Small, potentially water-bearing exoplanet

[The Singing Badger]

This is interesting. Gliese 581 C is only 50% bigger than Earth and in the star's habitable zone.

http://www.space.com/scienceastronomy/0 ... lanet.html

[chris]

Grant has already updated extrasolar.ssc with the new planets so that you can view them in Celestia:

http://celestia.cvs.sourceforge.net/cel ... asolar.ssc

--Chris

[chris]

The excellent extrasolar planet blog oklo has some interesting speculation on Gliese 581 c:

http://oklo.org/?p=205

The author speculates that the planet formed further out from the planet and has a deep ocean containing over an Earth mass of water. Here's an image of the planet's atmosphere generated by a hydrodynamic simulation

http://www.flickr.com/photos/oklo/471754646/

It's just a simulation of course, but it's still interesting to me that it doesn't show the planet as completely by a cloud layer.

--Chris

[eburacum45]

I don't get it; if this planet has a diameter of 1.5 Earths and a mass of 5 Earths, it will have a density close to that of iron and a gravity about twice that of our planet.
In that case, it should have retained hydrogen and would be a small gas giant. Its hard to figure the angles on these exoplanets...

[eburacum45]

I've made an image of star-rise as seen from the surface of GL 581 c, and compared it to Sunrise on Earth, using Celestia. (thanks, Grant).
I've assumed several things;

1/ the planet is a waterworld;
2/you can see the star from the surface
3/it is tidally locked (this means the star-rise goes on for ever if you are on the terminator)



Note how big the star looks compared to the Sun; it is, in fact, quite a bit smaller than our star, but the planet is much closer.

[chris]

The transits of planet Gliese 581 b would be spectacular . . . Although the size estimate for extrasolar planets is very rough, during transit b is likely to appear roughly 75% as large as the Moon does from Earth. Here's a view of a transit from just above the terminator of c:



--Chris

[ajtribick]

From the Extrasolar Planets Encyclopaedia, radius of the star is 0.38 times radius of the sun. From spectral type (M3), the temperature is around 3500 K.

This suggests a luminosity of 0.019 times solar, and scale factor for the habitable zone of 0.14.

Scaling Sol's habitable zone (0.95-1.65 AU according to some estimates) to that of Gliese 581 gives 0.13-0.23 AU.

This 5 Earth-mass planet (at 0.073 AU) is probably going to be unpleasantly hot, once the greenhouse gases are taken into account. In fact, scaling Venus's orbital distance to Gliese 581 gives 0.10 AU, which is still outside the orbit of the planet! So even if this planet has lots of volatiles (hot ocean planet), the oceans may be boiling or supercritical.

The 8-Earth mass planet in an orbit at 0.25 AU looks much more promising to be a habitable ocean planet once greenhouse gases are taken into account, especially since the eccentric orbit moves it into the habitable zone (periastron 0.2 AU).

[eburacum45]

The radius seems to have been worked out using this paper;
http://arxiv.org/abs/astro-ph/0511150
the gravity of such a super-earth would be about 2 gees. But if it is too hot, as you suggest, it could be a dry super-venus instead. I still think two gees is strong enough to hold on to a considerable amount of hydrogen; there must be a formula to work that out somewhere. If it has a lot of hydrogen, the radius could be a lot greater to the top of the atmosphere.

[chris]

From the Extrasolar Planets Encyclopaedia, radius of the star is 0.38 times radius of the sun. From spectral type (M3), the temperature is around 3500 K.

This suggests a luminosity of 0.019 times solar, and scale factor for the habitable zone of 0.14.

I get different values for the luminosity . . . The star is 6.26 parsecs from Earth, with a apparent visual magnitude of +10.55. This gives an absolute visual magnitude of 11.57. According to Celestia's tables (from Lang's Astrophysical Data: Planets and Stars), the bolometric correction for the star is somewhere between -1.89 and -2.15, giving a luminosity between 0.0114 and 0.0146 times solar.

The new paper by Udry et al. (http://exoplanet.eu/papers/udry_terre_HARPS-1.pdf) uses a value of 0.013 times solar for the luminosity of Gliese 581. The value of 0.013 comes from a 2005 paper by Bonfils et al, and is within the range I've come up with. But, even assuming this lower luminosity, the planet still lies inside the scaled solar habitable zone (0.11 - 0.19 AU).

--Chris

[AlexChan]

A question,
when I use Celestia to simulate the planet, the clouds look pink.
if the components of the atmosphere is the same as earth
can the sky still look blue?

[Cham]

as you suggest, it could be a dry super-venus instead. I still think two gees is strong enough to hold on to a considerable amount of hydrogen; there must be a formula to work that out somewhere. If it has a lot of hydrogen, the radius could be a lot greater to the top of the atmosphere.

The mass quoted for this planet already INCLUDES its atmosphere. So I don't think it has so much hydrogen, even with a 2g gravity.

[ajtribick]

I get different values for the luminosity . . . The star is 6.26 parsecs from Earth, with a apparent visual magnitude of +10.55. This gives an absolute visual magnitude of 11.57. According to Celestia's tables (from Lang's Astrophysical Data: Planets and Stars), the bolometric correction for the star is somewhere between -1.89 and -2.15, giving a luminosity between 0.0114 and 0.0146 times solar.

The new paper by Udry et al. (http://exoplanet.eu/papers/udry_terre_HARPS-1.pdf) uses a value of 0.013 times solar for the luminosity of Gliese 581. The value of 0.013 comes from a 2005 paper by Bonfils et al, and is within the range I've come up with. But, even assuming this lower luminosity, the planet still lies inside the scaled solar habitable zone (0.11 - 0.19 AU).

--Chris

First off, the planet distance is 0.073 AU, which doesn't lie within your scaled habitable zone, in fact it lies interior to the inner boundary.

I'm using the spectral class-temperature tables inside Celestia, and the fact that some sources give an M2 class, some give an M3 class, so I've taken 3500 K as an approximate value. Of course, blackbody approximation might be quite bad for very cool stars which can have molecles in their atmosphere, which complicate things a lot, so my blackbody model might be off quite a lot.

However, reading the discovery paper, the 0 degrees C value is assuming an equilibrium object with the same albedo as Venus, whereas if you take the actual bolometric albedo of Venus and use the solar luminosity, you end up with a temperature somewhere between minus ten and minus twenty degrees C! (In fact, the value in Celestia is around -40, but I guess that might be a visual albedo) The middle planet of Gliese 581 apparently receives more total radiation than Venus does, which should set alarm bells ringing!

If I use an empirical B-V correction (from here) to get the temperature, using the equation B-V=-3.684log(T)+14.551, and from SIMBAD, the magnitudes are B=12.17, V=10.56, so B-V=1.61, which yields T=3256 K

The bolometric correction is given by BC = -8.499 [log(T)- 4]^4 + 13.421[log(T)- 4]^3- 8.131[log(T)- 4]^2 - 3.901 [log(T)- 4] - 0.438

Plugging this in to the luminosity formulae gives a total bolometric luminosity of 1.9% solar, giving a scaled habitable zone of 0.13-0.23 AU, agreeing with my analysis taking a rough value of the temperature of the red dwarf star.

[chris]

First off, the planet distance is 0.073 AU, which doesn't lie within your scaled habitable zone, in fact it lies interior to the inner boundary.

That's actually what I was trying to say, but I hastily wrote 'inside', which sounds more like 'within' than my intended 'interior to.'

However, reading the discovery paper, the 0 degrees C value is assuming an equilibrium object with the same albedo as Venus, whereas if you take the actual bolometric albedo of Venus and use the solar luminosity, you end up with a temperature somewhere between minus ten and minus twenty degrees C! (In fact, the value in Celestia is around -40, but I guess that might be a visual albedo) The middle planet of Gliese 581 apparently receives more total radiation than Venus does, which should set alarm bells ringing!

Agreed that Gliese 581 c is likely to be a hot place--it seems optimistic to expect that the temperature of the planet will be comfortable. I wonder how the likely tidal locking affects surface temperatures of the planet. Venus is a very slow rotator, yet the atmosphere still manages to keep the temperature fairly consistent across the surface.

--Chris

[eburacum45]

as you suggest, it could be a dry super-venus instead. I still think two gees is strong enough to hold on to a considerable amount of hydrogen; there must be a formula to work that out somewhere. If it has a lot of hydrogen, the radius could be a lot greater to the top of the atmosphere.

The mass quoted for this planet already INCLUDES its atmosphere. So I don't think it has so much hydrogen, even with a 2g gravity.

This is the problem; we don't know the composition or density of this planet, so we can't make any reliable predictions about the gravity or the thickness of the atmosphere.

It appears that the estimate for the radius, and hence the density, was made using this paper
http://arxiv.org/abs/astro-ph/0511150
which includes graphs of increasing density for planets of an Earth-like composition. However we have no proof that this planet has an Earth-like composition; it could have a composition resembling that of Mercury, or Venus, or even the Moon.
Alternately it could be a Neptune like ice-cored giant planet which has migrated inwards; if so it could have a massive ice content, a diameter twice that of the Earth's, and a much lower gravity.

The range of possibilities is very wide. If it is a water world, the density implied in the announcement is probably wrong.

[eburacum45]

Incidentally I don't think that even a high atmospheric temperature necessarily rules out a water world; I envisage a planet with a thick atmosphere and superheated high-pressure oceans, perhaps overlaying a hot high-pressure ice mantle.
Of course all that water vapour would be a terrific greenhouse gas...

[Planet X]

A question,
when I use Celestia to simulate the planet, the clouds look pink.
if the components of the atmosphere is the same as earth
can the sky still look blue?

The pinkish color is actually what clouds would look like when illuminated by a red dwarf star. For comparison, a planet in one of my own systems has pale orange colored clouds due to it orbiting an orange K star.

Now, the sky of an Earth-like planet orbiting a red dwarf star would actually look virtually the same as our own sky. The sky would have just a faint purpleish tinge to it, with a slight pinkish color in the clouds when viewed from the surface. Later!

J P

[ajtribick]

Incidentally I don't think that even a high atmospheric temperature necessarily rules out a water world; I envisage a planet with a thick atmosphere and superheated high-pressure oceans, perhaps overlaying a hot high-pressure ice mantle.
Of course all that water vapour would be a terrific greenhouse gas...

From the paper "Volatile-Rich Earth-Mass Planets in the Habitable Zone", it seems that even at Earth distance from the sun, an ocean planet could end up with a supercritical fluid ocean, whereas Gliese 581 c receives more total radiation than Venus does. Gliese 581 d looks like it might well be the habitable world of this system.

Whether life is possible in a supercritical fluid is uncertain, perhaps this paper is cause for optimism, for planets such as Gliese 876 d and the inner two worlds of Gliese 581?

[bdm]

A question,
when I use Celestia to simulate the planet, the clouds look pink.
if the components of the atmosphere is the same as earth
can the sky still look blue?

It depends on the amount of blue light radiated by the star. If the star does not emit enough blue light, insufficient blue light would be available to make the sky blue. Instead, the sky may have other colours. I would guess that the sky colour would not be the exact shade of blue we see on Earth, but may instead be darker and have a slight greenish cast (assuming Earthlike pressure and gravity).

A higher atmospheric pressure would complicate the picture somewhat. The sky would be lighter, but I don't know for sure how this scales to atmospheric pressure.

Now I'm not really sure how correct his would be. Two or so years ago I was trying to find information about the sky colour on alien worlds (particularly for Earthlike worlds around red dwarf strs) but could not find good information on the Web. The best I could come up with was a computer program I wrote myself that attempted to replicate the sky colour based on blackbody curves, Rayleigh scattering and the frequency response of human eye pigments. It gave a sky colour that was greenish for a red dwarf star. I cannot vouch for the accuracy of the software because I have no way to validate its results.

If anyone does have good information on how to determine (or even compute) the sky colour on alien worlds, I would appreciate it.

[bdm]

Life requires more than just the presence of water. It likely requires a variety of carbon-based compounds that can join together in different ways, like amino acids and DNA, and an environment in which these molecules can settle down and fester.

This last point is often overlooked. It's easier to grow bacteria in quiet pools than turbulent environments. If an environment is amenable to the development and growth of life, it is unlikely to remain so if it is constantly agitated, dispersing its nutrients in all directions. An environment that has the right conditions to start life would be more likely to do so the longer those conditions persist.

Do such big planets with deep oceans have environments free from disruptive turbulence in which life is more likely to get started?

[eburacum45]

At the bottom of a waterworld ocean, I can envisage a slushy transition zone where the pressure is not quite adequate to solidify the ocean. This strange zone, apparently full of apolar water molecules according to Chaos Syndrome's link, might be the site for some strange chemistry; there could be meteoritic sludge including organic compounds brought in by comets, and minerals exhaled by thermal vents.

A site for abiogenesis?

Speculation, but not perhaps impossible.