George's Cosmic Treasure Hunt by Lucy Hawking

it?” said Emmett, standing up and looking into the tree. “And anyway, there are no aliens. We have no proof of another intelligent life-form in the Universe at this moment. We can only calculate the probability that on some other planets, there are conditions suitable for forms of extremophile bacteria. Which would have the approximate IQ level of Annie herself. Or probably a bit more. I can calculate the probability of intelligent life for you, if you like, using the Drake Equation.’
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    THE DRAKE EQUATION
    The Drake Equation isn’t really an equation. It’s a series of questions that help us to work out how many intelligent civilizations with the ability to communicate there might be in our Galaxy. It was formulated in 1961 by Dr. Frank Drake of the SETI Institute, and is still used by scientists today.
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    This is the Drake Equation:
    N = N* x f p x n e x f l x f i x f c x L
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    N* represents the number of new stars born each year in the Milky Way Galaxy
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Question:
What is the birthrate of stars in the Milky Way Galaxy?
Answer :
Our Galaxy is about twelve billion years old, and contains roughly three hundred billion stars. So, on average, stars are born at a rate of three hundred billion divided by twelve billion, equaling twenty-five stars per year.
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    f p is the fraction of those stars that have planets around them
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Question:
What percentage of stars have planetary systems?
Answer:
Current estimates range from 20% to 70%.
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    n e is the number of planets per star that are capable of sustaining life
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Question:
For each star that does have a planetary system, how many planets are capable of sustaining life?
Answer:
Current estimates range from 0.5 to 5.
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    f l is the fraction of planets in ne where life evolves
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Question:
On what percentage of the planets that are capable of sustaining life does life actually evolve?
Answer:
Current estimates range from 100% (where life can evolve, it will) down to close to 0%.
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    f i is the fraction of habitable planets with life where intelligent life evolves
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Question:
On the planets where life does evolve, what percentage evolves intelligent life?
Answer:
Estimates range from 100% (intelligence has such a survival advantage that it will certainly evolve) down to near 0%.
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    f c is the fraction of planets with intelligent life capable of interstellar communication
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Question:
What percentage of intelligent races have the means and the desire to communicate?
Answer:
10% to 20%.
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    L is the average number of years that a communicating civilization continues to communicate
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Question:
How long do communicating civilizations last?
Answer:
This is the toughest of the questions. If we take Earth as an example, we’ve been communicating with radio waves for less than one hundred years. How long will our civilization continue to communicate with this method? Could we destroy ourselves in a few years, or will we overcome our problems and survive for ten thousand years or more?
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    When all of these variables are multiplied together, we come up with:
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    N , the number of communicating civilizations in the galaxy.

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    â€œWell, thanks for that, Professor Emmett,” said Annie. “Your Nobel Prize is in the mail. So now why don’t you bacteria off yourself? Go find some of your own species to hang out with? Actually, George, there are aliens on Earth, and Emmett is one of them.”
    â€œNo, no, rewind,” said George urgently. “You’ve had a message from some aliens? Where? How? What did it say?”
    â€œThey sent her a text message to say they would be beaming her up to the mother ship at twenty-one hundred hours,” said Emmett.