

Scientists Estimate At Least 17 Billion Earth-Size Planets Inhabit Milky Way - G5ANDY
http://www.space.com/19157-billions-earth-size-alien-planets-aas221.html

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Symmetry
This is sort of terrifying, given that we haven't observed any space-faring
species from any of those other Earth-sized planets yet.

<http://en.wikipedia.org/wiki/Great_Filter>

~~~
anigbrowl
I don't worry about that too much.

Consider a community of people with little technology that live away from any
major trade routes, and thus don't see ships or planes. They would be
oblivious to all the internet, TV, radio and postal traffic going on around
them despite its huge volume. Now, I can't think of any such islands that are
totally out of contact in 2013, but there are still uncontacted tribes in the
Amazon basic (that we know about) and probably a few in other places (that we
don't know about).

Since interstellar travel would require something like a warp drive to be any
way practical, and we have yet to crack that problem, it seems quite likely to
me that we're also incapable of observing traffic of that kind going on in our
(galactic) vicinity. I don't see any particular reason that more advanced
civilizations would go out of their way to reveal themselves to us; indeed, it
bothers me that most alien invasion movies/novels tend to skip over the
question of what is so special about Earth that aliens should cross vast
distances to plunder our resources in particular when it would be so much
easier to just grab a passing asteroid or comet.

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BerislavLopac
"Now, I can't think of any such islands that are totally out of contact in
2013"

We can always count on Cracked to point the way. ;-)

[http://www.cracked.com/article_19976_6-isolated-groups-
who-h...](http://www.cracked.com/article_19976_6-isolated-groups-who-had-no-
idea-that-civilization-existed.html)

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ra
I'm in awe of how much we've discovered about the universe in the last decade
of science exploration.

The space programs Cassini, Kepler and Curiosity, as well as earth bound
programs like VLT and LHC really have expanded our understanding of the laws
of nature to levels I didn't think I'd see in my lifetime.

We live in fascinating times.

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k_kelly
There's always the problem of when.

While it's becoming more likely that a life supporting planet exists outside
our solar system, we have the problem that mankind's recorded history is
measured in thousands of years (and more than likely will only be measured in
thousands of years) while the universe is a tale of billions of years.

I'd still say we are likely to see artificial satellites at some point, as our
own would likely survive for millions of years in some capacity (even as
junk), but the possibility of direct communication is probably never going to
happen.

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mullingitover
I'm curious how many Earth-sized planets have a twin planet like our moon.
Without the moon we'd be tidally locked, and despite all our water we wouldn't
be nearly as life-friendly.

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ceejayoz
Mercury and Venus aren't tidally locked.

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CapitalistCartr
They're quite close to it. Mercury takes two of its own years to have one day,
and Venus's year is less than two of its days.

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nfg
This is generally suspected to be because of large primordial impacts, much
like the one which it is proposed created our moon. I don't think there's
anything intrinsic about Goldilocks zone orbiting rocky planets which would
lend to tidal-locking with the star.

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mullingitover
It's not their distance from the sun, it's the presence of liquid being
affected by gravity and creating a tidal bulge. The bulge slows down the
planet's rotation via friction, and eventually the planet stops rotating.

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jonknee
Fascinating. Our little rock could be "one in a billion" and we would still
have 16 other civilizations to meet. Just in our galaxy.

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mistercow
Actually we would expect to have 17 other civilizations to meet. The events
are independent, so observing our own civilization doesn't affect the odds for
the rest of the galaxy.

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marshray
But we're not interested in Earth, so we'd expect to see 16.999999999 _other_
civilizations.

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mistercow
Yes, but I think you may have a few too many sig figs there.

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marshray
So does 17 :-)

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mistercow
I guess the number of sig figs when you write out "a billion" is ambiguous.

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skwosh
Regarding the existence of life...

Seems all that's really required to kickstart life and evolution is any self-
replicating structure.

Chemicals are well suited for creating complex structures, but it doesn't rule
out self-replicating forms embedded in other kinds of matter held together
with other kinds of force.

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loudmax
This is interesting, but I think an estimate of the number of planets with
lots of liquid water on the surface would be more exciting. Liquid water is
key to life as we know it, so knowing how common it is in the galaxy might
give us some idea how likely we are to find extraterrestrial life.

~~~
andrewfelix
There's a lot more to life than just liquid water. In fact there are a huge
array of requirements including distance from the sun, right arrangement of
planets, tidal systems, plate tectonics...

~~~
noiv
Why there are so many constraints on life listed? Is it to make more us
special? Some people follow a different approach and say the purpose of life
is to hydrogenate carbon dioxide.

[http://blogs.discovermagazine.com/cosmicvariance/2010/03/10/...](http://blogs.discovermagazine.com/cosmicvariance/2010/03/10/free-
energy-and-the-meaning-of-life/)

I very much like the idea that life might be a consequence of entropy and
emerges everywhere where energy is in flux and matter poses a threshold
against the arrow of time.

~~~
andrewfelix
It's a nice idea, but I find the 'Rare Earth' hypothesis convincing:
[http://en.wikipedia.org/wiki/Rare_Earth_hypothesis#Rare_Eart...](http://en.wikipedia.org/wiki/Rare_Earth_hypothesis#Rare_Earth.27s_requirements_for_complex_life)

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pedalpete
I didn't think there was anything special about the size of the earth (and
apparently I'm right). Can anybody explain why this is important, I don't
think the article did a very good job of that.

~~~
mikeash
Too small, and the planet won't be able to retain an atmosphere (e.g. Mars).
Too large, and it'll probably retain too much.

~~~
mansoor-s
I have always been under the impression that the lack of atmosphere on mars
was due to its magnetic field weakening and allowing solar winds to carry away
the atmosphere over the billions of years.

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martinced
Regarding the Fermi paradox: one of the civilization has to be the most
advanced and has to be so intelligent that it obviously knows the Fermi
paradox ("probably" under another "name" ; ) all too well.

Must s*ck to be that one civilization: being right about the universe actually
holding several carbon-based lifeforms and yet scratching their heads thinking
"We can't be the most advanced, we can't be the most advanced".

: )

~~~
Udo
The universe _is_ relatively young in the sense that metal-rich solar systems
are probably a requirement for advanced lifeforms and we haven't had that many
generations of stars yet. Our sun is a "Population I" star (counter-
intuitively, astrophysicists are counting backwards here) that has taken heavy
elements as building material for the whole solar system from the remnants of
earlier stars.

However, the Milky Way is at the same time certainly old enough to suggest we
shouldn't be the first. Evolutionary timescales probably depend a lot on the
environment, but even if we're taking the development of our biosphere as one
of the fast examples, there should still be other comparable planets billions
of years younger than ours lending credibility to the assumption that there
should have been civilizations before us.

One of the big questions is also how long civilizations stay in the
"compatible" phase on average. If they disappear (either going extinct or
transcending into another stage) fast enough, finding and communicating with
them in time will be very difficult. Like I said, extinction isn't necessarily
the driving force. For example, our own sci-fi stories are absolutely rife
with slightly advanced civilizations that very likely would never communicate
with us.

Another favorite explanation of mine is that there could be some unknown local
issue in our stellar neighborhood. Candidates scenarios for this are myriad,
and they fall into two categories: there isn't anyone nearby, or we are
artificially isolated somehow.

~~~
marshray
Yeah, it's not like some alien benefactors would have placed a geodesic array
of transmitters in the Oort cloud that would cancel out all our inane radio
spectrum emissions.

Or could they? Hmmm...

