james_nicoll

I was just reading the BBC's article about this. Very, very cool. And naturally the first words out of solarbird's mouth were "Class M"! We are indeed a household of geeks. ^_^

If it is a bigger planet, it probably will be geologically active for a longer time than Earth (and thus habitable). Also, red dwarfs have a longer lifetime compared to our sun. Possibly an interesting place for life.

Andreas Morlok

Andreas -- not necessarily. Gliese 581 is somewhat metal-poor, so its planets might be too. If the planet is deficient in radionuclides, it could less internal heat than Earth despite the greater mass.


Doug M.

IIUC, the planet has around 5 solar masses, which suggests a surface gravity around 16 m/s^2.

Also, it's almost certainly a one face world, tidally locked to the primary. Which doesn't rule out habitability, though -- last I looked, the models were suggesting that one face worlds might not be that bad. (Cold on the nightside, hot on the dayside, but atmospheric circulation could prevent the atmosphere from snowing out.)

Also-also, it's orbiting a red dwarf star. So, no UV to speak of. Earth plants would need lamps, or genetic engineering.

It's not looking like a resort destination. But, hell, liquid water! Not certain, but more likely than otherwise. That's pretty cool right there.


Doug M.

Like a lot of red dwarves, Gliese 581 seems to be a variable. Brief googling hasn't found details, so I'm assuming it's a flare star.


Doug M.

My bet is that this is not going to be a big Earth, but a big Venus.

The "zero to 40" temperature estimate must be the planet's radiating temperature (for contrast the Earth is at -18) as they don't speculate about an atmosphere. So even with an earth-like atmosphere it would be very hot down there, probably past the runaway greenhouse point if there is enough water. But with five times Earth's mass and little or no UV to help particles reach escape velocity, I suspect it has a very thick atmosphere, and a surface temperature in the hundreds of degrees C.

Liquid water may exist in the upper atmosphere.

I could be wrong for a number of reasons. The solar wind from the nearby star might sweep away enough of the atmosphere to keep the surface temperature reasonable. I doubt this but I certainly don't know enough about this type of star to say for certain. Flares might thin the atmosphere enough. If the planet was this close to the star during formation much of the atmosphere may have been lost then.

Of course, any of the above processes might also thin the atmosphere too much, or entirely.

The newspaper story I saw must have printed the wrong luminosity for the star, at 1/50 solar. If that were true the planet would be getting about four times the solar energy per unit area that Earth gets for a black body temperature of about 360 C.

William Hyde

On the one hand, there's a lot less UV to crack H2O into H2 and O2. On the other, how large are the flares?

I think the escape velocity is something like 20 km/s. That should help retention of lighter elements some.

Maybe the right model isn't Earth or Venus but Tenebra....

I've just looked at the paper, and the temperature range is back-of-the-envelope. If it had Venus' albedo T = -3, if earth, T = 40. But no atmospheric effect is mentioned so these are radiative temperatures.

So more completely:

"The planet's outgoing radiation will be emitted at a temperature between -3 and 40C, while the surface temperature will be somewhere between 30 and 1000C depending on the optical properties of the atmosphere. If you buy the 30 number we have a bridge to sell you".

I'm not sure that Tenebra type planets can exist. It's been years since I read the book, and I was in Astronomy then, so naturally I knew nothing about planets.


William Hyde

I vaguely remember something about this, but completely misremember what.

Red dwarves emit in very different spectra than the Sun, and this affects how various atmospheric gases affect temperature. But I don't remember how.

Anyone?


Doug M.

"Red dwarves emit in very different spectra than the Sun, and this affects how various atmospheric gases affect temperature."

That is correct, of course, but just how would depend on the details of the spectrum and the gases involved. To take a trivial example, if a star puts out 25% of its power in the IR, and the planet has a bar of CO2 plus some H2O in the atmosphere, all that energy will be absorbed in the upper atmosphere, and mostly re-radiated to space without ever warming the surface. The result might be a huge stable "stratosphere" occupying much more of the atmosphere than ours does. That would in turn affect the distribution and nature of the clouds, which would then alter the visible portion of the spectrum, as well as feeding back into the IR. Could get complicated. Maybe the example isn't trivial, after all.

The particulate matter in the atmosphere of Venus allows some sunlight to reach the surface, not much, but it is crucial. If the sun's light were redder scattering would be stronger (I think the particles are in the right size range for that, anyway), less sunlight would penetrate and the climate of Venus would be different.

If I were a young and active researcher, I'd be hooking up a radiative-convective model to some plausible atmospheres and cranking out surface temperature estimates within the week. Luckily I don't have access to an RCM so I am spared even the remote temptation. But someone must be doing this as of now.

William Hyde

"First a few technical details. We model the planet’s lower radiative stratosphere with a 2D compressible hydrodynamics code. We use a time-dependent model for radiative heating and cooling. The planet is assumed to be spin-synchronous, so that it rotates on its axis once every 12.9 days. The planetary mass is five-Earth masses (I’m holding out for a transit on May 7th!), and we take a radius of 1.7 Earth radii. The orbit is assumed circular, the luminosity of the star is 0.013 solar luminosities, and the planetary “Bond” albedo is assumed to be 55%. At the layer we’re modeling, we assume a molecular weight of 25, and an atmospheric column depth of 2500 kg/m^2. This corresponds to an atmospheric pressure at the troposphere-stratosphere interface of order 400 Mbar. We assume an equilibrium night-side temperature of 250K (as a result of heat welling up from beneath)."

http://www.oklo.org/

Damn thing doesn't allow comments, though.


Doug M.

"Damn thing doesn't allow comments, though."

Yes it does, just click on the number next to the section title.

I don't have time to look at the pdf now, but this looks sensible. They agree with me, so they must be right.

They might be jumping the gun by going for a full upper-atmosphere 3d simulation before running the RCM, though. I don't know how they know there really *is* a stratosphere. But probably that is explained in the pdf.

The night-side assumption will have to be discarded eventually. In a very thick atmosphere the time constant of heat diffusion is much smaller than that of radiative loss so the day-night contrast is small. There's little temperature difference at the surface between night and day, or pole and equator, on Venus, for example.

William Hyde

I'm wondering how old the star is. Solstation.com says at least two billion years, which isn't very specific.

Might be a bit young for Gl 581c to be hosting anything more complex that bacteria/virii then?

That was what I was thinking. Since it's larger than Earth, it might take longer for oxygen to build up to levels that allow advanced life. So any native life might be more primitive than you'd expect even from the age.

Do we know how long the planet has been in the "habitable zone"?

We don't know where it formed in the system, so nope. If it formed where it is, it's been experiencing much the same conditions for its entire life, because red dwarfs evolve slowly.

How did they determine the radius of the planet to be 1.5 Earth radii?

Oh, I see. They followed Valencia, D., O'Connell, R. J., & Sasselov, D. Icarus, Volume 181, Issue 2, p. 545-554.

Bottom line: Radius scales like mass to the 0.27 power for super-Earths.

Therefore 5 Earth masses gets you 1.5 Earth radii. Will have to read paper to learn why.

Also, 1.36 times Earth density, or about 7.5 grams per cubic centimeter. Seems awfully high.

If surface gravity scales like M/(R^2) then I get about 2.0 Earth gravities.