Starring Intelligent Aliens

November 5, 2009 by Clara Moskowitz,
Distributions of mass and orbit size for the extrasolar planets so far discovered. The habitable zone is marked in green. Credit: NASA

The most probable place to find intelligent life in the galaxy is around stars very similar to our sun, a new study has found.

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 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 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 - 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 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 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.

Source: by Clara Moskowitz,

Explore further: Researchers Say Tides Can Cut Life Short On Planets Orbiting Smaller Stars

Related Stories

Most Milky Way Stars Are Single

January 30, 2006

Common wisdom among astronomers holds that most star systems in the Milky Way are multiple, consisting of two or more stars in orbit around each other. Common wisdom is wrong. A new study by Charles Lada of the Harvard-Smithsonian ...

Kepler Set to Launch Tonight on Planet Finding Mission

March 6, 2009

( -- The Kepler spacecraft and its Delta II rocket are "go" for a launch tonight that is expected to light up the sky along Florida's Space Coast at 10:49 p.m. EST as the rocket lifts off from Launch Complex 17-B ...

Twinkle, twinkle, any star - Sun not so special

May 21, 2008

ANU astronomers have found there is nothing special about the Sun after conducting the most comprehensive comparison of it with other stars – adding weight to the idea that life could be common in the universe.

Planetary systems can form around binary stars

January 10, 2006

New theoretical work shows that gas-giant planet formation can occur around binary stars in much the same way that it occurs around single stars like the Sun. The work is presented today by Dr. Alan Boss of the Carnegie Institution’s ...

Recommended for you

'X'-ploring the Eagle Nebula and 'Pillars of Creation'

July 13, 2018

The Eagle Nebula, also known as Messier 16, contains the young star cluster NGC 6611. It also the site of the spectacular star-forming region known as the Pillars of Creation, which is located in the southern portion of the ...

Observatories team up to reveal rare double asteroid

July 13, 2018

New observations by three of the world's largest radio telescopes have revealed that an asteroid discovered last year is actually two objects, each about 3,000 feet (900 meters) in size, orbiting each other.

First space tourist flights could come in 2019

July 13, 2018

The two companies leading the pack in the pursuit of space tourism say they are just months away from their first out-of-this-world passenger flights—though neither has set a firm date.


Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (1) Nov 05, 2009
The problem here is the definition of "intelligence". If we (us humans on this earth) are any indication of intelligence, then I hope we do not find any stars with the "right balance". So far, I am glad most stars do not have the "right balance". Carter's quote "intelligent life requires at least a few billion years to evolve". This sounds very scary if we call this (intelligence) is "evolving"!
5 / 5 (1) Nov 06, 2009
What this really tells us is the chances of finding life that is similar to our own. Who knows what might really be out there. On our own planet we've found extremophile bacteria that can live in sulfuric acid, extreme cold, extreme heat, total darkness, and/or crushing pressure. We really have no clue what there is out there, and how and where it might live. Just imagine what amazing things we will find.
5 / 5 (1) Nov 06, 2009
Models based on a single statistical point are crap as any statistician will tell you.

We don't know what forms life can take other than ours - so any prediction on where life (and with that intelligent life) may be found are pure speculation.

What you can say is that suns and planets like ours can support life like ours. Duh.
(We don't even know if suns/planets like ours are optimal for our kinds of life. Might be that we are a fluke at the bare tolerability limit for some parameter(s). Again: a statistic of one says NOTHING about a process other than: Yes, under these circustances the proces is possible)
not rated yet Nov 06, 2009
Hey danman5000,
You have to check out 'Camouflage' by Joe Haldeman. It has an alien being that evolved in VERY different conditions crash landing on earth and continuing its evolution here. It is right on point with what you're bringing up. And quite frankly it's fascinating.
not rated yet Nov 10, 2009
The problem here is the definition of "intelligence". If we (us humans on this earth) are any indication of intelligence, then I hope we do not find any stars with the "right balance". So far, I am glad most stars do not have the "right balance". Carter's quote "intelligent life requires at least a few billion years to evolve". This sounds very scary if we call this (intelligence) is "evolving"!

I think the fact that humans, who originally came from a mess of complex molecules, have eventually become what we are today - a society with each person being an individual that is self aware and capable of making decisions in how they choose to live out their life cycle and being able to determine the results of those actions as well as being able to develop tools that allow them to essentially see the early universe is certainly "intelligent". Are we perfect? No. Am I misunderstanding your post? Maybe.
not rated yet Nov 15, 2009
Mr. Man, your statement is what the author is describing. But, just because we have "...self aware and capable of making decisions..." does not make us intelligent. We have only evolved with these qualities and have gone the opposite direction. Claiming we are intelligent is no different than saying the earth is the center of the universe (history, remember?). Besides, it is impossible that we are intelligent. Look around ourselves, it is very obvious. Any intelligent life discovering human would destroy us out of existence. Perhaps, we are the species of virus in this universe! Let's not push our luck to think we are the all mighty evolution. So, we may find evolving life but I doubt very much intelligent life is our capacity.
not rated yet Dec 09, 2009
I think Milou will eventually change his mind.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.