Communion Of Dreams


30 billion Earths.

How many Earth-like planets are there in our galaxy? Ones which are reasonably like ours, in terms of size, density, and location relative to their sun’s ‘habitable zone’? That’s one of the basic components of the Drake Equation, and until fairly recently all estimates were little more than speculation.

Enter Kepler:

Expanding our view from Kepler’s corner of the galaxy to show more of the Milky Way, the sky fills with billions of potentially life-bearing worlds. If we showed them all, the sky would be a mass of green. So now the green dots illustrate stars that might host such planets, visible with a good pair of binoculars on a dark night here on Earth.

From this perspective, the chances that we’re alone in the cosmos seem very slim, indeed.

The final answer? 15 – 30 billion Earth-like planets.

Of course, that doesn’t include the rest of the Drake equation values. Such as: what percentage of planets which could potentially develop life actually do so? Then what percentage of those planets develop intelligent, technologically sophisticated life? Then what percentage of such intelligent species develop interstellar communication capabilities? Then how long will such a civilization survive, out of the billions of years of history?

The last time I played with the numbers, the best guess from Phil Plait was that there were some 2.5 billion potentially habitable planets. Kepler indicates that number was too conservative, by something on the order of a factor of 10. Running the rest of the equation is largely just an experiment in gut feelings (since we don’t yet have any real data), but what is impressive is that at each stage as solid data has become available, we’ve had to adjust our “best guesstimate” numbers *upwards*. Meaning that the the total number of technologically sophisticated civilizations capable of interstellar communications out there at this point in time also goes up.

From Chapter 4 of Communion of Dreams:

“But in any event, as Arthur Bailey said this morning ‘where are they?’ Where are the aliens? That’s what’s bothering me.”

Where, indeed? I came up with my own answer, explained in Communion.

But I wonder what the real answer will be.

Jim Downey

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