-ID: 1607-7962/gra/EGU2007-A-07850
© European Geosciences Union 2007

http://www.cosis.net/abstracts/EGU20...25d18001ea2a33

[whimsyfree]

Geophysical Research Abstracts, Vol. 9, 07850, 2007
SRef-ID: 1607-7962/gra/EGU2007-A-07850
© European Geosciences Union 2007

http://www.cosis.net/abstracts/EGU20...25d18001ea2a33

That's just another link to the abstract, not a published paper.

[Trakar]

Most things seem to point that way: higher mass and lower irradiance than Earth. Thermal escape doesn't seem likely. M dwarfs can have high episodic emissions of EUV and x-rays though, which would contribute to atmospheric loss. I've also read that terrestrial planets much larger than Earth would have internal dynamics not conducive to a strong magnetic field.

And if it initially formed in the outer reaches of the embryonic system, its composition (particularly in a low metallicity birth cloud) is going to be drastically different than the inner system heavy element mix rocks we see as forming close in, within our system. There is actually a pretty extensive body of work dealing with the potential of Hot Ice Giants that result from just this sort of senario. If I'm not mistaken, several have already been tentatively identified in similar Red Dwarf exoplanetary systems, if not within this very system.

[Trakar]

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.

Its impossible to say for certain with what is currently known, but the knowns; tidal lock, circular orbit, high probability for outer system formation in a low metallicity system, stellar class variability and characteristics of the parent star, all, seem consistent with a greater than Earth mass body with little or no tectonic activity or internally driven magnetosphere of significance. Its probably and equally erroneous stretch of reality, but more a Super Venus with a much smaller core and fewer radioactives.

[Hungry4info]

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
http://arxiv.org/abs/0710.0699

[Trakar]

That's just another link to the abstract, not a published paper.

Ah, I thought you were merely having trouble verifying the authenticity of the paper. Curiously I only have an online link to the abstract, not sure why you shouldn't be able to pull it up on a University database access EGU work is mostly open source with access. Regardless, I do have some links to other papers by the several of the same main authors dealing with related subject matter if you'd rather pull them up for a peek:

" 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.

[Trakar]

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
http://arxiv.org/abs/0710.0699

Not if they formed in low metallicity systems, and particularly not if they originally formed in the outer regions of such systems and then shed mass down to their current levels post migration.

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).

[Hungry4info]

What is the mechanism through which the system metallicity enhances plate tectonics?

[Trakar]

What is the mechanism through which the system metallicity enhances plate tectonics?

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.

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.

[m1omg]

Perhaps so, if we had the capability to search for life on Gliese 581g, but since our technology will not yet allow that, other than perhaps very basic spectroscopy, I vote for Mars or Europa.

I get your point, but what if we achieve the technology soon? And I wouldn't bet on Europa or Mars.

[m1omg]

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.

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.

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/

[whimsyfree]

Ah, I thought you were merely having trouble verifying the authenticity of the paper. Curiously I only have an online link to the abstract, not sure why you shouldn't be able to pull it up on a University database access EGU work is mostly open source with access. Regardless, I do have some links to other papers by the several of the same main authors dealing with related subject matter if you'd rather pull them up for a peek:

...

Do any of those papers support the idea that an ice giant situated like Gl 581g would experience massive atmosphere loss?

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.

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?

[MaDeR]

I never wrote that every planet or moon that lies in the habitable zone is going to be habitable. Moon...

Moon was just example.

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.

We cannot know for certain. What atmosphere this planet have depens on very many variables, like starting conditions, history of solar system (like changing positions, maybe flyby of nearby stars etc). Thick atmosphere is not certain. While I agree thick atmosphere have high chance to be present, its composition, presence of watter etc are at the moment complete unknowns.

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.

I do not claim this is carbon planet or block of iron. I do not even claim this is unhabitable Venusian-like hell or whatever. I claim there is too few data to any serious speculation, not mentioning very bold claims that this planet have higher chance for life than Mars or Europa. Without data about atmosphere, we cannot go anywhere.

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.

Venus was in habitable zone in past, and even today you can find habitable zone high in atmosphere, where pressure and temperature theoretically could be conductive to some werid life (not that I think there is such a life in Venusian clouds, but currently we cannot exclude this possibility).

It is true that we don't know almost anything about the Gliese 581 system. However, the laws of the physics are the same,

Yes! But, unfortunately, with currently known data, there is just too much possible scenarios of development of this system, with wildly varying results.

so we can assume Gliese 581 g is not likely to have underwent the runaway greenhouse effect required for a Venuslike atmosphere.

I claim we cannot assume this. Insufficient data.

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.

Well, I am sorry for rude first post. Unfortunately, I still stand by gist - that there is no basis to many of your claims. Speculation can be useful, but only if there are any basis for that. I do not see basis for claim that this planet have higher chance of life than Mars or Europa. Disclosure: I personally think life on Mars and Europa have very high chance of existing. Putting Gliese 581g above them is very risky, in my opinion.

[Trakar]

Do any of those papers support the idea that an ice giant situated like Gl 581g would experience massive atmosphere loss?

Several of them speak of the stripping of atmosphere from inner orbit jupiters, neptunes and super earths and in fact suggest that super Earths in many cases may be the stripped cores of inner orbit Neptunes.

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?

The core is smaller because there are less heavy elements to form the core.
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.

[Trakar]

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.

Last to first, the old "it is difficult get a Jupiter around a red dwarf" supposition is not really relevent to this discussion (at least not so far). That is an argument about formation methodology and the association of such with system metallicity. The instability (flares, CMEs, High UV output, etc.,) are a common feature of all M class stars, but these factors are most predominant early in the star's life. This is particularly relevent to Gliese581 in that the migration to inner orbits had to have happened before the star blew off its planetary nebula. As indicated in the papers listed, these types of stars are then capable of eroding even a Jupiter class giant to its core if it is in a close in (< several tens of days) orbit in a relatively short period of time (IIRC, a couple of Gy). Gliese581 is a typical M class red dwarf.

[whimsyfree]

Several of them speak of the stripping of atmosphere from inner orbit jupiters, neptunes and supper earths and in fact suggest that super Earths in many cases may be the stripped cores of inner orbit Neptunes.

That's avoiding the question, so I'll take it as a no.

[Trakar]

That's avoiding the question, so I'll take it as a no.

That is a direct answer to the question asked. These were the issues under discussion and the reasons the references were given in the first place.
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.

[eburacum45]

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.

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.

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.

[m1omg]

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.

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.

Well it is a possibility, but Gliese 581 is very close to us, so I don't think it would have a radically different composition, perhaps some planets near stars that are close to rapid massive star formation regions would have a higher chance of being carbon or iron planets, or planets close to the galactic core where there are a lot of old massive stars going supernova.

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.