
First habitable exoplanet discovered? - ssclafani
http://www.sciencedaily.com/releases/2011/05/110516080124
======
jerf
"After its discovery, it was generally believed that any atmosphere thick
enough to keep the planet warm would become cold enough on the night side to
freeze out entirely, ruining any prospects for a habitable climate.... To test
whether this intuition was correct, Wordsworth and colleagues developed a new
kind of computer model capable of accurately simulating possible exoplanet
climates.... To their surprise, they found that with a dense carbon dioxide
atmosphere -- a likely scenario on such a large planet -- the climate of
Gliese 581d is not only stable against collapse, but warm enough to have
oceans, clouds and rainfall."

Question. How the _bloody hell_ do you validate a computer model of how an
atmosphere unlike anything we can observe in our solar system works? Did
someone fly out there and verify it before publishing? And I'm supposed to
believe your computer model iterates correctly over _geologic time_? Really?
_Really?_

(Obligatory-but-totally-serious that this is still cool work not counting the
computer model part, and to be honest I don't really care either way what the
computer model says; I would be equally skeptical if they produced a model
that claimed any outcome at all.)

~~~
saulrh
Sensitivity analysis. Elements of the simulation are based wholly on proven,
low-level physics (for example, calculating the amount of heat absorbed by a
given amount of CO2 from a given amount of sunlight) whenever possible, and
when that's infeasible the modelers test a range of possible values to see
what happens. For example, you can't exactly calculate the density of
condensation nuclei in the atmosphere (used for calculating cloud formation
behavior), so they just ran the simulation with a bunch of believable values.
Another example is pretty clearly laid out in the first sentence of the
"results" section:

    
    
      We performed simulations with 5, 10, 20 and 30 bar
      atmospheric pressure and 1:1, 1:2 and 1:10 orbit-rotation
      resonances for both rocky and ocean planets (see Table 1).

~~~
jerf
There is not enough data in a vague orbit and vague size (in a dataset that
until recently contained a spurious planet) to feed a simulation like this,
which means that the simulation was, mathematically necessarily (and I do not
use this term lightly), made up from nearly whole cloth. And then we have no
way to verify anything that it did say, assuming it even started from the
correct place, especially with regards to stability on geologic time. Even an
99.9999999%-per-hour perfect simulation will still diverge on geologic time
scales. For all we know, an atmosphere with a mountain range _here_ will
freeze out in a century, whereas a mountain range _there_ could in fact result
in a stable atmosphere over geologic time. This is all information-theoretic
necessity. This is entertainment, not science.

That may be all the scientists claimed. It's still a _fun_ result, even if I
wouldn't put my grandchildren on a flight to Gliese on the strength of it. I'm
reacting against the certainty expressed in the article.

~~~
saulrh
Ok, I think I see what you mean now, and I agree with you. They've shown that
the distribution assuming some set of priors includes some particular,
interesting outcome. This is the first planet we've ever seen where we can say
that given how much information we have about it, though, which means that
it's still cool.

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DavidSJ
The actual paper: <http://arxiv.org/pdf/1105.1031>

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D_Alex
Great! Let's go and check it out!

~~~
pjscott
It's 20 light years away, so that could take a while. (I still want to do it,
though.)

~~~
palish
Question:

In the year 2200, if a 10 year old child were to travel to the planet at a
speed of 0.9999c, then were to immediately turn around and travel back to
Earth, they would be approximately 50 years old. However, what year would they
arrive back on Earth?

Assumptions:

\- no acceleration or deceleration time.

\- the planet is exactly 20 light years away.

\- the velocity of the ship remains exactly 0.9999c while in transport.

(The answer is _not_ 2240\. It's much greater. My question is, how _much_
greater?)

~~~
btilly
Your problem statement is wrong.

The child is traveling 40 light years at 0.9999c, which takes about 40 years +
35 hours. However the child won't arrive back 50, the child will not yet be
11. See
[http://www.wolframalpha.com/input/?i=time+dilation+traveling...](http://www.wolframalpha.com/input/?i=time+dilation+traveling+at++.9999c+&a=*FS-_**RelativisticTimeDilationFormula.to-.*RelativisticTimeDilationFormula.t--&f2=40+y&x=6&y=5&f=RelativisticTimeDilationFormula.t_40+y)
for the exact age.

See <http://en.wikipedia.org/wiki/Twin_paradox> for more on this, including an
explanation of why your belief about the age of the traveler is wrong.

~~~
palish
How can this be?

If the child were moving at 0.9999c (which is less than 1.0000c) and the
planet is 20 light-years away, then how could it take _less_ than 20 years for
the child to reach it? Let alone ~0.5 years?

EDIT: Here's an explanation from a friend:

    
    
      light always travels at c even if you're already moving close to c
      but it's impossible for anything to travel faster than c
      so if you're traveling at .9999c
      the passage of time must be scaled for the traveler
      to make light on the ship appear to move at c
      even though it's only moving at 1-.9999c
    

That's... awesome.

~~~
Confusion

      If the planet is 20 light years away, you're saying we 
      could reach it in less than a year if we attain a velocity
      of 0.9999c?
    

Yes. In the reference frame of the traveler, very little time passes. However,
when he returns, people on earth will have aged more than 40 years.

This is also why particles traveling at _c_ cannot possibly decay: no time
passes for them. A photon is everywhere at once, from its own point if view.

~~~
aik
Amazing. I hope to some day wake up and have a deep conceptual understanding
of the relationship between velocity and time in relation to the various
bodies at play.

I also hope to have an understanding of what it means for a photon to be
everywhere at once, from its own point of view.

In the end, I guess I just want to understand the universe just a bit more
than I do.

~~~
dodo53
Special relativity is actually quite accessible and really interesting - find
a good book or set of lectures on iTunes U or something. (General relativity,
the generalised version including acceleration and gravity, is what makes
people think relativity is hard).

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Rariel
Great. Now all we have to do is wait until Zefram Cochrane invents the warp
drive in 2063. :-)

~~~
troels
Actually, being just 20 lyrs away, sub-warp speed would be sufficient for us
to get there. Well, not _us_ , but our grand children anyway.

~~~
Fargren
What's the state of the art in cryostasis?

