Gliese 581 d and e announced, e is ~ 2 Earth masses.

[Spaceman Spiff]

Breaking news, it seems, which demands an urgent 'inside office hours' post!

I wasn't sure about this when I spotted an update on exoplanets this morning:

The Extrasolar Planets Encyclopaedia: Gl 581 (http://exoplanet.eu/star.php?st=Gl+581)

but it seems it's official.

Universe Today : Nearly Earth-sized Planet, Possible Watery World Spotted Near Another Star
(http://www.universetoday.com/2009/04/21/nearly-earth-sized-planet-watery-world-spotted-near-another-star/)

BBC: Lightest exoplanet is discovered
(http://news.bbc.co.uk/2/hi/science/nature/8008683.stm)

Spiff.

[t00fri]

I want a little cottage on habitable planet d! I love hiking

Fridger

[Hungry4info]

d has been known, but this new fourth planet is certainly new. An exciting find!

I want a little cottage on habitable planet d! I love hiking

Watch out for loose and out of place rocks. I hear falling on GJ 581 d hurts.

[Spaceman Spiff]

I want a little cottage on habitable planet d! I love hiking

Ah, but Fridger, this is likely an ocean world. You'll probably have to put the cottage inside a boat, then swim about.

Anyway - huff - the deceivers! Those dreams of habitability the press are pushing: 'd' has an eccentricity of 0.38! It probably moves out of the habitable zone for days on end!! Imagine the storms!!! The freezing and melting floes!!!!

Hmm, Svalbard++ I hope the sea's the right shade of pink for you, certainly not green .

d has been known, ...

Oops, yes. I saw later that 'd' only had its orbit parameters updated. It's a shame with that exoplanet site: you can only see there's a 'latest update', not what the data was before, if any.

Spiff.

[t00fri]

Spiff,

recognize this florishing world with green meadows and blue rivers?
Just a little GIMP makes everything habitable...


Fridger

[Spaceman Spiff]

Hey, but for all we know (the original images were monochrome), Titan's surface really could be those colours! That smog condenses into Fairy Liquid, see.

Spiff.

[t00fri]

Hey, but for all we know (the original images were monochrome), Titan's surface really could be those colours! That smog condenses into Fairy Liquid, see.

Spiff.

Apart from the orange skycolor which was measured...

Fridger

[AVBursch]

Gliese 581d may be the first Earth-like world found outside of the solar system.

Given the distance of Gliese 581d from the star, this planet would be in the outer habitable zone. This world would be colder than Earth, but much warmer than Mars. The planet's semi-major axis is 0.2153 AU. However, the Sun is 78 times more luminous than Gliese 581, so the planet receives stellar insolation equivalent to a distance of 1.901 AU from the Sun.

Gliese 581d likely has an atmosphere considerably thicker than Earth's, which would help warm the planet and moderate temperature fluctuations. Accordingly, Gliese 581d would have a subarctic climate in the equatorial zone, a tundra climate in the temperate zone, and an icecap climate in the polar regions. Temperature differences between summer and winter would mainly be due to the planet's eccentricity (this can be observed on Mars). Since Gliese 581d likely has a thicker atmosphere than Earth, daily temperature fluctuations would be smaller than Earth's.

In the summer season, the equatorial regions would have a daytime temperature of 50F - 55F (10C - 13C) and a nighttime temperature of 40F - 45F (4C - 7C); The temperate regions would have a daytime temperature of 40F - 45F (4C - 7C) and a nighttime temperature of 30F - 35F (-1C - 2C); The polar regions would have a daytime temperature of 25F - 30F (-4C - -1C) and a nighttime temperature of 15F - 20F (-9C - -6C). Temperatures during the winter would be about 30F (17C) lower than in the summer.

The equatorial zone of the planet, in theory, would be warm enough for trees to be able to grow. However, the growing season would be short, about 18 days out its "year" of 66 days. There may be issues with biological life adapting to a relatively rapid fluctuation of the planet's seasons. On Earth, such climates are found in Alaska, Canada, and Russia, but the seasons change much more slowly, allowing for a ~100 day period every year where life can grow.

With all that said, Gliese 581d is likely to have a global layer of permafrost going many miles deep, have lakes of liquid water in the equatorial region, and have icecaps on the poles. Gliese 581d can be classified as Earth-like (until a spectral analysis shows otherwise). On Earth, there is life both in the subarctic and tundra zones. This means that Gliese 581d has the potential to support life as well.

Now for Gliese 581e. This planet would almost certainly be rocky, much like Mercury. The planet is very close to the star, only 30% farther away than Iapetus is from Saturn. Since Iapetus is already tidally locked, then it is reasonable to assume that Gliese 581e would be tidally locked as well. If the planet has a composition similar to Mercury, then the planet would have a diameter of around 8700 miles (14000 km). The planet recieves stellar insolation equivalent to 0.253 AU from the Sun. Gliese 581e would have an escape velocity of 9 miles/sec (14.4 km/s), which would allow the planet to hold onto an atmosphere of carbon dioxide and nitrogen in spite of the 500K equilibrium temperature. The planet's atmosphere would be too hot for clouds to form. The planet could have a strong magnetic field as well. Terraforming this world would be impossible.


Earth-mass planets are detectable through the doppler method. Given the level of precision needed to find Gliese 581e, it should be possible to detect the wobble induced by an Earth-mass planet orbiting a 170 Jupiter Mass star (about spectral type M6V) with today's technology.

[Spaceman Spiff]

Apart from the orange skycolor which was measured...

Ah yes, so true, my dear Fridger, but only the sky. As we discussed for Venus and Titan, the colour of the overcast sky does not reveal the true rock colour (yes, I know you know! ) so the surface of Titan could be bright green! Those were frozen fairy liquid boulders in front of Huygens, oh yes!

More seriously, I wonder if the dune seas of Titan appear dark in the Cassini ISS images because they are made of bluish water ice grains...

Spiff.

P.s., Next, I deal with extrapolating climate models from single data points.

[Spaceman Spiff]

Greetings AVBursch!

I see you must have joined this forum today because of the recent exciting news on Gl. 581?

That's a mightily detailed write-up on Gl. 581 d, but I fear too much extrapolation for concluding 'd' is anything like Earth-like. Here are some caveats against that which I'd like to toss in, if I may.

Without knowledge of the orbital inclination, there's a 50% chance that 'd' is more than 13 Earth masses. That's a Uranus-like world. Even if such a world orbited as close as 'e', it would be able to keep all its water and all its gases, right down to molecular hydrogen, in great abundance.

Even a lucky 90° orbital inclination gives no less than 7 Earth masses. I think a global ocean hundreds of kilometers deep is very likely. I would not expect any land.

Accordingly, Gliese 581d would have a subarctic climate in the equatorial zone, a tundra climate in the temperate zone, and an icecap climate in the polar regions.

If we do have any land, then if you're familiar with the carbonate-silicate cycle, this sets the level of carbon dioxide in any atmosphere in the habitable zone to the right level for the greenhouse increment to keep the average global surface temperature somewhere above 0°C (18°C for Earth at present). The CO2 abundance for 'd' is likely to be high, way more than the maximum 5% that humans can cope with (ask your average deep sea diver).

Paradoxically, without land, a slower sinking of carbon dioxide could make 'd' a hot world - the volcanic outgassing from below the deep ocean would first make the ocean fizzy and acidic, but eventually the CO2 should build up in the atmosphere.

Temperature differences between summer and winter would mainly be due to the planet's eccentricity

We have no idea if 'd's obliquity is about 90°.

Lastly, there's a little problem of 'aliasing' with the Doppler technique. It is possible for two planets in very circular orbits with a 2:1 mean motion resonance to impersonate one planet with a moderate eccentricity. I note that there's room for a smaller planet at 33 day period at 0.14 A.U. so maybe we'll have a lighter 'd' and a truly Earth sized 'f' in more circular orbits. That 'f' would be at a Solar System equivalent distance of 1.2 A.U., and probably 1-2 Earth masses. Much more yummy, don't you agree?

All goes to show what we really (don't) know!

Spiff.

[t00fri]

Spiff,

thanks for formulating what I would have written otherwise...

Fridger

[Romie]

Hi guys, Here i got some info of our newly discovered exoplanet Gliese 581/e .. so i just edit my extrasolar.ssc and append this info for- Gliese 581/e .

File: extrasolar.ssc
==============================================================
"e" "Gliese 581" # HO Lib
{
Texture "asteroid.*"

Mass 1.9 # M.sin(i) = 1.9 earths
Radius 12160

InfoURL "http://exoplanet.eu/star.php?st=Gl+581"

EllipticalOrbit {
Period 0.008627
SemiMajorAxis 0.03
Eccentricity 0.01
}

# likely to be in captured synchronous rotation
}
==============================================================

so if anyone got any extra information or exact texture for it then please post it here or pm me.....

[t00fri]

Why didn't you quote the scientific sources for your data. Only this way your data become really useful. It seems you didn't take them from here:

http://exoplanet.eu/star.php?st=Gl+581

which refers to an update as of April 21, 2009.

Fridger

[Spaceman Spiff]

Greetings Romie,

Actually, a broadly all right SSC given what we are told, but some comments. I see you included the very link t00fri mentioned in the InfoURL anyway, but how come some numbers differ slightly?

Mass. The Exoplanet site states mass of 'e' is 0.006104 ? Jupiter's mass = 1.94 Earth's mass. Although errors are not given (yet) on the Exoplanet site, it's wise to keep precision throughout calculations until the final answer.

Radius. It is usually a big mistake to assume radius varies as mass: 12,160km is clearly 1.90 ? 6,378km. You've just made your planet 'e' as undense as petrol! Radius would scale as the cube root of mass when keeping the same average density. Probably bigger planets are more dense. Let's allow density to be proportional to mass: then radius scales as fourth root: 1.94^(1/4) = 1.1802. Likely radius 1.1802 ? 6,378km = 7,527km.

Period. 3.14942 divided by 365.2564 days gives 0.008622 years. You had 0.008627 years. This time there's an error (±0.000001232 years), but it's negligible compared to the precision we've been given, so it's important to keep the digits correct to this precision.

Semi-Major Axis. This is tricky. The precision of reported values is deliberately kept low because of uncertainty of the star mass (we are tossed a low precision "0.31"). Yet to be consistent, if 'd' is at 0.22 A.U., and given the ratio of orbital periods for 'd' and 'e', 'e' must be at 0.029 A.U., not 0.030 A.U. (0.03 and 0.030 are Not The Same Thing!).

Eccentricity. There's no reported detection of eccentricity. Why 0.01? It should be just 0.

Rotation. "# likely to be in captured synchronous rotation". Yes! Very likely! I think tides from 'b' are too weak compared to those from the star to spoil that, even though 'b' appears three times larger at opposition than our full moon!

Texture. "... exact texture ..." Interesting concept . A surface texture might not be needed: we could have an ocean world, with 100% cloud cover. I suspect that for the estimated age of Gl. 581 at 8 billion years (11 billion years upper), any global ocean being lost through a Venus-like wet runaway greenhouse would still exist if it started off deeper than 320km (~450 km upper) (assuming Venus lost an Earth-comparable 4km deep ocean in 100 million years - I have to refresh myself on that). That could be quite possible, but the alternative is the oceans are gone and we have a dry greenhouse just like Venus. An ocean texture is much easier than a land texture . In any case, as I say, the most likely appearance of 'e' would be 100% coverage by uniform white clouds.

So...

Code: Select all

"e" "Gliese 581" # HO Lib
{
Texture "Gl581-e-ocean.*"

Atmosphere {
# blah blah blah
Cloudmap "Gl581-e-clouds.*"
}

Mass 1.94 # M.sin(i) = 1.94 Earth masses.
Radius 7527 # Guesstimate from taking fourth root of mass ratio to Earth.

InfoURL "http://exoplanet.eu/star.php?st=Gl+581"

EllipticalOrbit {
Period 0.008622
SemiMajorAxis 0.029
Eccentricity 0
}

# Likely to be in captured synchronous rotation.
}


Spiff.

[chris]

Period. 3.14942 divided by 365.2564 days gives 0.008622 years. You had 0.008627 years. This time there's an error (±0.000001232 years), but it's negligible compared to the precision we've been given, so it's important to keep the digits correct to this precision.

This is the only problem in an otherwise very useful decronstruction of an exoplanet ssc file. In Celestia, 'year' always refers to a Julian year of exactly 365.25 days (with a 'day' equal to 86400 SI seconds.) This gives a period of 0.0086226 years, quite close to the original value. I think that this use meaning of year is the convention in astronomy, but it should still be documented more clearly in Celestia. Also, we need some means of specifying units in ssc files to help prevent these sorts of errors.

--Chris

[Spaceman Spiff]

Ah Chris!

thanks for letting me know the Celestia definition! That is key above all else.

Shamelessly wikipedia-ing up right as I type, I see about astronomy generally taking the year to be 365.25* days exactly - oh but that's just so Julian! Even Gregorian doesn't relate truly to what we want.

I would have thought the appropriate period we need is the very one for finding where Earth is in its orbit in the grand XYZ scheme of things: the period of our (unchanging) Kepler orbit - the sidereal year. That's what we use 'year' for in Celestia, really.

Of course, I would want to see a move to something harsh: all quantities in SI units and the Earth year in SI seconds (http://en.wikipedia.org/wiki/Sidereal_year has it at 31,558,149.7635 seconds at 12:00, 1 Jan 2000 (hey, UTC or wot?), or 31,558,149.7676 depending on whether you believe their other page - Gah! We'd better find a better source).

I accept that introducing SI Units and all SSCs needing a niggling Period correction due to changing year would unsettle a lot of users though, sigh.

Spiff.

* and Wikipedia says 365.25 days is used to define the distance of the Light Year.

[Romie]

Yes, i took those readings from http://exoplanet.eu/planet.php?p1=Gl+581&p2=e and just put it in my extrasolar.ssc
but Spiff it is said on many places that the size is around 1.5 times of Earths radius that is- 6,378 * 1.5 = 9,567 radius
and by your calculations

1.94^(1/4) = 1.1802. Likely radius 1.1802 ? 6,378km = 7,527km.

That is 9,567 - 7,527 = 2040 error, and that's huge!
correct me if i am wrong
http://en.wikipedia.org/wiki/Gliese_581_e -says "it is unlikely to possess an atmosphere due to its high temperature, small size, and strong radiation from the star." so no atmosphere no clouds then....

sorry about the mistakes in finding radius of 'e' as 12160

You've just made your planet 'e' as undense as petrol!

ahhh so much petrol, $$$$
I just want to visit those exo-worlds and want to find out how life evolved there

[chris]

Ah Chris!

thanks for letting me know the Celestia definition! That is key above all else.

Shamelessly wikipedia-ing up right as I type, I see about astronomy generally taking the year to be 365.25* days exactly - oh but that's just so Julian! Even Gregorian doesn't relate truly to what we want.

I would have thought the appropriate period we need is the very one for finding where Earth is in its orbit in the grand XYZ scheme of things: the period of our (unchanging) Kepler orbit - the sidereal year. That's what we use 'year' for in Celestia, really.

Celestia has its share of quirks, but I don't think the use of a Julian years throughout isn't one of them. All of the ephemerides I've used in Celestia that use a value in years as input specify a Julian year (and often a Julian century with 36525 days.)

Of course, I would want to see a move to something harsh: all quantities in SI units and the Earth year in SI seconds (http://en.wikipedia.org/wiki/Sidereal_year has it at 31,558,149.7635 seconds at 12:00, 1 Jan 2000 (hey, UTC or wot?), or 31,558,149.7676 depending on whether you believe their other page - Gah! We'd better find a better source).

I think you've hit on the problem with using a Sidereal year as a unit of time: the exact number of seconds in a standard sidereal year is dependent upon a lot of factors, giving people like me that many more ways to screw up a time calculation. Also, I imagine it might even be possible that future observations would allow us to determine the orbit of the Earth more precisely and revise the length of the J2000 sidereal year. Better just to use a convenient multiple of the SI time unit and be done with it.

I accept that introducing SI Units and all SSCs needing a niggling Period correction due to changing year would unsettle a lot of users though, sigh.

Not just unsettle--I'd go mad if I had to convert to sidereal years when all my libraries and data sources are using Julian years. But lately I've been living a sheltered life in the Solar System--Fridger or someone else will know better than me what they mean by 'year' when they measure times out there beyond the Kuiper Belt.

--Chris

[Hungry4info]

A discussion regarding the habitability of GJ 581 d.

Why are people saying that revising the period to 67 days brings
581d into the habitable zone? With a semimajor axis
of 0.22AU, the stellar flux at the orbit is only 370 W/m**2,
and even if the planet had zero albedo, when you average this
over the planet's surface you only get a blackbody temperature
of 200K. If the planet had an ocean it would be a snowball,
bringing the temperature down even more.

You could try to bring up the temperature by putting in a lot
of CO2, but at 370 W/m**2, you have an even harder problem
for Gliese 581d than you do for Early Mars, which had an
incident radiation of 412 W/m**2 . It's a real stretch to
say that it's in the hab zone, unless you consider Early
Mars to be in the hab zone.

Raymond,

we support your concern. Dimitris Mislis and I, we used the formula given in Kasting et al. (1993) [1993Icar..101..108K] to compute the period of a planetary companion to be in the habitable zone around a MS star.

To obtain the extent of the HZ, we took the values for S_eff as provided by Kasting et al. for a Venus-like planet to maintain liquid water at the inner edge of the HZ (S_eff = 1.9114) and a martian planet at the outer edge, modified to exhibit a maximum greenhouse effect (S_eff = 0.36).

As you stated, Raymond, Gl581d is located beyond the HZ of its host star.

[selden]

Hungry,

Where did you get those quotes?

It's usually not appropriate to copy people's conversations from one site to another for several reasons.
1. the original authors might not want it
2. it's out of context, and there might have been something important written that you overlooked and didn't copy
3. It's unlikely, I'm sure, but for all we know, you might have modified the text, hopefully by accident.

A link to the original discussion would be much better, along with your own comments.