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.
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."
The study is detailed in the September 2009 issue of the Astrobiology
Journal.
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