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Posts posted by DanielC

  1. Hello,


    I'm new in this forum. I just thought this thread and I thought I might reply. I am currently doing a masters in astrophysics and I'm very interested in exoplanets. :)


    It's a bit puzzling why they say "squarely in the middle of the habitable zone". Maybe someone here can comment. Water freezes at 273 kelvin, so the equilibrium temperature they give, 228 kelvin, seems quite cold.


    There is a difference between "equilibrium temperature" and "surface temperature". Earth's equilibrium temperature is 255 K which is also below freezing. More interesting is the surface temperature. For Earth that is 288 K and for Gliese 581g the current guess is about 247 K. That's still below freezing, but it is not nearly as bad as it sounds. For reference, this value puts the planet's temperature right in the middle between Earth and Mars. We know that Mars once had liquid water, so Gliese 581g probably has/had water. Furthermore, what went wrong with Mars is that it was just a bit too far from the sun and it didn't have enough gravity to hold on to its atmosphere. Gliese 581g doesn't have either of these problems, so it's a pretty exciting candidate.


    That is assuming the planet is not librating so much as to make these goldilocks zones migrate so much over the 37 day orbit as to make the planet uninhabitable. This planet is a bit too much like the joke about the two statisticians who went duck hunting.


    If you can, read the paper by Joshi, M. 2003 Astrobiology, 3, 415. He did a climate simulation of a tidally locked planet. His main finding is that they'd actually be quite habitable. As long as it has a decent atmosphere and and active hydrology, the temperature differences between cold and warm parts are not too extreme. Basically, the more water the better. So, for example, if Gliese 581g has a very deep ocean, I wouldn't worry much about libration.

    so would any regions on this planet exhibit seasons? If not, and there was no day/night cycle, it would be strangely static to be there, I think. I wonder if plants can even grow without night-time and seasons.

    Earth plants need a day/night cycle, but there is no fundamental reason why alien plants have to be like that. Evolution makes life adapt to whatever conditions are prevalent in the area. I'm not a biologist, but I'd imagine that not having a night would not be much of a bother for Gliesean flora. I imagine that Gliesan astronomers are ignoring big type G stars because obviously a planet with a day/night cycle could not support life :)



    There was no mention of Kepler, so was this discovered by wobble? How long until Kepler delivers similar discoveries? Does Kepler's field of vision include nearby stars or only a region of stars very far away? What range of distances are the stars in Kepler's field of vision?


    No Kepler. Yes, this was discovered by wobble. Kepler should deliver similar discoveries in a couple of years. AFAIK Kepler's field only includes pretty far away stars.


    Gliese 581 is the 86th star in distance from us at 20 ly. Would Gliese 581g have tremendous winds at the border between the hot and cold sides?


    Actually, it is the 116th star. Wikipedia is wrong on this one. The paper I mentioned earlier (Joshi 2003) does not predict very strong winds at the border. It actually predicts the strongest winds at the substellar point (where the star is straight up). His paper assumes a planet somewhat warmer than Gliese 581g and it predicts mean wind speeds of 15 m/s, which is a strong wind, but it's not a hurricane.


    Are there any actual photos of this planet/star or are there only artistic renderings? That's all I can seem to find on google anyway.


    We don't have the technology to photograph a planet like this. In 8 years, when the E-ELT comes online we'll be able to photograph these planets - and by that I mean that we'll have one pixel or two. That will be a very cool time because when we can get a pixel of light from a planet, you can do spectroscopy and figure out what the atmosphere is made of. And if it is very far away from chemical equilibrium (like ours) that would be a tell-tale sign of life.



    too small to view directly.


    Actually, that's not the problem. The problem is that it is right next to something really really bright.


    That's a point. There's apt to be a lot of available energy, if there were complex life-forms able to utilize it.

    But those same conditions (as I imagine it) might make it hard for complex organisms to evolve in the first place!


    I suppose there are always caves, and cracks in the rock, and craters---providing shelter where primitive single-cell goo can develop. Microbes have colonized all sorts of niches on earth from hot to cold and wet to dry. But if there are these strong winds always blowing across the borderland from cold to hot, transporting water, as vapor, hotwards away from the borderlands, and then bringing the vapor back at higher altitude to be cycled down on the border and colder regions as precipitation. If there is this constant maybe gale-force rainstorm&blizzard---and maybe even sandstorm---how does even multicell life get started?


    If the model from Joshi is valid (see above), the winds will not be particularly strong. The strongest winds are in the hottest spot, and with an average of 15 m/s. And that model was assuming an Earth-like energy flux. Since Gliese 581g is colder than Earth its winds would be correspondingly slower than that.


    Could ocean exist btw?


    According to Joshi's model, yes. His models included both a water world, and a half-water / half-land world. Both were habitable, but the water world had a better climate (not surprising).

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