# Planet Earth/7a. How Rare is Life in the Universe?
## Is Life Unique to Earth?

Since the advent of radio-telescopes, scientists have been scanning the skies for messages from the depths of outer space, especially of other intelligent beings in the universe. In the endless quest to find even the simplest lifeforms on other planets scientists have gone to extraordinary lengths to send spacecraft and landing rovers to other planets in our own solar system. Only to find them empty, barren of even the simplest of single celled lifeforms. What makes life on Earth so unique and so special? And what is the possibility that there may be other planets in the universe with lifeforms like those found on Earth? Of intelligent life, like humans able to communicate between the stars using light. For Frank Drake these questions haunted him, and since the 1950s he has developed tools to try and observe radio transmissions from other planets, and listen for signals from space. So far, they have been eerily quiet. Radio waves travel at the speed of light, faster than any space craft, and listening for these signals is the best way to observe extraterrestrial intelligent life, able to produce such communications. So far only silence.

## The Drake Equation

In 1959, Frank Drake held a meeting of some of the top scientists of the day including Carl Sagan, to work on quantifying the possibility of life on other planets. He was likely discouraged by the lack of evidence for extraterrestrial life, but eager to encourage others to look for life beyond Earth, especially at the beginning of the Space Age of the 1960s.
A few years before, the nuclear physicist Enrico Fermi made a statement during a lunch conversation at the Los Alamos National Laboratory, among his fellow scientists, he asked them “Where are they?” Noting the lack of evidence for extraterrestrial life beyond Earth, but also remarking on the likely high probability of life existing at least somewhere in the universe, since there are so many stars and so many possible planets. This idea has been called the Fermi Paradox, a paradox that states that despite there being a high probability of life existing on other planets, there is no evidence for life found beyond that found here on Earth, even despite our best efforts in searching for it. A few years later Frank Drake wanted to estimate what this probability is for extraterrestrial life, and he prepared a mathematical equation to approximate this probability to present to his friends and colleagues at a meeting. This mathematical equation has become known as the Drake Equation. Since its first quantification in 1959, the equation has been modified and studied by those scientists who want to try and understand the rarity of life in the universe, but it is an incomplete model, and maybe in error.
The basic Drake Equation is $N=R*\cdot fp\cdot ne\cdot fl\cdot fi\cdot fc\cdot L$
where $N$ is the number of civilizations in the universe with interstellar communication possibilities. $R*$ is the rate of star formation in the universe, $fp$ is the average fraction that have planets, $ne$ is the average number of planets that can support life, $fl$ is the fraction of planets that develop lifeforms, $fi$ is the fraction of planets that develop intelligent life, $fc$ is the fraction of civilizations that develop a technology that releases signals of their existence, and $L$ is the length of time those civilizations exist in time before becoming extinct.

### R*, rate of star formation

Estimated values for R* are likely very large, the Milky Way Galaxy produces between four to seven new stars a year (https://www.nasa.gov/centers/goddard/news/topstory/2006/milkyway_seven.html), and with a universe with an estimated 100 billion galaxies, the net increase of stars per year is about 400 to 700 billion stars per year. A very large number, and this increases the probability of life somewhere in the trillions of stars of the universe. The estimated percentage of these stars with planets is also estimated to be a fairly high percentage, as study of exoplanets have revealed that many if not most stars have planets that rotate around them, a generous 90% of stars might have planets, and hence solar systems.

### ne, is the number of Earth-like planets

The next value ne is the number of Earth-like planets, a value that is much more difficult to determine. In our own solar system Earth sits in the triple junction for the presence of water as solid, liquid and gas phases. Some use this critical zone as the possible region from a star that a planet would exhibit a liquid ocean, and ice caps, with some atmospheric water, and hence necessary for life. This is a fairly narrow distance from a sun, and some planets even within this zone might be too large or too small in mass. Study of exoplanets reveals a huge variation in the configuration of solar systems, and this value is much lower than 1. Maybe an Earth-like planet is found in 1 in 1,000 solar systems, a value of 0.1%.

### fl, life originating on these Earth-like planets

The next three values to estimate are fl the percentage of life originating on these planets, and fi the percentage of intelligent life originating on these planets, and fc the ability of this intelligent life to be able to send communication signals between planets. Of the three values dealing with life, fl is one of the most likely, as life on Earth originated very early in its history about 3.8 billion years into Earth’s beginnings there is indirect evidence for the existence of life, and by 3 billion years life is common. The study of other planets, although a very limited sample, suggests that life has very narrow conditions for its presence, as we have not found any organic molecules on Mars or the Moon. If the fraction is on the order of 1 in every 1,000 Earthlike planets, fl would be near 0.1%. This would mean that within the universe there are 630 billion planets with life existing on them. Unintelligent, single celled lifeforms that can’t communicate would be the dominate mode of life in the universe. Of the parameters most changing to estimate, fi and fc are the most critical.

### fi, the percentage of planets with intelligent life

The percent of these planets with intelligent life and life able to communicate between stars using radio waves. Some scientists argue that these percentages are very small, as demonstrated by the long length of geological time on Earth for an intelligent species, humans, to evolve and be able to send radio signals. Donald Brownlee and Peter Ward argue for very tiny numbers for these values, making the possibility of intelligent life on other planets very unlikely, and have coined the hypotheses called Rare Earth. They view the complex series of events that has led to the advent of humans as being highly unlikely to occur on another planet. The 3.8 billion years that it took for humans to appear on Earth is a testament to the low probability of a similar series of events occurring on another planet. Life does not inherently become intelligent over time, it just survives and makes do with its environment. If fi is 1 in 3.8 billion chance (using the yearly probably since the origin of life) of intelligent life evolving, then fi is an extremely small number (0.0000000026%).

### fc, capacity for interstellar communication

The next value fc is the percentage of these intelligent life planets having the capability of interstellar communication. If humans have been around for 250,000,000 years, and radio signals were discovered only in the last 150 years, a 150 years of communication divided by 250,000,000 years of human existence as a species as the intelligent life on the planet, the value for fc is 0.000006%.

### L, the duration of interstellar communication on a planet

The last value is L, the length of time that interstellar communication exists on a planet. This was a value that intrigued and bothered Carl Sagan, for he worried that one of the most limiting of the variables of the Drake Equation was the length such highly intelligent species are able to maintain interstellar communication, the value of L. Global war, pandemics, extinction due to limited resources, climate change and other disasters might befall such advanced civilizations, ending their abilities to communicate beyond the stars. A fairly conservative number would be 1,000-years, although such units of time might range across widely different units of time depending on the intelligence and nature of these alien species. We can add a few years, maybe 17. But a 1,017-year length of time is a fairly long civilization on Earth, longer than most empires on Earth, and all the while maintaining the ability to detect and send these signals into space.
If we solve for N, where the number of civilizations in the universe with interstellar communication possibilities, using these estimated, guessed at, or even just jolted down based on non-existent data, we get a simple value of N, of just 1.

## Is Earth unique in harboring life?

One civilization in all the universe with the ability of interstellar communication, but without any other planet to communicate with. These estimates of the Drake Equation could be, and likely are wrong, widely miss-representing the true probabilities, but they highlight something really important, the key to really getting at the sense of whether Earth is the only planet with intelligent life in the universe depends how we estimate the rarity of the existence of intelligent life through billions of years in the process that life underwent on Earth to lead to the advent of intelligent lifeforms, such as yourself. What bizarre world that we live in where you can read these words, comprehend them, and gain knowledge through them. How has life gone from a single complex molecule, to cellular organisms, to multicellular organisms, to animals that move and swim, to a tool builder, and eventually to a creature such as yourself.
This is the story of life on planet Earth, and why it is so precious, and possibly so rare. Earth is unique not because of its dimensions, atmosphere, oceans, continents, or rocky and molten interior, but it is unique because it is the only planet we know of that harbors intelligent life.
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