Drake’s Equation

Ken Tapping, August 21 2013

In the sky this week…

  • Venus lies low in the west after sunset.
  • Saturn lies in the Southwest in the evening.
  • Jupiter rises in the early hours. Mars lies in the dawn twilight.
  • The Moon will be Full on the 20th.

In the late 1950's scientists concluded that if a then state-of-the-art radio transmitter were located on a planet orbiting a nearby star, it would be possible to pick up its signals here on Earth, using a similarly up-to-date radio telescope. Then, in the early 1960's, radio astronomer Frank Drake tried it. He used a 25-m diameter radio telescope located at Green Bank, West Virginia, and pointed it at two nearby Sun-like stars: Epsilon Eridani and Tau Ceti. Although not successful, this project paved the way for an ongoing search for signals from other intelligent species out there in the universe. Today's technology is millions of times more capable than the equipment used by Frank Drake, and given the current rate of progress, if there are radio signals from alien races coming in from space, within the next decade we will detect them.

Frank Drake is famous for coming up with the “Drake Equation”, which is a formula for estimating how many worlds out there might have beings transmitting radio signals in our direction. It is more than 50 years since Drake did his calculation. Over that time our knowledge has improved. We know more about how stars work and we now know that most stars have planets, whereas in the 1960's we were only guessing whether other stars might have planets at all. We can update the calculation a bit.

Our galaxy, the Milky Way, contains somewhere between 100 and 400 billion stars. However, many of those stars are unlikely to have planets with intelligent beings living on them. Some of those stars are white dwarfs, which are the burned-out remains of old stars, with energy outputs so low their planets would be frozen. Blue giant stars consume their fuel so quickly they will run out of fuel and blow up after just a few million years. It took around three billion years for life on Earth to progress to where it is now. Some stars are red giants. These are ageing stars that have swelled up, incinerating any planets they might have, and will not live for much longer, so they are not good candidates to monitoring for alien radio signals. This leaves the “Sun-like” stars, which shine fairly reliably for billions of years. We can class at least 30% of the stars in our galaxy as being “Sun-like”. That gives us at least 30 billion candidates.

Today, our searches for planets orbiting other stars suggest all stars have planets. If we assume just three planets per star on average, then there are 90 billion planets out there orbiting Sun-like stars. If we believe our Solar System is typical, there could be almost 10 billion Earth-like planets in the Milky Way, suitable for life as we know it.

Unfortunately, from this point the arguments get a lot weaker. At the moment we have no idea what percentage of Earth-like planets have living creatures on them, because we know of only one example – our world. On the other hand, we do know that the clouds of dust and gas occurring almost throughout our galaxy are loaded with organic chemicals that can easily be persuaded to react together to form amino-acids, the building blocks of proteins, which are the basis for our form of carbon-based life. If the same raw materials for making worlds are available everywhere, then we might reasonably expect carbon-based life to be fairly common in the universe. If life appears on just 1% of Earth-like planets, there could be around 100 million worlds in our galaxy bearing carbon-based life. Unfortunately, it is very hard to estimate how many of these life forms are using radio technology to send signals in our direction. Of course, if we do not search for those signals, we will never find out, and we really should know.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355

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