by: Rick Johnson
PO Box 40451
Tucson, Az.

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In 1961 a bunch of scientists tried to calculate the chances of finding intelligent life in the Milky Way Galaxy. They came up with the Greenbank Formula which heavily influenced Gene Roddenberry as he created the classic television series Star Trek.
The formulae in its simplest form may be found here but it goes something like this:
N = R*fp ne fl fi fc L where:
  1. N = number of civilizations in galaxy capable of interstellar communication.
  2. R* = average rate at which stars develop during lifetime of galaxy.
  3. fp = fraction of stars with planets.
  4. ne = average number of planets, per star, capable of supporting life.
  5. fl = fraction of planets with life.
  6. fi = fraction of planets with intelligent life.
  7. fc = fraction of planets with intelligent life capable of interstellar communication.
  8. L = average lifetime of society capable of interstellar communication.

The big problem with the Greenbank Formula is that so many of the variables are totally guestimates. We cannot even be certain of the number of stars in the Milky Way Galaxy so how can we be certain how many of these will have a star-faring society like the Vulcans? The estimates of intelligent life ranged from 1,000 to 1,000,000 with estimates of a star-faring world every 100-500 light years.

What I have done here is to do some very rough math of my own to support my fan-fic. I am less concerned with space-faring civilizations than repeating the random musings of a main character who explains why the Commonwealth hasn’t bothered to return to the Earth in the last thousand years.
So the math is crude and guestimated and so can be changed easily.


The Milky Way Galaxy is a spiral galaxy consisting of two arms tightly coiled around a core. Surrounding this disc is a halo of sparse stars and orbiting within and without this halo are multiple satellite galaxies.
There are:
100,000,000,000 (100 billion) stars in the disc
100,000,000,000 stars in the halo
100,000,000,000 stars in satellites = 300,000,000,000 stars in Milky Way Galaxy

# stars in disc = 100,000,000,000
50% are G-type = 50,000,000,000
*10% have stony iron planets in the terran ecosphere = 5,000,000,000
10% are Class-M = 500,000,000
**50% are left-hand molecules = 250,000,000
10% have proteins we can consume = 25,000,000

Therefore: there are a potential 25 million Class-M worlds in the disc of the Milky Way
Galaxy that can support Human life.
Triple that to include the halo and satellites.

The total area of the Disc alone is: A=pi (rxr)(h) or the area of a disc is equal to pi times the radius squared times the thickness of the disc.
D = 100,000 c by 10,000 c thick
R = 50,000 c
R squared = 2,500,000,000 c
Pi x (rxr) = 7,850,000,000 c
A = 7,850,000,000,000 c

Therefore: there is a Class-M world every 314,000 cubic light years (c). Or one every 68 light years.

There are 10,000 light years between Earth and Gaea = 147 Class-M worlds between Earth and Gaea.
if the trip takes one year, a 365 day trip = you meet one Class-M every 2.4 days.
10,000 c divided by 365 = 27.4 c/day = a velocity of 1.1 c/hr

A Morgor ship from Jupiter in the Burroughs Universe (the Florina from the Spy series) travels 19,000,000 miles per day based on the estimate from when John Carter was abducted by the Morgers in Skeleton Men of Jupiter.
79,166,666 miles per hr
13,194 miles per min
220 miles per second
Which means you can go from Earth to:

  1. Moon = 18 minutes
  2. Venus = 1 day to 9 days
  3. Mars = 2 day to 14days
  4. Jupiter = 20 days to 30 days
  5. Edge of solar system: = 260 days


Note: Stars are classed by letters based on their position n the HR Diagram.
Planets are classed by letters based on their size and environment.
Sol is a G-type star. Earth is a Class-M planet. Amtor and Barsoom are borderline Class-M. If we can live on it, it is Class-M. If it has life similar to Earth, it is an Earth-clone. Star Trek was running across dozens of Earth-Clones like the world of the Roman Empire.
Most worlds like to Terra-Form worlds into Class-M for future colonization. This messes with the stats but allows more planets to colonize. Drakonis terra-forms at least one world in every star system for that reason. The Demons terra-formed thousands or millions of worlds then forgot they existed. Someday they will return to reclaim their property and, hopefully, give the human colonists a chance to move away. If not they will simply retake their property which includes any colonists on their world.

*note: The chemicals that create life can exist only in a specified temperature and environment. This is called the “Ecosphere of a star”. For a G-Type star like Sol, the Ecosphere is between 80-110 million miles. Earth being at 93 million miles. Any closer and it is too hot to create and support Terran life. Any father and it is too cold to allow the molecules to form.
But once created, these living organisms can be transported to a similar world outside our ecosphere and take hold. This is a problem with Mars where the Russians did not sterilize their Martian Probes and so may have introduced Terran bacteria to Mars. Most of these will die but some will survive and adapt to colonize the Red Planet.
Carbon-based life lives on a stony-iron world like Earth. Silicon life exists in an ammonia-methane atmosphere like Jupiter or underground on Earth (yes, we have silicon life-forms in the caves of Earth). Hydrogen-life exists in space, Helium life exists on a world like Pluto… you get the idea.

**note: Every molecule exists in a right-hand or left-hand form. In some molecules like water, the left-hand is the same as the right-hand version. In more complex molecules like the basic proteans of live, the two forms are different. Earth originally had both left and right-hand versions but the left-hand molecules ate all the right-hand molecules so we are a world of left-hand molecules.
We cannot eat right-hand molecules. They are not poison, they just don’t match so it’s like eating dirt. Fills your stomach but provides no nutrition.

To contact me or to request topics to be covered, send to RikJohnson@juno.com
by: Rick Johnson
PO Box 40451
Tucson, Az.

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