The Big Question

There is a great question which almost everyone has pondered, one which, if answered, would change our perception of human life forever – are we alone?

The observable universe is 90 billion light-years in diameter, containing at least 100 billion galaxies, each with up to one trillion stars. The Milky Way alone consists of at least 4 billion sun-like stars, many of which have planets in the habitable zone (the area around a star that is not too hot and not too cold for liquid water to exist on the surface of the surrounding planets). Many of these stars and their planets are much older than our sun, meaning intelligent life will have had substantial time to arise. As the Milky Way can be traversed in just a few million years (even at the slow pace of currently envisioned interstellar travel), the Earth should in theory have already been visited by extraterrestrials or their probes, or we should be able to point to some sign of their existence. However, there is no convincing evidence of either.

What are the Numbers?

Although the question seems very vague and even somewhat incomprehensible to our limited human minds, a thorough scientific approach can help us to tackle the question in different ways. First, we can attempt to estimate the number of potentially active, communicative extraterrestrial civilizations in the Milky Way. This is the purpose of the Drake Equation.

The Drake equation is:

Where:

• N = the number of civilizations with which humans could communicate
• R*  = the average rate of star formation in our galaxy
• f  = the fraction of stars which have planets
• ne  = the average number of planets that can potentially support life (per star that has planets)
• fl  = the fraction of those planets that actually develop life at some point
• fi  = the fraction of those planets (with life) which 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
• L  = the length of time for which these civilizations release these signals into space

R*

Current NASA and ESA estimates put this around 1.5 – 3 stars per year. This can be estimated by observing supernovae in other galaxies, or tracers such as infrared and ultraviolet light associated with the production of stars. The reason why we cannot just divide the number of stars in the Milky Way by the age of the Milky Way is that the rate of star production has greatly decreased over time as the galaxy is much less active than it was billions of years ago, and as it has expanded, so less material is constantly being recycled into new stars.

fp

This is currently estimated to be around 1, based on research by the PLANET Collaboration and data collected over 6 years through Gravitational Microlensing, a process that allows astronomers to observe planets and other massive objects through the light from background sources, which they ‘bend’ due to their gravity.

ne

The number of habitable planets per star is difficult to estimate because of the problem in defining what is a habitable planet. One aspect is that the only reference we have is life on Earth. We have a sample size of 1 and thus cannot make accurate assumptions on what life would be like and where life could exist. Even now we are still finding life in frozen Antarctic cores, or in the deepest depths of ocean hydrothermal vents, all of which we thought impossible a few years ago. All life on Earth requires water, but we do not know whether extraterrestrial life could use a different chemical solvent instead. Other problems with defining habitability include tidally-locked planets (where one side always faces the star and is scorching hot, and one side never faces the star and is permanently frozen – which makes habitability uncertain). There is also the consideration that some moons may be habitable. We currently believe that life could begin on planets in the habitable zone (also known as the “Goldilocks Zone”) of the star, on planets roughly resembling Earth.

An average estimate for ne, therefore, is around 0.2.

fl

With Earth as our reference, the chance that life forms on a habitable planet at all seems quite high. This is because life on Earth appears to have begun very soon after conditions on Earth became favourable for organisms. Identifiable life on Earth is around 3.8 billion years old, and the Earth itself is around 4.5 billion years old. Despite the very high asteroid and volcanic activity during Earth’s early stages, there is reason to believe that life arose as early as the first 100-200 years that Earth was habitable.

On the other hand, Earth could be rather unique. The Rare Earth Hypothesis argues that the conditions needed for the formation of life – abiogenesis – are exceedingly rare. For example, the Earth just so happens to be relatively far away from any potential supernovae, whereas planets nearer the centre of the Milky Way may be completely hostile to life if a nearby supernova explodes. Furthermore, Earth also has a relatively strong magnetic field, protecting it from the sun’s solar wind, which would otherwise strip off most of the ozone layer, exposing the planet to harmful UV radiation. Solar flares would be devastating to early lifeforms, particularly on planets around smaller stars, which can have especially violent flares.

Furthermore, the origins of life, and the exact conditions required for it, are still unclear, and thus estimates for fl range from as high as 0.1 to as low as 0.00001.

As a somewhat average assumption, let us say that the fraction of habitable planets which will develop life at some point is 0.0001 = 1/10,000.

fi

There are two main sides to the debate around this value. Some point out that, out of the billions of species that have existed on Earth, only one has become truly intelligent. Judging by Earth, this calls for a tiny value of fi. Moreover, the Cambrian Explosion (the rapid development of wide variation within multicellular life ~541 million years ago) happened very long after the first development of life, suggesting that for variation and intelligence to arise, very specific conditions are necessary. In addition, life seems relatively fragile and vulnerable to extinction events, so life on a planet could die out before intelligence is reached. For example, if Earth had not been protected by Jupiter’s gravity from asteroid impacts, complex life could have been wiped out before intelligence evolved.  However, others argue that this value should instead approach 1, as all life naturally tends towards complexity or intelligence, and thus it is inevitable that at some point intelligent life arises from non-intelligent life. This is also because intelligence is advantageous for natural selection, and so evolution should favour the most intelligent, assuming other characteristics are equal.

As there is so much controversy around this value, we will make the median assumption that it is 0.01.

fc

The fraction of intelligent species that communicate deliberately beyond their planet would most likely be relatively small. For example, humans, despite a few messages into the cosmos, do not send out much deliberately detectable activity. However, we inadvertently would be relatively easy to detect to civilizations not much more advanced than our own, due to signs such as radio traces, our changing of the composition of the atmosphere, or sending out spacecraft, among many others. However, the distance from which our communications and signals can be observed is a large limiting factor, and all of the radio signals we’ve ever transmitted into space only extend over about 200 light-years, practically nothing on a cosmic scale. Moreover, after only a few light-years, our signals decay into noise, which is very difficult to identify as coming from an intelligent species.

A common estimate for this value is 0.2.

L

L is the length of time for which a civilization is communicating. This number depends on what is classed as a civilization. If the Roman Empire counts as a civilization, then, using human civilizations as examples, L is around 400. However, in this context, we should either consider all intelligent life ever on a planet as a civilization or introduce nr, which is the number of times a civilization re-emerges on a certain planet. It is easier to just consider one civilization over a longer time, in which case L is estimated to be up to 10 000 years.

What does this mean?

Using the numbers above, we can say that:

$N = 3 \times 1 \times 0.2 \times 10^{-4} \times 0.01 \times 0.2 \times 10 000$

$N = 1.2 \times 10^{-3} = 0.0012$

Using these values, it is rather improbable that there are other communicating civilizations in the Milky Way at this point in time. It appears that we are likely to be alone.

Using other estimates for the values

R* is estimated by some to be as high as 7 stars per year.

fp is sometimes estimated to be as low as 0.3.

Values for ne range from 0.01 to 5.

Approximations for fl range from 10-2 to 10-5

One of the most controversial values, fi ranges anywhere from 10-5 to 1.

Another common estimation for fc is 0.01.

L is sometimes estimated to be up to 109 years, or even just a few hundred years. However, taking into account nr, it is likely to be to the upper side of the approximations.

Using these higher estimates, we get:

$N = 7 \times 1 \times 5 \times 10^{-2} \times 1 \times 0.2 \times 10^9$

$N = 7 \times 10^7 = 70,000,000$

Suggesting that we are most definitely not alone.

A Note on Shortcomings

Despite all its positives, the Drake equation does have many shortcomings. It makes many assumptions, the most notable of which being that broadcasting and listening for radio signals is the method by which an intelligent species would choose to communicate across interstellar space. They may be communicating right now, but using a method which we are not searching for, or trying to detect. This is one explanation for why SETI has not produced any confirmative data proving the existence of aliens.

Attempts have been made to rectify these problems, such as the Seager equation, which calculates the number of detectable inhabited planets taking into account our own limitations in detection and observation of distant planets). However, these often have yet more shortcomings of their own.

A Conclusion

The Drake equation is by no means perfect. In fact, it is very far from that, but it does demonstrate how the scientific method can make a seemingly imponderable question into something that can at least be reasoned with. It is obvious that the range of differences of values compounds to give huge variations in the estimate of N, and that it thus provides little to no certainty at all. However, as more data is collected, and as humans explore, develop and advance, perhaps we may narrow down the answer.

At this point in time, I believe we should keep an open mind. It may be unlikely that we find little green (or red) men on Mars, but it is not a stretch to think that somewhere, on one of the 500+ billion planets in the Milky way, or even in in the 90 billion light-years of the Observable Universe, there is life of some form.

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