Whereas I believe that the planet's 'surface' is a place of scalding heat and crushing pressure.

The most likely formation scenario is that the planet formed further out and then migrated inwards, carrying about one Earth mass of H2O with it. The current orbital insolation is similar to that of Venus. In which case, since water vapour is a greenhouse gas, the outer layers of the several-thousand-kilometre-thick ocean would have boiled into the atmosphere, while the lower layers would be under too much pressure to boil -- although it would not be an actual liquid as it would be far above the critical point.

I see it as a cross between Venus and Neptune, with a super-thick, super-dense, super-hot atmosphere composed mainly of supercritical steam.

Bill

A red dwarf's large convective zone (relative to its radiative zone) stores lots of magnetic energy, causing constant flaring and CMEs. A typical red dwarf flare makes the Sun's X-class flares look puny by comparison...

I was thirsty to know what our Sun would look like from there, and I get a magnitude of about +3.5. With "C" in a zodiac constellation, anyone there who is looking would easily detect the radial acceleration effects of Jupiter and know there was a planetary system in the neighborhood. "They" would have to be impressed by how circular the orbit of Jupiter is (for an orbit out there), and Saturn's when they detected that.

No one's addressed this, but I would expect, based on a very simple model, to be very hot geologically, with too many volcanoes if anything. The worlds in our solar system, apart from tidally-driven ones, have "heat" to the extent of their mass. Earth is bigger than Mars, and is still active. Mars is bigger than the Moon, etc. This system is about the same age as ours... I would expect "C" to be pretty active.

Considering the lower metallicity of the star wrt the Sun, I would expect that the radioactive elements would likewise be depleted - in what proportion I couldn't guess, but I could certainly envision this compensating for the greater mass. I'm more concerned with the thickness of the atmosphere - I could easily see a Venus-like greenhouse effect underway on a body of this scale. The paper predicting depleted volatiles in the "habitable" zones about red dwarf stars leaves me feeling a little more optimistic. Seems likely this will all be resolved sometime in the next 5-10 years, with new interferometric scopes coming online.

Some say Gliese 581 C will be made uninhabitable by fire,
Some say by ice.
From what I've tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.

This is not directly related to Gleise 581 C, but here are three interesting new papers on transiting extrasolar planets, all of them appearing at arXiv tonight:

XO-2b: Transiting Hot Jupiter in a Metal-rich Common Proper Motion Binary

Christopher J. Burke, P. R. McCullough, Jeff A. Valenti, Christopher M. Johns-Krull, Kenneth A. Janes, J. N. Heasley, F. J. Summers, J. E. Stys, R. Bissinger, Michael L. Fleenor, Cindy N. Foote, Enrique Garcia-Melendo, Bruce L. Gary, P. J. Howell, F. Mallia, G. Masi, B. Taylor, T. Vanmunster

We report on a V=11.2 early K dwarf, XO-2 (GSC 03413-00005), that hosts a Rp=0.973+0.03/-0.008 Rjup, Mp=0.57+/-0.06 Mjup transiting extrasolar planet, XO-2b, with an orbital period of 2.615838+/-0.000008 days. XO-2 has high metallicity, [Fe/H]=0.45+/-0.02, high proper motion, mu_tot=157 mas/yr, and has a common proper motion stellar companion with 31" separation. The two stars are nearly identical twins, with very similar spectra and apparent magnitudes. Due to the high metallicity, these early K dwarf stars have a mass and radius close to solar, Ms=0.98+/-0.02 Msolar and Rs=0.964+0.02/-0.009 Rsolar. The high proper motion of XO-2 results from an eccentric orbit (Galactic pericenter, Rper<4 kpc) well confined to the Galactic disk (Zmax~100 pc). In addition, the phase space position of XO-2 is near the Hercules dynamical stream, which points to an origin of XO-2 in the metal-rich, inner Thin Disk and subsequent dynamical scattering into the solar neighborhood. We describe an efficient Markov Chain Monte Carlo algorithm for calculating the Bayesian posterior probability of the system parameters from a transit light curve. System parameters and confidence intervals from a chi^2 minimization are also provided.

On constraining a transiting exoplanet's rotation rate with its transit spectrum

David S. Spiegel, Zoltan Haiman, B. Scott Gaudi

We investigate the effect of planetary rotation on the transit spectrum of an extrasolar giant planet. During ingress and egress, absorption features arising from the planet's atmosphere are Doppler shifted by of order the planet's rotational velocity (~1-2 km/s) relative to where they would be if the planet were not rotating. We show that, in the case of HD209458b, this shift should give rise to a small net centroid shift of ~60 cm/s on the stellar absorption lines. Using a detailed model of the transmission spectrum due to a rotating star transited by a rotating planet with an isothermal atmosphere, we simulate the effect of the planet's rotation on the shape of the spectral lines, and in particular on the magnitude of their width and centroid shift. We then use this simulation to determine the expected signal-to-noise ratio for distinguishing a rotating from a non-rotating planet, and asses how this S/N scales with various parameters of HD209458b. We find that with a 6 m telescope, an equatorial rotational velocity of ~2 km/s could be detected with a S/N~5 by accumulating the signal over many transits over the course of several years. With a 30 m telescope, the time required to make such a detection reduces to less than 2 months.

HAT-P-2b: A Super-Massive Planet in an Eccentric Orbit Transiting a Bright Star

G. A. Bakos, G. Kovacs, G. Torres, D. A. Fischer, D. W. Latham, R. W. Noyes, D. D. Sasselov, T. Mazeh, A. Shporer, R. P. Butler, R. P. Stefanik, J. M. Fernandez, A. Sozzetti, A. Pal, J. Johnson, G. W. Marcy, B. Sipocz, J. Lazar, I. Papp, P. Sari

We report the discovery of HAT-P-2b, a massive (Mp=8.17+/-0.72 M_Jup) planet transiting the bright (V=8.7) F8 star HD 147506, with an orbital period of 5.63 days and an eccentricity of e=0.5. From the transit light curve we determine that the radius of the planet is Rp = 1.18+/-0.16 R_Jup. HAT-P-2b has a mass about 9 times the average mass of previously-known transiting exoplanets, and a density of rho = 6.6gcm^-3, similar to that of rocky planets like the Earth. Nevertheless, its mass and radius are in accord with theories of structure of massive giant planets composed of pure H and He. The high eccentricity causes a 9-fold variation of insolation of the planet between peri- and apastron.

Bill

p.s. I have compiled a document containing 482 (so far) papers on arXiv, relating to extrasolar planets, in a format similar to those above. The list is not exhaustive -- I listed only those papers in topics I personally found interesting, so a lot of papers involving such things as theories of orbital evolution were not included. However, the number of papers about direct observations of, and theories about, the currently known extrasolar planets posted to arXiv should be almost complete. Would there be interest in me posting the list on this board?

Using the information in Valencia et al as a guide, I came up with a simple model (nicknamed the Onion Model, to differentiate from the Valencia model) for predicting super-Earth structure for a 5 Earth mass planet like Gliese 581c. [The Onion Model assumes the planet is like an onion, figure the volume of the layers, then figure the radius – everything in similar states and densities as Earth’s layers]. I played with a few scenarios and came up with the following table:

Click to view attachment

From the fact that Gleise 581 is a metal poor star, I allowed the core mass fraction (CMF, CMF = 32% for Earth) to drop to lower values. (Entry 3 in the attached table is for the bizarre scenario where Gliese 581c is a pure iron-nickle ball)

If Gleise 581c had very similar makeup of Earth (ice mass fraction [IMF] = 0.01% and core mass fraction [CMF] = 30%, entry 4), it would have an average ocean depth of 2 km (2 x that of Earth). Given the stronger gravity of a 5 Earth mass planet, the relative topography would be less, and the planet would likely have a global ocean with little if any landmass poking above the surface. It’s weather would be dominated by the ocean. (Earth’s ocean drives our climate, and it covers only 70% of our surface)


Changing the amount of heavy metal core down to 10% does not seem to affect the predicted depth of the ocean. (Entries 4-6).

Locking the CMF at 20% and increasing the amount of water up to 50% has a dramatic effect on the depth of the ocean (entries 8-12). Even an IMF of 10% (entry 8) gives a thousand km deep ocean!

Assuming that a metal poor star would thus have a greater relative proportion of dusty volatile ices compared to heavy metals, I allowed the IMF to stay at a 10x higher than Earth’s level of 0.1% and the CMF to vary downwards from 20% down to 0% (entries 15-24). What is interesting is that even a CMF of 5% furnishes a final core size that is the same size as Earth’s core.

I would assume that although the heavier radionucleides might not be present, there would still be enough residual heat from the initial formation/bombardment to keep the core molten for a long period of time. This should give an Earth size dynamo and magnetic field. It’s also interesting that even a 0.1% CMF (320 x less metal fraction than found on earth) can still provide 800 km core. Is 800 km sufficient for a decent magnetic field?

All the scenarios with 0.1% IMF (entries 15-24) have a 15-20 km deep ocean.

Bottom line: Given a metal poor star but a 5 earth mass planet, it is still likely that it could have a magnetic field (thus hold on to it’s water over the long term) and is also likely to have a deep ocean.



Please find attached an EXCEL table that predicts radii and ocean depths with both the Onion Model and Valencia model. All you need to do is enter IMF (%), CMF (%), and the Mass (in earth masses).
Click to view attachment

(Also attached please find a word document describing Onion Model calculation set up)
Click to view attachment

Have fun, create worlds!

-Mike

I added a surface gravity column - interesting that the surface gravity ranges from 0.3g to ~2.8g depending on the CMF\IMF ratio. Some of those options would certainly have difficulties holding on to an atmosphere. I wonder if there would be any risk of losing much of the oceanic mass given the combination of low surface gravity and relatively high surface temperature.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

Gliese 581c made APOD for May 2.

The skew comes from lower inclination orbits having a lower threshold of absolute stellar acceleration orbits. A near-perpendicular orbit's planet would only be detected if the absolute stellar acceleration were exceptionally high. A side-looking orbit would be detected in those conditions AND conditions of lower absolute stellar acceleration. Therefore, we'll see more side-looking ones than perpendicular, the the perpendicular ones will be more biased towards large and close-in planets than the side-looking ones.

Nice work, juramike. As luck would have, there is a new paper in press in Icarus:

Mass-radius curve for extrasolar Earth-like planets and ocean planets
C. Sotin, O. Grasset and A. Mocquet

The biggest difference between your model and their model appears to be the inclusion of a HP-ice layer between the liquid water layer and the silicate mantle. Assuming 50% H2O content in Gliese 581c, they predict a radius of around 12,750 km (reading from one their graphs).

The skew comes because the ratio of detected acceleration vs actual acceleration is a cosine function of the angle. So, for a range of inclinations

So given a certain radial acceleration, it is pretty likely that the true Mactual is within a factor of 4 of the Mdetected.

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I don't know for sure, but I don't think so. You'd need a known quantity on the star's surface. Imagine a star that is half white/half black split on a longitude. In that case, you could tell the rotation axis based on the light curve. But starspots could be on any latitude, so it would impossible to correlate a light curve to a tilt at a specific angle, since the spot could be larger or smaller and further or closer to the pole and cause the same effect.

That's what made the discovery of transiting planets such a big deal, since it vastly reduces the probability bars on the mass, since being in line with the star gives you a small I, with a cos I very close to 1. Along with that, it's possible with sufficient sensitivity, I think, to get which star latitude the planet is crossing based on the shape of the transit curve and orbital speed.

It is a good point, though, that any reported detection of a low-mass planet really could be a high-inclination planet, unless it's transiting. Why do we not generally see that caveat, or is there some additional test that lets them eliminate this possibility? I'm not able to think of anything right off the top of my head though, and all published numbers seem to be "M Sin i" the "minimum mass."

http://exoplanets.org/planets.shtml

--Greg

Thanks! So I guess the Valencia modification is taking the HP-ice layer model by Sotin, and then adding bonus (denser) layers due to the HP-silicate phases.

-Mike

Here's a quick Q'n'A I was able to do with Xavier Bonfils, one of the astronomers envolved in the discovery.
I don't know if this has been discussed but he makes reference to the fact of, and I quote: "Interestingly, if you take into account an important greenhouse effect, Gliese 581 d (the outer planet in the system) may be more hospitable!"

I'm sorry for the unstable updating on the blog but "real life" takes most of my time and doesn't allow me to dedicate the attention I would like to put on it...
I hope things will improve in the following days, there also some news outside the spaceurope's realm...

This links, provided by Lewis Dartnell might be useful since they're focused on M-dwarf planet habitability...

5 earth mass planet orbiting an M-dwarf star identified by perturbations of a Neptune-sized world previously detected. It is the smallest extrasolar planet detected to date. The planets is designated GJ 436 c.

The surface temperature is predicted to be a toasty 400-700 K (127-427 C)

http://www.physorg.com/news126960070.html

Link to original article abstract here.

-Mike

Yet another " Earth-like " planet GJ 436c

<<< THEATRICAL SIGHS>>>

I don't know why, except for PIO press-relase hype and press igno-blather, they have to keep referring to these monstrosities as "Earth-Like".

The standard term is "Terrestrial Planet".

At a rather arbitrary level, one defination of "Earth-like" (ignoring climate.... Venus is Earth-like except for climate) would be a mass of 0.5 to 2.0 Earth masses.

Smaller, you get to Mars and Mercury/Lunar type planets, larger, you get to Super-terrestrials.

Some blather goes that anything over 1.00000000 Earth masses is super-terrestrial. That's so stupid it's silly.

Something that's 5 Earth-masses will start to have substantially different geologic histories and atmospheric retention histories than Earth, even if it received an equivalent stellar-irradiance flux as Earth.

0.5 to 2.0 E.M. is a crude approximation of a range where given appropriate external conditions, which Venus lacks <!>, something quiet Earthlike and non-space-suit inhabitable may result.

The actual press release is in Spanish.

http://www.csic.es/wi/VisualizarDocumento....cbase=CSIC_PROD

I think the relevant line is:

Según los modelos actuales, el planeta sería de tipo rocoso, con una masa de cinco tierras y un periodo orbital de 5,2 días.

Which I translate as "According to current models, the planet would be the rocky type, with a mass of five Earths and an orbital period of 5.2 days."

Weirdly, the press release claims the planet is NOT tidally locked -- that it has a siderial period of 4.2 days and a tropical day of 22 days. Wonder how they figured the rotation rate?

Probably the most interesting thing is that this seems to be the first claimed discovery of an exoplanet via orbital perturbations of other exoplanets.

The full report (presumably in English) appears in the latest edition of Astrophysical Journal Letters.

--Greg