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

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

    Yet another planet around Gliese 581. 3.1 Earth masses, projected surface gravity 1.1-1.7 g.

    Probably tidally locked, though, and assuming an Earthlike greenhouse effect the mean temperature would be below freezing. Still, it seems well within what life could handle.

    paper
    UCSC press release
    http://news.sciencemag.org/scienceno...arth-like.html

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    Considering the mass and higher gravity, the atmosphere is hopefully much thicker. That would keep the planet warm, and also spread the warmth around so that it doesn't turn into a larger version of Mercury (although it would still be rather marginal, with most potential life living in the twilight area between Perma-Day and Perma-Night).

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    Can I point out that the star Gliese 581 has lower metallicity than the sun, M/H = -0.33. Yet here we have at least 6 planets, including at least 5 "rocky" planets, each of which exceed the mass of the Earth, the most massive terrestrial planet in our system.

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    On the other hand, Mars is quite habatable tempereture wise nearer the equator, yet is not known to harbour life. So just because it is in the right temperature range is no guarantee of life.

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    Scientists seem to warming up to the possibility of extraterrestrial life. First there was Chris McKay a leading expert on Mars saying he now believes Viking really did detects organics on Mars, now one of the scientists for this discovery says this:

    Odds of Life on Newfound Earth-Size Planet '100 Percent,' Astronomer Says.
    By Jeanna Bryner
    LiveScience Managing Editor
    posted: 29 September 2010
    05:03 pm ET
    "Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent," said Steven Vogt, a professor of astronomy and astrophysics at the University of California, Santa Cruz, during a press briefing today. "I have almost no doubt about it."
    http://www.space.com/scienceastronom...le-100929.html

    Bob Clark

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    Quote Originally Posted by RGClark View Post
    "Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent," said Steven Vogt, a professor of astronomy and astrophysics at the University of California, Santa Cruz, during a press briefing today. "I have almost no doubt about it."
    http://www.space.com/scienceastronom...le-100929.html

    Bob Clark
    I would have thought an astrophysicist would be a little better with statistics!

    I HOPE there is life on Gliese 581g, but any statement suggesting we have evidence of such is nothing more than hopeful speculation.

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    RGClark. Maybe, maybe not. It's somwthing we need to be very sure of before we jump to conclusions. After all, discovering life on other worlds, even microbial life, would be among the, if not the, greatest discoveries ever. No one wants to be burned by a false positive.

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    Quote Originally Posted by stoic View Post
    "Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent," said Steven Vogt, a professor of astronomy and astrophysics at the University of California, Santa Cruz, during a press briefing today. "I have almost no doubt about it."
    I would have thought an astrophysicist would be a little better with statistics!
    I had exactly the same thought when I first read his comments. He's speaking emotionally, not rationally. That should be a clue about the weight one should put on his prediction as a whole.

    Have to admire the enthusiasm though, just like one can't help but respect Carl Sagan.

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    If this planet has enough CO2, it could be as warm as Earth. With a thick,rotating atmosphere it could be warm all over, more or less. Mind you, if photosynthesis converts the CO2 to oxygen, this would reduce the greenhouse effect considerably and could lead to significant cooling on this world. Maybe the self-regulating effects of the Earth's biosphere don't work equally well on other worlds.

  11. #11
    Quote Originally Posted by Vultur View Post
    Gliese 581. 3.1 Earth masses, projected surface gravity 1.1-1.7 g.
    This is something I need a little help with because obviously there is an elementary gap in my understanding. If the planet has the mass of over 3 Earths, why would its surface gravity only be 1.1 - 1.7 x that of Earth? Shouldn't its surface level gravity be roughly 3 times that of Earth's, or does the distance from the centre of gravity explain the difference?

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    Quote Originally Posted by Canis Lupus View Post
    This is something I need a little help with because obviously there is an elementary gap in my understanding. If the planet has the mass of over 3 Earths, why would its surface gravity only be 1.1 - 1.7 x that of Earth? Shouldn't its surface level gravity be roughly 3 times that of Earth's, or does the distance from the centre of gravity explain the difference?
    That means the planet is bigger than earth.

    Look at the moon. It has 1% of Earth's mass, yet 16% of its surface gravity. The moon is made of the same stuff as the Earth, I would guess.

    This beauty of a planet would be 1.35 to 1.68 the size(diameter) of the Earth.

    By comparison, if it were the same size as the Earth, then the surface gravity would be.....3.1g!!

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    I see this 581g as the center of attraction here today...

    Look at the pages unfolding...

    This is encouraging for the finding of life.

    Might I suggest a little calm. Return to the truly scientific method...

    Study, Analise and, observe. Leave the wild speculative pros to the fiction writers.

    These are indeed exciting times. We might well be on the brink of significant discovery...

    Policing the emotive response to a level of science... please.

    ... or, is this all just a ploy to encourage funding... Hmmm...:

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    Density is important to surface gravity. The Saturn cloud tops have the same gravity as Earth's surface mostly because Saturn has the lowest density in our solar system. I agree, three times the mass gives more than 1.7 g surface gravity unless the average density is very low, perhaps impossibly low without lots of gas and not much solid and liquid. Possibly Gliese 581g has a much hotter interior than Earth.
    Earth has almost 3 times the average density of our moon. Neil

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    The uncertainty in the surface gravity is entirely due to questions about this planet's density. Since astronomers haven't got a transit to work on, no-one knows the diameter, and so no-one knows the density. The surface gravity is dependent on the density.

    If this planet is mostly made of water ice, it will have a gravity of 1.1 or thereabouts, quite similar to Earth's if it is mostly made of iron and nickel, it will have a surface gravity of about 1.7 or maybe a bit more. A rocky planet will lie somewhere in between. So the composition of this unknown world greatly affects its characteristics; any concept of surface conditions on this world needs to take that into account.

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    It's being assumed this planet would be tidally locked into a permanent light side and a permanent dark side. However is this necessarily the case ? Mercury ought to be locked like this, and for many years up to not very long ago it was assumed to be the case. Now we know it is in a stable resonance.

    Another thing, whilst agreeing we don't know the composition of this planet very well, all things being equal it should have held onto more heat than the Earth and be more geologically active. The assumption of climate zones remaining unchanged over billions of years may be wrong, because there could be very active continental drift, and the climate could be chaotic because of all the volcanoes.

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    However is this necessarily the case ?
    Yes, if it's orbit is circular, which it most certainly is. The reason for Mercury not being locked is its excentric orbit.

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    If Gliese 581g is tide locked. the spot where the sun is directly overhead could average 68 f = 20 c even if the average temperature is -39 f = -39c for the whole planet. Such extreme temperature could mean most of the water is trapped in a huge ice cap, with no rainfall in the warm spot. A thick atmosphere with some green house gases will make the temperature more uniform and better adapted to life.
    Since Earth's tide locked moon is receding about one centimeter per year, this may be true of tide locked planets, thus compensating for the sun getting hotter over the very long term = good for life.
    Earth's moon has some liberation. If liberation is common to tide locked planets, this makes the habitable spot a bit larger with more climate variations.
    Planets that are not tide locked with negligible axial tilt (such as Mercury) can have habitable polar regions even if the rest of the planet is much too hot.
    Planets of multiple suns (such as Centauri A,B.C) Typically have temperature changes over decades or centuries which can be bad for life, if the rare hot times are too hot. Yes, at least one planet each is possible for A,B and C in the habitable zone. They likely are small planets, if any, as detection attempts, so far have failed. Neil
    Last edited by neilzero; 2010-Oct-01 at 02:12 PM.

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    Quote Originally Posted by stoic View Post
    I would have thought an astrophysicist would be a little better with statistics!

    I HOPE there is life on Gliese 581g, but any statement suggesting we have evidence of such is nothing more than hopeful speculation.

    He did qualify it by saying that it was his own personal feeling.

    Bob Clark

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    Quote Originally Posted by Canis Lupus View Post
    I had exactly the same thought when I first read his comments. He's speaking emotionally, not rationally. That should be a clue about the weight one should put on his prediction as a whole.
    Have to admire the enthusiasm though, just like one can't help but respect Carl Sagan.
    I don't agree with that. He is making a prediction on what will be found in the future based on the evidence we have so far.
    Robert Goddard making a prediction that we will at some point have rockets to the Moon carries alot more weight than the editor of the New York Times proclaiming that is impossible because "in space there is nothing to push against".


    Bob Clark

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    Re the tidal locking: are the measurements accurate enough to tell them if the eccentricity is small enough to ensure tidal lock?

    Another small hope may be if it has a massive moon, if so the day could equal the month ?

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    Gentlemen, may I point out that we do not know what vulcanicity the planet has. That could also provide extra warmth.

    Of course, this planet may have other problems according to other reports.

    HOPES of intelligent life on planet Gliese 581g were dashed yesterday as scientists revealed it is actually a bit like Sunderland.

    Images from the Keck telescope clearly show deep grooves carved into the planet's crust by aliens dragging their knuckles back and forth, waiting for a human bouncer to punch.

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    Quote Originally Posted by RGClark View Post
    I don't agree with that. He is making a prediction on what will be found in the future based on the evidence we have so far.
    No. That is what you take it to mean. But I can't see any prediction there. All I see is a clear, definite statement and nothing short of that. A statement "that the chances of life on this planet are 100 percent" (not, that they will be), and, of course, this is his feeling, whose else should it be? Either way, chances of 100% amount to certainty - ergo he's talking nonsense. Why should a scientist, or an astrophysicist at that, even be immune to such? So, why not call a spade a spade then? I just don't get it. As if this was the first time someone spoke out without much thinking beforehand.

    Robert Goddard making a prediction that we will at some point have rockets to the Moon carries alot more weight than the editor of the New York Times proclaiming that is impossible because "in space there is nothing to push against".
    What's that got to do with alleged instances of life on other planets? A bit far-fetched comparison, isn't it?
    The effort to understand the universe is one of the very few things that lifts human life a little above the level of farce, and gives it some of the grace of tragedy.
    Steven Weinberg

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    Of course, there is always the possibility that this is the left over core of a neptune sized ice-giant that migrated in to it's current location and has already lost most of its mass to stellar-wind erosion from not having a magnetic field and being in an orbit that is half the distance of Mercury's orbit. or some such similar senario.

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    Quote Originally Posted by Substantia Innominata View Post
    No. That is what you take it to mean. But I can't see any prediction there. All I see is a clear, definite statement and nothing short of that. A statement "that the chances of life on this planet are 100 percent" (not, that they will be), and, of course, this is his feeling, whose else should it be? Either way, chances of 100% amount to certainty - ergo he's talking nonsense. Why should a scientist, or an astrophysicist at that, even be immune to such? So, why not call a spade a spade then? I just don't get it. As if this was the first time someone spoke out without much thinking beforehand.
    What's that got to do with alleged instances of life on other planets? A bit far-fetched comparison, isn't it?
    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?

    Robert Clark
    Last edited by RGClark; 2010-Oct-02 at 01:15 PM. Reason: typo

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    "Barely"? They said this planet is exactly in middle of habitable zone. Weasel speak much?
    Note that if you take in account only distance and mass, Venus and Mars are in our stars' habitable zone too. We already learned necessity of knowing atmosphere - and with same star system (Gliese 581 d versus e)!
    And about Vogt... well, he said this is his personal opinion. Sciencists are humans too. I like his enthusiasm, even if chances are significantly less than 100%. So what, we have bilions other planets in habitable zone in this galaxy alone.

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    Quote Originally Posted by kzb
    Re the tidal locking: are the measurements accurate enough to tell them if the eccentricity is small enough to ensure tidal lock?
    I didn't see a mention of the eccentricity value of the planets in the six-planet model, but we're told they're circular. In a circular orbit, a 1:1 spin-orbit resonance would be the preferred state. So yes, we can expect this planet to be tidally locked.

    Quote Originally Posted by kzb
    Another small hope may be if it has a massive moon, if so the day could equal the month ?
    Yes. Though this planet's Hill sphere is a bit smaller than Earth's.

    Quote Originally Posted by Trakar
    Of course, there is always the possibility that this is the left over core of a neptune sized ice-giant that migrated in to it's current location and has already lost most of its mass to stellar-wind erosion from not having a magnetic field and being in an orbit that is half the distance of Mercury's orbit. or some such similar senario.
    Not likely. Remember there's a hot Neptune further inward toward the star. Gliese 581 g has enough mass to hold onto a decent atmosphere, much like Earth and Venus do.
    Quote Originally Posted by Trakar
    and being in an orbit that is half the distance of Mercury's orbit
    But orbiting a star that is much dimmer, and according to the MOST satellite, much less active. It's inappropriate to just scale the effects of such things with the semi-major axis.

    Quote Originally Posted by MaDeR
    Note that if you take in account only distance and mass, Venus and Mars are in our stars' habitable zone too. We already learned necessity of knowing atmosphere - and with same star system (Gliese 581 d versus e)!
    Nitpic: Gliese 581 e is the innermost, hot Earth just within the hot Neptune's orbit. It's c that is just inward of the habitable zone.

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    Quote Originally Posted by Hungry4info View Post
    Not likely. Remember there's a hot Neptune further inward toward the star. Gliese 581 g has enough mass to hold onto a decent atmosphere, much like Earth and Venus do.
    But orbiting a star that is much dimmer, and according to the MOST satellite, much less active. It's inappropriate to just scale the effects of such things with the semi-major axis.
    Actually it is the hot inner remnant giants that made me consider that this was formerly the core of an ice giant. The whole system migrated inward. and yes, distance to star is terribly important when looking at the atmosphere of a planet without a magnetic field and subject to the effects of stellar wind upon that atmosphere. This planet formed as an icy ball in the outer regions of its system and migrated inward following its sister planets. There is no way it could have formed in this spot and then held its position as the other planets migrated inward past it. the only factor that makes it seem "earthlike" is that it currently resides in an orbit that happens to be close to the triple point range of water, and to me, given the very un-earthlike conditions of its mass, composition, proximity to stellar parent, etc., this one similarity is not enough to much excite or interest me.

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    Let us say that a particular sun is a variable star, and that gas is blown away. Wouldn't the moons of a super-jupiter move inward from drag, then merge with an exposed core to form such a "rock giant"/ superearth? This would keep the core molten for a long time, and still leave a goodly amount of atmosphere once a tighter magnetosphere is established--and if the star quiets down?

    Also, could an Earth type planet closer inward lose atmo to be picked up by objects farther out?

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    Quote Originally Posted by Trakar View Post
    Actually it is the hot inner remnant giants that made me consider that this was formerly the core of an ice giant. The whole system migrated inward. and yes, distance to star is terribly important when looking at the atmosphere of a planet without a magnetic field and subject to the effects of stellar wind upon that atmosphere. This planet formed as an icy ball in the outer regions of its system and migrated inward following its sister planets. There is no way it could have formed in this spot and then held its position as the other planets migrated inward past it. the only factor that makes it seem "earthlike" is that it currently resides in an orbit that happens to be close to the triple point range of water, and to me, given the very un-earthlike conditions of its mass, composition, proximity to stellar parent, etc., this one similarity is not enough to much excite or interest me.
    Where are you getting your information? From what i've read, its mass is in the range considered 'Earthlike', we know nothing directly of its composition, and if it had an orbit like Earths it would be well away from the H2O triple point region, which really is the major defining characteristic of an 'Earthlike' orbit as far as discussions of planetary habitability go.

    I'm hoping its something really weird with a mysterious history to be honest: A mercury sized world loaded with heavy elements, constant volcanism and lots of short lived radio isotopes. Something we can't easily explain, although a tru;y habitable planet would be nice to.

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