
Small stars may have Habitable Zones, but habitable planets might not be common - fcsuper
http://fcsuper.blogspot.com/2016/12/small-stars-may-have-stable-habitable.html
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fusiongyro
I'm not an astronomer, though I do work with them. My impression from talking
to them is that there is no consensus around whether or not habitable planets
are common or rare.

Our principal detection method for finding planets excels at finding gas
giants with masses similar to their host star. This is not a great tool for
finding habitable planets. Direct imaging of planets is not feasible at an
intergalactic scale; I'm not sure it's even really possible for stars further
out than a few hundred lightyears. This greatly limits our sample size. The
less you know about your distribution, the less accurate your simulation is
going to be.

A priori arguments from simulations have the problem of either producing an
absurd cornucopia of habitable planets or making too few. Steven Dole's
_Habitable Planets for Man_[1] discusses the odds and convincingly argues for
a large number of habitable planets, but it's pretty out of date. His ACCRETE
simulation is great at producing solar systems that resemble our own, but it
is not well-respected by astronomers. The time I spoke with an astronomer
about this, he said that current simulations have trouble getting from rocks
to planets. ACCRETE is a pretty crude simulation; to Dole and his coauthors
the interesting thing about it was that such a crude instrument could produce
such legitimate-looking results.

One of ACCRETE's problems is that there are a number of parameters, and if you
change them slightly you get very different solar systems. I would think a
good simulation would have a small number of these parameters but they usually
give you enough rope to hang yourself. In any event, to choose the right
settings you must have either physics dictating them or accurate numbers from
samples, and we still have a very small set of examples.

As an aside, the sheer volume of stars makes a strong argument that even if
the odds are vanishingly small, there ought to still be a mind-numbing of
habitable planets.

[1]: [https://www.amazon.com/Habitable-Planets-Man-Stephen-
Dole/dp...](https://www.amazon.com/Habitable-Planets-Man-Stephen-
Dole/dp/0833042270/ref=sr_1_1?ie=UTF8&qid=1480950883&sr=8-1&keywords=habitable+planets+for+man)

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Florin_Andrei
> _As an aside, the sheer volume of stars makes a strong argument that even if
> the odds are vanishingly small, there ought to still be a mind-numbing of
> habitable planets._

In a Universe that's extremely large, or potentially infinite, the main
question is not the number of habitable planets. It's the density, or the mean
distance between neighbors - in space as well as in time.

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camelNotation
The issue here is not so much the presence of a planet with the right mix of
elements and environmental properties. The size of the universe indicates this
is likely, just as you said. The issue is abiogenesis. We have absolutely zero
data on how often abiogenesis occurs in nature. We don't actively observe it
occurring on earth and have yet to encounter it elsewhere even once.
Therefore, no matter how common a "habitable" planet is, without living things
to terraform its surface and atmosphere, it's not actually habitable. It needs
ecosystems to sustain human beings. And we have no way of knowing how likely
or unlikely it is for life to exist.

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agentgt
There was a Scientific America article that discussed something similar but
the major point of the article was that there will more likely be habitable
planets in the future. That is the universe is still in its early days of
habitable planets/zones.

I read the article casually before going to bed and can't recall that many of
the details.

I'll try to find the article and append a link (I believe the article even
referenced the same study).

EDIT I guess it was the smithsonian but I swear it was SA:
[http://www.smithsonianmag.com/science-nature/why-universe-
be...](http://www.smithsonianmag.com/science-nature/why-universe-becoming-
more-habitable-180959972/)

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Rhapso
The whole "colonize planets" won't pan anyway. By the time it is viable a
O'Neill cylinder will be a better place to live and we can put them in ideal
orbits.

~~~
fcsuper
Right now, this is more about searching for other life.

