MEMO COPY in re News -- Stars May be Cosmic Road Signs to Intelligent Aliens.
Kent Paul Dolan
John Harshman wrote:
> Ye Old One wrote:
>> Stars May be Cosmic Road Signs to Intelligent Aliens
>> By Clara Moskowitz
>> Astrobiology Magazine
>> posted: 05 November 2009
>> 08:03 am ET
>> http://www.space.com/scienceastronomy/091105-am-intelligent-aliens.html
or
http://preview.tinyurl.com/yag7ytk
>> When scientists search the heavens for habitable
>> worlds beyond Earth, they don't necessarily know
>> what to look for. A new study has found that the
>> most probable place to find intelligent life in
>> the galaxy is around stars with roughly the mass
>> of the sun, and surface temperatures between
>> 5,300 and 6,000 Kelvin (9,100 and 10,300 degrees
>> Fahrenheit) - in fact, stars very similar to our
>> own sun. Learning that sun-like stars are good
>> candidates for life may not sound surprising, but
>> it isn't always what scientists have thought.
>> "The principle of mediocrity says that, barring
>> any evidence to the contrary, our observations
>> should be typical among those of all intelligent
>> observers," said researcher Daniel Whitmire, a
>> physicist at the University of Louisiana at
>> Lafayette. "But the typical star is not like the
>> sun - the typical star is a low mass star. We
>> don't find ourselves around a typical star and we
>> show the reason why in this paper. Our results
>> confirm the principle of mediocrity as applied to
>> the sun."
>> Sun-like stars are actually a minority in the
>> galaxy - 93 percent of stars in the Milky Way are
>> less massive, less luminous and cooler than the
>> sun. Though the typical star in the galaxy weighs
>> between one-tenth and half the mass of the sun,
>> life is more likely to be found around the more
>> unusual variety of stars like our own, the
>> researchers found.
>> To make their calculation, Whitmire and colleague
>> John Matese combined models of how planets form
>> with data on the distribution of stars in the
>> galaxy as a function of mass. The planet models
>> show when worlds are most likely to form in the
>> habitable zone - a Goldilocks region around a
>> star in which a planet would be just right for
>> life - not too close that its surface would be
>> boiling, and not too far that it would be frigid
>> either. Planets in the habitable zone are the
>> best candidates for having liquid water, which is
>> thought to be a prerequisite of life.
>> In general, the planet-formation theories predict
>> that more massive stars are the most likely to
>> have planets in the habitable zone. So the larger
>> a parent star is, the more likely its planets
>> will have environments conducive to life.
>> But this advantage of larger stars is
>> counteracted by the fact that more massive stars
>> are less abundant - there are fewer big stars out
>> there. In addition, the more massive a star is,
>> the shorter its lifetime. That makes it hard to
>> find very massive stars that have lived long
>> enough for complex life to develop.
>> The researchers weighed these factors against
>> each other to calculate the distribution of stars
>> most likely to host thinking, living creatures.
>> "It's a tradeoff between the numbers of stars out
>> there and the probability of habitable planet
>> formation increasing with mass." Whitmire said.
>> "We show it's no accident we find ourselves
>> around a star like the sun." The distinction
>> between habitable planets and planets harboring
>> intelligent life is based on the fact that
>> intelligent life requires stars with lifetimes
>> greater than the time required for intelligence
>> to evolve. For example, in the case of this solar
>> system, we could not find ourselves around a star
>> with a lifetime less than 4.5 billion years.
>> Indeed, sun-like stars seem to have the right
>> balance: They are of high enough mass that they
>> are more likely to host habitable planets, but
>> they are of low enough mass that they live long
>> enough for intelligent life to develop, and are
>> not extremely scarce. Whitmire estimates that 10
>> percent of the Milky Way's stars might fall into
>> the category they've outlined. This would still
>> leave over 10 billion candidate stars in the
>> Milky Way alone.
> Why was this surprising enough to be publishable?
>> Learning that sun-like stars are good
>> candidates for life may not sound surprising,
>> but it isn't always what scientists have
>> thought.
Because it counters other arguments that stars
capable of supporting stellar-habitable-zone
earth-like planets are relatively scarce.
Previous thinking was that the best candidate stars
were ones considerably smaller (and thus longer
lived) than our sun, because the limiting factor for
intelligent life arising seemed to be the sun
that irradiates its planet surviving long enough for
that evolution to run its course.
We've been finding extra-solar planets at all for
only a brief time, (a couple of decades, maybe) and
we've found as yet no extra-solar planets
sufficiently like earth in mass and distance from
their stars, to support life, at all.
More recent findings, that "planetary systems at
all" per star are probably common, combined with the
current finding that stars where having planetary
systems that could support life also work from the
viewpoint of the stellar characteristics, in
particular lifetime of the star plus no intefering
stellar companion plus stability of the habitable
zone, are plentiful, counters one part of the Carter
argument.
>> The results mitigate the most commonly used
>> argument that intelligent life must be extremely
>> rare, Whitmire said. This idea, based on the
>> anthropic principle, was outlined by
>> astrophysicist Brandon Carter. There is an
>> approximate coincidence between the time it took
>> intelligence to evolve on Earth and the lifetime
>> of the sun. Assuming these two timescales are
>> independent, this coincidence makes sense if
>> intelligent life is extremely improbable, Carter
>> argued. In most cases, he claimed, the time it
>> takes for intelligent life to emerge is much
>> longer than the portion of a star's existence
>> that is conducive to such life.
>> "In the paper we explain one number in the
>> coincidence -- why the lifetime of the sun is
>> what it is," Whitmire said. "The additional
>> assumption necessary to counter the Carter
>> argument is that intelligent life requires at
>> least a few billion years to evolve, as expected
>> if we are typical."
> Can someone explain why this counters the Carter
> argument and makes intelligent life common?
For a gloss of the Carter argument, see for example
section 3 of:
http://saj.matf.bg.ac.yu/175/pdf/045-050.pdf
Carter's argument, which breaks down to "why
_coincident_ intelligent life forms on stars within
communication range are scarce (why SETI isn't
succeeding) depends both on the times when
intelligent life can appear on planets of a galaxy
being independent and uncorrelated, and on the
number of candidate locations within the galactic
habitable zone where it can appear being relatively
scarce.
The "coincidence" mentioned is that the time to
develop intelligent life on earth equates within a
factor of two with the expected lifetime of our sun.
Considering that stellar lifetimes can range from
tens of millions of years to hundreds of billions of
years, that "mere factor of two discrepency" is
indeed a pretty astonishing concidence.
This current result reduces the "relatively scarce"
and "unlikely coincidence" parts of Carter's
argument, the one in the paper I referenced reduces
the "independent and uncorrelated" part of Carter's
argument.
The warning that Eta Carinae is a ticking time
bomb waiting to sterilize at least one
hemisphere of earth is a rather sobering one.
[This combination of debunkings of Carter's argument
seems to throw us right back into the Fermi paradox:
http://www.space.com/searchforlife/shostak_paradox_011024.html
where the heck are the other forms of intelligent
life we should be expecting to find with, say,
SETI, or that should already have found us?]
>> The study is detailed in the September 2009 issue
>> of the Astrobiology Journal.
FWIW
xanthian.