The Drake Equation: Are We Alone in the Universe?

The search for extraterrestrial intelligence (SETI) began in the 1950s with the suggestion of using microwaves for interstellar communication. Frank Drake, a physicist, conducted the first systematic search for SETI in 1960, known as Project Ozma.

He focused on two nearby stars, ε Eridani and τ Ceti, but he have found no signals. Drake then organized a meeting on SETI with 10 invitees, including Philip Morisson and Carl Sagan, where he introduced his famous equation to estimate the number of detectable civilizations in our galaxy.

The Drake equation, a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy, has captured the imagination of scientists and the public alike.

The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life. It is more properly thought of as an approximation than as a serious attempt to determine a precise number.

Image-1. Frank Drake stands in front of a whiteboard with his eponymous equation, which estimates the number of detectable alien civilizations in the Milky Way galaxy.

History of the equation

In September 1959, physicists Giuseppe Cocconi and Philip Morrison published an article in *Nature* titled “Searching for Interstellar Communications.” They argued that advancements in radio telescopes had made it possible to detect transmissions from civilizations orbiting distant stars.

They suggested that such messages might be sent at the wavelength of 21 cm (1,420.4 MHz), corresponding to the radio emission of neutral hydrogen, the universe’s most abundant element. They reasoned that other intelligent beings might consider this wavelength a logical point of reference in the radio spectrum.

Two months later, Harvard astronomer Harlow Shapley speculated on the number of habitable planets, suggesting that among the universe’s 10^18 stars, perhaps 100 million worlds could harbor life as we know it.

Seven months after Cocconi and Morrison’s article, Frank Drake launched Project Ozma, the first systematic search for extraterrestrial signals. Using the 85-foot dish at the National Radio Astronomy Observatory in Green Bank, West Virginia, Drake monitored two nearby Sun-like stars—Epsilon Eridani and Tau Ceti—scanning frequencies near the 21 cm wavelength for six hours daily from April to July 1960. Although the project was well-executed and cost-effective, it detected no signals.

Following this, Drake hosted the first conference on detecting extraterrestrial radio signals at the Green Bank facility in 1961.

During preparations, he realized that a formula could be created by multiplying several factors to estimate the number of detectable civilizations in our galaxy—this became the famous Drake Equation, focused on the radio search for extraterrestrial intelligence rather than primitive life forms.

The conference attendees included Frank Drake, Philip Morrison, J. Peter Pearman (the conference organizer), businessman and radio amateur Dana Atchley, chemist Melvin Calvin, astronomer Su-Shu Huang, neuroscientist John C. Lilly, inventor Barney Oliver, astronomer Carl Sagan, and radio astronomer Otto Struve.

These participants, later known as “The Order of the Dolphin” due to Lilly’s work on dolphin communication, commemorated their meeting with a plaque at the observatory.

Drake Equation

At its core, the Drake equation posits that the number of extraterrestrial civilizations can be calculated by multiplying several probabilities together. N represent the number of civilizations in our galaxy with which we might be able to communicate. These factors include:

• R*: The rate of star formation in our galaxy (typically stars per year).
• fp: The fraction of those stars that have planets.
• ne: The average number of planets that can potentially support life per star with planets.
• fl: The fraction of planets that actually develop life at some point.
• fi: The fraction of planets with life that go on to develop intelligent life.
• fc: The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
• L: The length of time for which such civilizations release detectable signals into space.

Original Estimates

• R∗ = 1 yr−1 (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative),
• fp = 0.2 to 0.5 (one fifth to one half of all stars formed will have planets),
• ne = 1 to 5 (stars with planets will have between 1 and 5 planets capable of developing life),
• fl = 1 (100% of these planets will develop life),
• fi = 1 (100% of which will develop intelligent life),
• fc = 0.1 to 0.2 (10–20% of which will be able to communicate),
• L = somewhere between 1000 and 100,000,000 years.

Using the minimum values in the equation results in a minimum estimate of N being 20. When the maximum values are applied, the estimate for N reaches up to 50 million. Given the uncertainties involved, Drake noted that the original meeting concluded with the approximation that N is roughly equal to L, suggesting there could be anywhere from 1,000 to 100 million planets with civilizations in the Milky Way Galaxy.

The equation doesn’t consider time explicitly. This might be an issue because stars forming civilizations might be very old, and the star formation rate might have been different back then. However, for our Milky Way galaxy, the star formation rate seems to be relatively constant, making the equation a good approximation.

Sara Seager’s Version of the Drake Equation and Biogenic Gases

Astronomer Sara Seager introduced a modified equation that concentrates on identifying planets with biosignature gases. These gases, produced by living organisms, can accumulate in a planet’s atmosphere to detectable levels using remote space telescopes. The Seager equation is formulated as follows:
where:

• N = the number of planets with detectable signs of life,
• N∗ = the number of stars observed,
• FQ = the fraction of stars that are quiet,
• FHZ = the fraction of stars with rocky planets in the habitable zone,
• FO = the fraction of those planets that can be observed,
• FL = the fraction that have life,
• FS = the fraction on which life produces a detectable signature gas.

Seager emphasizes, “We’re not discarding the Drake Equation, which addresses a different question.” She explains, “Since Drake formulated the equation, we’ve discovered thousands of exoplanets, and our understanding of what might exist out there has been transformed. Now, we face a new challenge—one that doesn’t focus on intelligent life: Can we detect any signs of life in the near future?”

Carl Sagan’s Version of the Drake Equation

American astronomer Carl Sagan introduced some modifications to the Drake Equation, which he presented in the television series Cosmos: A Personal Voyage. The revised equation is shown below; where:

• N = the number of civilization in the Milky Way galaxy with which communication might be possible
• N∗ = Number of stars in the Milky Way Galaxy
• fp = the fraction of those stars that have planets.
• ne = the average number of planets that can potentially support life per star that has planets.
• fl = the fraction of planets that could support life that actually develop life at some point.
• fi = the fraction of planets with life that go on to develop intelligent life (civilizations).
• fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
• fL = fraction of a planetary lifetime graced by a technological civilization

Image-2.The Search for Extraterrestrial Life.

Usefulness

While the Drake equation offers a useful framework for considering the possibility of extraterrestrial life, it is important to recognize that many of its variables are highly speculative and come with significant uncertainty. Our understanding of the factors that drive the origin and evolution of life is still incomplete, and we have limited data on the abundance of exoplanets that could support life.

Despite these uncertainties, the Drake equation has been a powerful motivator for scientific inquiry. It has inspired astronomers to search for exoplanets and develop methods to detect potential biosignatures in their atmospheres. It has also driven astrobiologists to study the conditions under which life might emerge and evolve on other planets.

As our understanding of the universe grows, so does our ability to refine the Drake equation and assess the likelihood of extraterrestrial life. Although the question of whether we are alone in the cosmos remains unanswered, the Drake equation provides a valuable tool for exploring this profound question and guiding future research.

The Drake Equation provides a framework for summarizing the factors that influence the likelihood of detecting radio communications from intelligent extraterrestrial life. The final three parameters fi,fc,fL are largely unknown and difficult to estimate, with values that can vary widely.

Thus, the value of the Drake Equation lies not in providing definitive answers but in encouraging scientists to consider the wide range of concepts involved in the search for life beyond Earth, giving the question of extraterrestrial life a foundation for scientific analysis.

Even after more than 50 years, the Drake Equation remains crucial because it serves as a roadmap for understanding what we need to learn to address this profound existential question. It also helped establish astrobiology as a scientific discipline, which, while open to speculation for context, primarily focuses on hypotheses grounded in existing scientific theories.