Are we alone?
Astronomers have pondered this question for years. Many more have wondered this more recently after seeing movies like Close Encounters of the Third Kind, E.T. the Extra Terrestrial, Independence Day, and most recently, Contact (Contact being the most realistic of all alien encounter movies, in my opinion). What does it take for life to develop at all?
First, you would need a life-site, i.e., a planet.
Second, you would need time for evolution.
Third, you would need favorable temperatures which include circular stable orbits; a stable star (one that is on the main sequence); a stable atmosphere (a runaway greenhouse effect like that found on Venus is no good); and an orbit in the "ecosphere."
Fourth, you would need shielding from ultraviolet radiation, primary cosmic rays, and micro meteoroids.
Lastly, the chemistry has to be there. Carbon, Hydrogen, Oxygen, Nitrogen, and others, but Hydrogen is the key.
But is there really life elsewhere in our galaxy? If so, are they trying to communicate? Many of these questions can be answered by plugging in numbers in the famous Drake equation:
where N is the number of communicating civilizations in our galaxy right now (the answer we are trying to obtain), N* is the number of stars in the galaxy we are studying, of these fs is the percentage of solar type stars, of these Np is the average number of planets per solar system, of these fe is the number of planets in the "ecosphere", of these fl is the percentage of these where life develops, of these fi is the percentage where intelligent life develops, and L is the length of time the civilization communicates divided by Lg which is the lifetime of the galaxy.
The Numbers
N*
We know there are about 300 billion stars in our galaxy.
fs
We also know that about 33% of these stars are solar type stars, that is, like ours. What this means is several things: First, the star isn't part of a system. If a star has one or more stars in the system, the orbits of the planets would be too unstable to support life. Also, if the star too massive, it will have too short a life span to allow life to develop.
Np
The number for average number of planets gets to be quite tricky. Until recently, there have been no discovered planets outside our solar system. This is for two reasons. First, planets are among the dimmest objects in space. Second, they are really close to extremely bright objects (their respective star). So until we can develop telescopes that can see these ultra dim objects, we have to resort to other means. For example, we can detect the wobble of stars caused by the mass of planets orbiting around them. And if we happen to be facing the plane of the planets orbit head on, then we can detect the loss of light from the star as the planet passes in front of it. So what does this all mean for the Drake equation? Your guess is as good as anyone's. Is our solar system typical of most, averaging 9 or 10 planets? We might be an exception, maybe the average star has only 2 or 3 planets, or maybe 30 to 40? There really is no way to know right now. The last I heard, we have confirmed about 9 planets outside our solar system (not around the same star, though).
fe
The number of planets in the so-called "ecosphere" is a little more stable, but not much more. It can be refined better after the average number of planets is known better. The ecosphere is simply the area around a star where water can exist as a liquid. Figuring out this area is just a simple method of plugging in numbers for some scary looking math. There is a formula for finding the average temperature for a planet a given distance away form a star with a given size and a given surface temperature. That equation is:
Where r is the distance of the planet from the star, R* is the radius of the star we are studying, T* is the surface temperature of the star we are studying, and Tp is the average temperature of the planet we are studying. All distances are in solar radii (our sun's = 1), and temperatures are in Kelvin.
To find the ecosphere of a given star, we solve the equation once with a planet that has an average temperature of 273 degrees K (0C, 32F; i.e. freezing point of water), and solve it again using a planet with an average temperature of 373 degrees K (100C, 212F, i.e. boiling point of water). With this we have our ecosphere. If we use this equation on our own sun, we get an ecosphere that looks like this:
The gray area is the ecosphere. As you can see, it extends from just inside the orbit of Venus to just inside the orbit of Mars. Yet, there is no life on Venus. So is the equation wrong? Not really. There are many more variables that determine the temperature of a planet than just its distance for its sun. Taking in all the additional variables possible, we end up with a picture that looks more like this:
The ecosphere for our solar system is just 4% the width of our distance from the sun. Lucky for us!
Let's say we have a star which has a radius of 10 (ten times the size of our sun) and a surface temperature of 30,000 degrees Kelvin. What would the size of its ecosphere be? We just solve the equation twice, as is here:
So the ecosphere for this star begins 150 A.U's from the star, and ends 281 A.U.'s from the star (1 A.U., or Astronomical Unit, is the average distance the Earth is from the sun).
fl
Another mystery. If we are the only planet in our sun's ecosphere, then is life going to pop up 100% of the time? There is no way to know, yet. We might be the norm, or the exception.
fi
Again, we don't know. Our planet let us evolve into intelligent beings, capable of communicating with the outside universe. But is this always the case?
L/Lg
And how long do the aliens try to communicate? We've only been leaking messages to outer space for about 50 years, and listening for even less. But the lifetime of the galaxy is 10 to the 10th power years (100,000,000,000 years).
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The "ecosphere" images on this page are from the Macintosh astronomy software Voyager II, version 1.00. Copyright Carina Software, 1992 - 1993. Touched up by Adam Sena, 1997.