Originally Posted by Antarctica!

The students ended up having a few questions after all. "To your first question... Yes, there are a lot of boxes that need to be ticked before a planet qualifies as hospitable, you are correct. In this case, though, all those details are included in the phrase 'hospitable planet'. As far as the equation goes, a planet needs to be in the right place, have the right size, have an atmosphere, and so forth," he waved a hand, "to be considered hospitable."

He tapped a slender finger against the relevant line on the blackboard. "If it's, say, too far away from the parent star, it's not considered hospitable and doesn't appear in this factor of the equation. As for your second question... It is generally assumed that it does, yes. Life begins simple and then, in the correct circumstances, evolves more complexity. Actually," he added, "bacteria can be fairly complex already, they're not the first forms of life on which everything else is built, but for shorthand, we'll refer to them. If you're alright with that?" He gave her a questioning look, smiling slightly.

Originally Posted by Antarctica!

"But!" He straightened up. "We will now tackle the equation itself. Here it is." He tapped the blackboard once more.

N = R* x fp x ne x fl x fi x fc x L

"First of all, the obvious. N is the end result, the number of alien civilisations with which we might be able to communicate." He tapped at the letter with a finger. "The other letters are more or less those factors we already discussed earlier." His hand slid lower to indicate the list the students and he had put together. "R* is the rate of star formation. And then, one after the other, stars with planets, planets that can support life, planets that actually have life, planets with intelligent life, that's fi, fc is the intelligent life that sends out signals into space, and finally, L is the length of time an alien civilisation actually sending signals."

He paused. This sounded very confusing, but, with the equation on the blackboard, hopefully not impossible to understand. "There is one more subtlety here that I need to talk about. These values are all built upon each other. We sort of neglected this in our discussion, but the original Drake equation did not, which complicates things. See the little f signs? This means we are talking about fractions. For example, if you take the first fractioned value, fp, then this is the fraction of all stars that have planets, not the number of stars with planets itself."

He paused again. "And so, further on, fl isn't just the number of planets with life, it's the fraction of all planets, and fi is not the sum of all intelligent life in the galaxy but the fraction of planets with any kind of life in them. This makes figuring out the values even more difficult since, as I said, they all rely on each other."

Originally Posted by Antarctica!

"You're very welcome," he said with a smile. "Erm, before we continue, let me just complete our list on the blackboard..." He turned around and tapped the blackboard several times, brow furrowed in concentration. "First, we have the list of parameters for our very own equation... That looks about right." Risu stepped back.

Calculating the number of extraterrestrial civilisations we can communicate with

• Rate of star formation
• Number of stars with planets
• Number of hospitable planets
• Number of hospitable planets that develop any kind of life
• Number of planets with life where the life becomes advanced enough to send out signals
• Length of time the advanced life spends on sending signals

He nodded to himself. "Yep. And now... The Drake equation..." Writing appeared on the right side of the blackboard, each line corresponding to the list on the left as closely as possible.

The Drake equation; N = number of extraterrestrial civilisations we can communicate with
N = R* x fp x ne x fl x fi x fc x L

• R* = rate of star formation
• fp = fraction of those stars with planets
• ne = average number of hospitable planets per star with planets
• fl = fraction of those hospitable planets that actually have life
• fi = fraction of those hospitable planets with life that develop intelligent life
• fc = fraction of intelligent life that sends signals into space
• L = length of time this intelligent life continues sending signals

Risu put his wand down on the desk and stepped aside so that everyone had a clear view of the blackboard. "That should be right," he announced, wiping his hands on his robes, where they left grubby lines of chalk.


"Well, that's the question!" Risu said, leaning against the wall next to the blackboard. "And it's a question I pose to you and everyone else in this class. With the specifics of the Drake equation figured out, I would like to ask you to discuss the value of it. As I commented earlier, the equation was first proposed as a way of making the search for alien life scientific. In your opinion, does this equation achieve this?

"Further, if you disagree, why? We've already mentioned a number of criticisms of the equation. The specific values for those factors are practically impossible to figure out accurately. The equation assumes that life on other planets is similar to life on Earth, and so on, needing water, an atmosphere... Are there any other reasons why the equation doesn't seem convincing to you?

"And, on the other hand, are there arguments against those criticisms? For example, isn't it a good idea to search for the kind of life we are familiar with first, since it gives us a possible starting point? Yes? No?" He shrugged. "What do you think?"


"And while you do that, I'll try to answer any remaining questions. Yes. What can I help you with?"