#### Max Hersov

The Big Question

There is one great question which almost everyone has pondered – one which, if answered, would change our perception of ourselves forever. Are we alone?

The observable universe is huge – 90 billion lightyears in diameter. It contains at least 100 billion galaxies, each with up to one thousand billion stars. The Milky Way alone contains at least 4 billion sun-like stars1, many of which have planets in the habitable zone (the area around a star where it is not too hot and not too cold for liquid water to exist on the surface of surrounding planets). Many of these stars and their planets are much older than the sun, giving intelligent life many opportunities 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. Firstly, we can attempt to estimate the number of potentially active, communicative extraterrestrial civilisations in the Milky Way. This is the purpose of the Drake Equation.2

The Drake equation is:

$$N = R_* \cdot f_p \cdot n_e \cdot f_l \cdot f_i \cdot f_c \cdot L$$

$N =$ the number of planets with detectable signs of life
$R_∗ =$ the number of stars observed
$f_p =$ the fraction of stars that are quiet
$n_e =$ the fraction of stars with rocky planets in the habitable zone
$f_l =$ the fraction of those planets that can be observed
$f_i =$ the fraction that have life
$f_c =$ the fraction on which life produces a detectable signature gas

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 one 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 it is much less active than it was billions of years ago, and it has also 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 which allows astronomers to observe planets and other massive objects through the light from background sources which they ‘bend’3  due to their gravity.

ne
The number of habitable planets per star that has planets 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 a few years ago we would think impossible. 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. What we do currently believe is that life could begin on planets in the habitable zone (also known as the “Goldilocks Zone”) of the star, on planets roughly resembling Earth, especially in size.
An average estimate for ne, however, 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/habitable. Identifiable life on Earth is around 3.8 billion years old, and the Earth itself is around 4.5 billion years old. Despite the fact that during Earth’s early stages there was very high asteroid and volcanic activity, there is reason to believe that life arose on Earth as early as in 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 made 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 it to harmful UV radiation. Solar flares would also be devastating especially 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 10% to as low as 10-3%. As a somewhat average assumption, let us say that the fraction of habitable planets which will develop life at some point is 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 favor the most intelligent, assuming other characteristics are equal.
As there is so much controversy around this value, let us 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, we Humans, despite a few deliberate messages into the cosmos, do not actively send out much deliberate 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 inadvertent radio traces, our changing of the composition of the atmosphere, and sending out spacecraft, among many others. However, the distance from which our communications and signals can be observed is a big limiting factor, and all of our radio signals ever transmitted into space only extend over about 200 light years, which is practically nothing on a cosmic scale. Moreover, after only a few lightyears, our signals decay into noise, which is surely 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 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:

$$N = 3 \times 1 \times 0.2 \times 10^{-4} \times 0.01 \times 0.2 \times 10 000$$ $$N = 1.2\times10^{-3} = 0.0012$$

Using these values, it is rather unlikely that there are other communicating civilizations in the Milky Way at this point in time. It would appear 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.
$f_p$ is sometimes estimated to be as low as $0.3$.
Values for ne range from $0.01$ to $5$.
Approximations for $f_l$ range from $10^{-2}$ to $10^{-5}$.
One of the most controversial values, $f_i$ ranges anywhere from $10^{-5}$ to $1$.
Another common estimation for $f_c$ is $0.01$.
$L$ is sometimes estimated to be up to 109 years, or even just a few hundred years. However, taking into account $n_r$, it is likely to be to the upper side of the approximations.

Using these higher estimates, we get: $$N = 7 \times 1 \times 5\times10^{-2} \times 1 \times 0.2 \times 109$$ $$N = 7\times10^7 = 70,000,000$$ 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, for example the Seager equation4 (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 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 93 billion light years of the Observable Universe, there is life of some form.

#### Notes:

1Although some estimates put this number as high as 20 billion.

2Actually, it was intended as a discussion topic at the SETI (search for extraterrestrial intelligence) meeting in 1961. It is very important to note that the estimates of the various values do vary greatly. Furthermore, it is only one equation among many which calculate the probability of life, but it is probably the most well-known and easiest to understand.

3It is important to make clear that the light does not bend, but appears to bend around the planet/massive body being observed due to the distortion of spacetime which is caused by that object’s mass.

4 The Seager Equation is: $$N = N∗ \times F_Q \times F_{HZ} \times F_O \times F_L \times F_S$$
$N =$ the number of planets with detectable signs of life
$N_∗ =$ the number of stars observed
$F_Q =$ the fraction of stars that are quiet
$F_{HZ} =$ the fraction of stars with rocky planets in the habitable zone
$F_0 =$ the fraction of those planets that can be observed
$F_L =$ = the fraction that have life
$F_S =$ = the fraction on which life produces a detectable signature gas

#### References:

“100 Billion Galaxies” En.wikipedia.org. 2021. Observable universe – Wikipedia. [online] Available at: <https://en.wikipedia.org/wiki/Observable_universe> [Accessed March 2021].

“100 Billion Galaxies” Eicher, D., 2020. How Many Galaxies Are There? Astronomers Are Revealing the Enormity of the Universe. [online] Discover Magazine. Available at: <https://www.discovermagazine.com/the-sciences/how-many-galaxies-are-there-astronomers-are-revealing-the-enormity-of-the> [Accessed March 2021].

“4 Billion Sun-Like Stars” Patel, N., 2020. Half the Milky Way’s sun-like stars could be home to Earth-like planets. [online] MIT Technology Review. Available at: <https://www.technologyreview.com/2020/11/06/1011784/half-milky-way-sun-like-stars-home-earth-like-planets-kepler-gaia-habitable-life> [Accessed March 2021].

“Gravitational Microlensing” NASA. 2021. The Milky Way’s 100 Billion Planets. [online] Available at: <https://www.nasa.gov/multimedia/imagegallery/image_feature_2233.html> [Accessed March 2021].

“400 [Years per Civilisation]” Shermer, M., 2002. Why ET Hasn’t Called. [online] Available at: <http://www.michaelshermer.com/2002/08/why-et-hasnt-called/> [Accessed March 2021].

“500+ Billion Planets in the Milky Way” Kurzgesagt – In a Nutshell, n.d. Kurzgesagt – In a Nutshell. [Video Catalogue] Available at: <https://www.youtube.com/channel/UCsXVk37bltHxD1rDPwtNM8Q> [Accessed March 2021].

In Particular:

Also Referenced:

En.wikipedia.org. 2021. Fermi paradox – Wikipedia. [online] Available at: <https://en.wikipedia.org/wiki/Fermi_paradox> [Accessed March 2021].

Nasa.gov. 2021. NASA – Milky Way Churns Out Seven New Stars Per Year, Scientists Say. [online] Available at: <https://www.nasa.gov/centers/goddard/news/topstory/2006/milkyway_seven.html#:~:text=Scientists%20estimate%20formation%20rates%20through,the%20heat%20these%20stars%20make.> [Accessed March 2021].

Astro.umd.edu. 2021. [online] Available at: <https://www.astro.umd.edu/~miller/teaching/astr380f09/lecture20.pdf> [Accessed March 2021].

Sierra, L., 2021. Are we alone in the universe? Revisiting the Drake equation. [online] Exoplanet Exploration: Planets Beyond our Solar System. Available at: <https://exoplanets.nasa.gov/news/1350/are-we-alone-in-the-universe-revisiting-the-drake-equation/> [Accessed 29 April 2021].

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