These pages guide you through a modified version of the Drake Equation. The formula provides a crude estimate of the number of technological civilizations in the Visible Universe. It does this by first multiplying together a series of numbers that estimate the quantity of habitable planets in the Universe and then multiplying this by a series of probabilities concerning the history of life on those planets.

We have included several simple exercises at the end to help you appreciate what your estimate implies about how close the nearest alien civilization may be, how common life itself may be, and how many technological civilizations have come and gone through the 13.8 billion years of cosmic history.

For more information about the original version of the Drake Equation and its history, use the "More About" button below. If you would like to read a personal statement by the person who designed this modified Drake Formula, use the "Statement" button below.

Astronomy buffs and professionals will notice that we make several crude approximations to keep the formula simple. We hope we will be forgiven for glossing over some complexities for the sake of obtaining a ballpark estimate.

We also apologize for software limitations on the way numbers are represented and displayed. We were unable to use sensible scientific notation throughout.

Use the Calculations button in the right corner above to start your Drake Equation Calculations.

The Universe is 13.8 billion years old, because the Big Bang occurred 13.8 billion years ago. It is not possible for us
to receive light from any part of the Universe more than 13.8 billion light years away,
because there has not been sufficient time for light to reach us from further away. This
distance is called our "Cosmic Horizon." As a result, the part of the Universe observable
by our telescopes, which we call the "Visible" Universe, is large but finite and contains a
finite number of galaxies. Deep imaging with the Hubble Space Telescope suggests that the visible universe contains about 10^{12} galaxies.

Enter for N^{0}.

x 10

=

1

Then move to the next variable.

Our own Milky Way galaxy is fairly typical in terms of both diameter and number of stars. So enter here the approximate
number of stars in the Milky Way. When you multiply N^{11} stars.

Enter for N

x 10

=

1

Then move to the next variable.

Not all stars are likely to be suitable for supporting life. Some stars emit too much ultraviolet light; others do not
last long enough. Some stars are binary or multiple stars in which planetary orbits at
comfortable distances from the stars are not stable. Exotic types of stars, like neutron
stars, are usually formed by violent processes that would destroy pre-existing planets.
So, many stars are unlikely to be good hosts for planets or moons capable of
sustaining life. However, if a planet's internal heat at the bottom of a deep ocean
can support life, it is possible that the nature of the star does not matter too much.
Considering all these factors and others you may know about, what fraction of the
stars in the Visible Universe do you think are suitable hosts for "habitable" bodies, that
is, planets or moons on which life could exist. (Note: f

Enter for f

=

1

Then move to the next variable.

By multiplying out N

Enter for n

=

1

Then move to the next variable.

Conventional thinking on the origin of life supposes that this happens through a chemical
evolution from pre-biotic materials to living organisms. However, it is also possible that life
can be seeded from one body to another. When you multiply N

Enter for f

x 10

=

1

Then move to the next variable.

N

Enter for f

x 10

=

1

Then move to the next variable.

Dogs are intelligent and can manipulate objects
in creative ways at times, but they do not build cities or grow crops. Whales are
probably more intelligent than dogs and may even have a culture of sorts, but they are
also not technological. So, on what fraction of the worlds that develop intelligent life
forms does at least one of them develop a technological civilization? By multiplying
N

Enter for f

x 10

=

1

Then move to the next variable.

Technological civilizations, like everything else in the Universe, probably have a
finite lifetime.
Estimate how long you think the average technological civilizations might last.
A thousand years? Ten thousand years? A million? A billion?
Our Sun is about half way through its life cycle. If you think our technological
civilization will survive as long as the Sun survives, then it will have a total life time
roughly half the life of the Sun, or L = 5 x 10^{9} years).

It might help to decide what you mean by a technological civilization. If you define societies with cities and agriculture as technological, then humans have had a technological civilization for about 10,000 years. For instance, if you think our civilization is around 10,000 years old, but about to destroy itself, then L = 10,000 years. This is crude, but we are after an estimate here, not a precise number.

Enter for L your estimate for the lifetime of a technological civilization in years, including both a number and a power of ten.

x 10

1

You can go back to any variable or on to the final calculation.

How many technical civilizations now in the visible universe?

How many technical civilizations per galaxy now?

What is the average distance to the nearest civilization in light years?

How many technical civilizations have ever existed in the current volume of the visible universe? Learn more

The number of active technological civilizations in the Visible Universe depends on how long you think they last, on average. Most likely, alien civilizations come and go. We could ask, in all of cosmic history up until now, about how many technological civilizations have ever existed in the volume of our current Visible Universe? We obtain that number by NOT considering the finite lifetime of intelligent civilizations.

The approximate number of technological civilizations that have appeared throughout the current volume of the Visible Universe in all the 13.7 billion years of cosmic history.

You can go back to any variable and change your entry.

How many technical civilizations have ever existed in the Milky Way? Learn more

In our Milky Way galaxy alone, the number of technological civilizations that have appeared so far.

How many habitable bodies exist in the visible universe? Learn more

The number of habitable planets and moons in the Visible Universe depends only on multiplying together N

How many habitable bodies developed simple life in the visible universe? Learn more

Of these habitable bodies, the number of worlds that have had at least a simple form of life depends on multiplying together N

How many habitable bodies exist in the Milky Way? Learn more

Numbers within our own Milky Way may be easier to grasp. We get estimates for these through not multiplying by N

How many habitable bodies have simple life in the Milky Way? Learn more

The number of worlds in the Milky Way that have had at least a simple form of life depends on multiplying together N

How many habitable bodies in the visible universe ever developed intelligent life? Learn more

The number of habitable planets and moons in the Visible Universe on which life progressed from simple to "intelligent" forms (cows, wallabies, dogs, dolphins, apes...you decide) depends on multiplying together only N

How many habitable bodies in the Milky Way ever developed intelligent life? Learn more

If you find numbers within our own Milky Way galaxy easier to grasp, then do not multiply by NG. The numbers of planets or moons in the Milky Way that have ever had intelligent life depends only on multiplying together N