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

[Spaceman Spiff]

Greetings again, Romie,

but Spiff it is said on many places that the size is around 1.5 times of Earths radius

but how was that figure arrived at? Plus, does seeing the exact same figure many times on the internet make it any more true?

One suspects a wet-finger-in-the-air guess, copied and pasted right across the internet. Note too that that radius implies a mean density of 3.2 times water. So, one would have to question what 'fluff' can exist on this rocky/metallic planet when it's supposedly "too hot / too small" for an atmosphere?

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....

The only 'serious' reference I see on that wikipedia page is Mayor's, but I've been trying to download that PDF for three days now without success: it always stalls at 67%. I'm therefore left guessing that the quote "it is unlikely to possess an atmosphere due to its high temperature, small size, and strong radiation from the star." is either made up there on Wikipedia by whoever wrote it, or a quote from a semi-informed journalist.

Hmm, let's see (getting detailed!). Earth's escape velocity is 11.2km/s. R.M.S. velocity of a CO2 molecule by Maxwell-Boltzmann distribution is around 0.43km/s, so CO2 is slow to escape. If 'e' has the mass and radius I say (1.94 and 1.18 times Earth's respectively), then its escape velocity is 1.28 times higher: 14.3km/s. Let's allow 'e' a really hot atmosphere: 1,000K. Then the R.M.S. velocity of a CO2 molecule there is 0.79km/s. That's because R.M.S. velocity only goes up by the square root of absolute temperature. OK, 0.79/14.3 = 0.055, more than 0.43/11.2 = 0.038. However, running the same calculations for Venus (750K), I get 0.68/10.3 = 0.066, that is, Venus has a higher fraction of fast CO2 molecules than 'e'. This is oversimplified of course, because it's exosphere temperature that matters, but you see: it's not a given that 'e' can't keep a thick atmosphere.

correct me if i am wrong

For the arithmetic, you are certainly correct (I hope mine is too!). It's that '1.5' figure that's very likely wrong. So it's not you that is wrong, it's whoever came up with that '1.5' figure!

I just want to visit those exo-worlds and want to find out how life evolved there

'e': Been there. The beer was too warm...
'd': Been there too. My beer glass blew away in the wind!

Spiff.

[Spaceman Spiff]

Greetings chris,

Celestia has its share of quirks, ...

No, no, Chris! I think you'll find "features" is the accepted term in software development .

I think you've hit on the problem with using a Sidereal year as a unit of time: ...

Yes, shocking to discover that Earth's orbital period is a (weak) function of time, tsk! Those pesky perturbations, et al.!

... 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.

"The wonderful thing about standards is that there are so many of them from which to choose." - Grace Hopper, I believe.

Better just to use a convenient multiple of the SI time unit and be done with it.

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.

I concur. So, to avoid getting lost in the trees that is the woods, if we make the option for SI units, then we can allow the Earth's period to be written in its SSC as <insert official sidereal year length> seconds, and can also allow all other bodies' periods to be stated in seconds, circumventing the need to ever know how a 'year' is defined by Celestia. Thus we avoid needing to 'hardwire' a sidereal year into Celestia libraries, and the Julian year remains a straightforward 31,557,600 seconds for converting civil calendar dates.

I hope no one is going to insist you set up a caesium clock at your home to define the official Celestia second, Celestia time versus Barycentric Dynamic Time, etc.!

Spiff.

[Spaceman Spiff]

Woah! Hit and run!

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.

S_eff: Hmm, undefined here. I'm guessing it's an Earth-normalised solar constant. If so, I think Heller here has misapplied Kasting's models. Heller seems to think that Venus defines the inner HZ edge at 0.7233 A.U., Mars the outer HZ edge at 1.5237 A.U. They do not! and Kasting found the inner HZ edge for our Sun to be likely (at current solar luminosity) 0.95 A.U., while the outer edge is likely beyond 2 A.U. depending on just how much greenhouse increment a planet can make or do away with to keep water liquid on its surface.

I note (1/0.7233)^2 = 1.9114. Meanwhile, (1/1.5237)^2 = 0.4307, so they must assume a factor of 0.4307/0.36 = 1.2 as the maximum greenhouse effect. What!? Venus has a factor 5 greenhouse effect...

Spiff.

[Hungry4info]

I have indeed gained permission to post what I did. The conversation is from e-mail responses to the Extrasolar Planet Newsletter provided at exoplanet.eu.

[selden]

Hungry,

Did you make a typo in the name of the Newsletter? Google can't find it.
I can't find any mention of a newsletter on exoplanet.eu, either.
I'm probably overlooking it, though. Sometimes I can't see things that are right in front of me.

Found it!
It's not on exoplanet.eu, though.
http://listes.obs.ujf-grenoble.fr/wws/a ... 00000.html

p.s. A subsequent message from Derek Homeier explains the HZ discrepancies. That is an example of why it's better to provide a link to discussions rather than copying them.

s.

[Hungry4info]

That is an example of why it's better to provide a link to discussions rather than copying them.

Yes, if I were able, I would have indeed provided a direct link to the discussion, however the only place that it existed (to my knowledge at the time) was in my Gmail inbox. Providing a link wouldn't work, it would require the log-in.

Thanks for the link, I wasn't aware that the discussion was there, o_O. I guess it makes sense, too. I also apologize regarding the location of the Extrasolar Planets newsletter.

[selden]

You're forgiven.
Thanks for making us aware of the discussion.
Determining the size of the Habitable Zone obviously isn't easy.

[bdm]

BBC: Lightest exoplanet is discovered

From the article:

Astronomers have announced the discovery of the lightest planet ever detected outside our Solar System.

It annoys me when the media stuff up the facts like this. There are lighter planets known around the pulsar PSR B1257+12. Although pulsar planets are not as interesting because the parent star is not sunlike, they do count as planets.

[ajtribick]

One of the slightly annoying points about the discovery paper is the lack of information about where the planets are in their orbits at a given time.

Using the Systemic Console and the previously-published dataset for Gl 581 from when planets "c" and "d" were discovered (the new updated radial velocity dataset is unfortunately not yet available), and constraining the parameters for the 4 planets to their values in the new fit*, I find the following best-fit values for the mean anomaly at epoch 2453152.712894 (the first datapoint in the set) as follows:

Planet b: 162.1 degrees
Planet c: 134.5 degrees
Planet d: 110.6 degrees
Planet e: 134.9 degrees

(unfortunately the Systemic Console's bootstrap and Markov chain facilities seem to be broken with regards to angle parameters, so no error estimates here). The following code will add these to Celestia, it can be placed in a .ssc file in your extras directory:

Code: Select all

Modify "b" "Gliese 581"
{
    EllipticalOrbit {
        Epoch 2453152.712894
        Period 0.0147
        SemiMajorAxis 0.04
        Eccentricity 0
        MeanAnomaly 162.1
    }
}

Modify "c" "Gliese 581"
{
    EllipticalOrbit {
        Epoch 2453152.712894
        Period 0.0354
        SemiMajorAxis 0.07
        Eccentricity 0.17
        ArgOfPericenter 210
        MeanAnomaly 134.5
    }
}

Modify "d" "Gliese 581"
{
    EllipticalOrbit {
        Epoch 2453152.712894
        Period 0.1829
        SemiMajorAxis 0.22
        Eccentricity 0.38
        ArgOfPericenter 287
        MeanAnomaly 110.6
    }
}

Modify "e" "Gliese 581"
{
    EllipticalOrbit {
        Epoch 2453152.712894
        Period 0.0086
        SemiMajorAxis 0.03
        Eccentricity 0
        MeanAnomaly 134.9
    }
}

* the innermost planet can actually be detected in this dataset, provided you fit planet d to the ~67 day period which appears weaker than the ~84 day period which was originally assigned to planet d.

[ajtribick]

Well the arXiv version does include the value of T. Yay!