Are We Alone in the Universe?

By Kenny Gan

Starry, starry night! Away from bright lights of the towns and cities at the beaches and the countryside, one can look up into the clear night sky to marvel at nature’s glittering light show; dense clouds of sparkling diamonds scattered liberally across the inky dome of the sky, each point of light a fiery sun with its own worlds, its light has travelled countless miles of space and eons across time to reach us. Are there beings in other worlds looking at their own stars and wondering the same thing?

The thought of life on other worlds makes our fights and quarrels seem small, our fears and our tribulations seem trivial. The cut and thrust of politics seem to pale into insignificance and our differences seem to melt away into a “united planet” instead of fragmenting into national boundaries. How will extraterrestrial life look like? Will they be more advanced than us and able to teach us a thing or two about living in peace and using our planet in a sustainable way?

In this essay, I will discuss the question of whether we are alone in the universe from a scientific viewpoint and if we aren’t, what are our chances of coming into contact with other intelligent beings? There is no doubt that the discovery of extraterrestrial life if it happens would be the most important scientific and social event in human history and may cause us re-appraise our religious beliefs.

Carbon Based Life

Life on earth is carbon-based and there is compelling scientific evidence to believe that if it exists elsewhere, it will also be carbon based. Carbon is abundant in the universe, being produced naturally in stars and it is the only element which has the ability to form very complex molecules with other abundant elements like oxygen, hydrogen and nitrogen which are the building blocks of organic life.

Silicon looks like a promising alternative as it can also form complex molecules but it is nowhere as good as carbon. Unlike carbon, silicon molecules do not exhibit left and right handedness which is necessary for many chemical processes that regulate life. Silicon also suffers from a fatal flaw in that the product of respiration to release energy would be a solid whereas for carbon it is carbon dioxide, a gas which is easy for organisms to expel.

Elements do not bind with one another readily in air or earth so a liquid medium is needed to allow the myriad complex chemical reactions to occur. For carbon based life forms, the most suitable liquid would be water as carbon does not dissolve in it and the temperature range of liquid water is optimum for carbon-based chemistry to occur.

So we can infer with a high degree of plausibility that if we share the universe with other life forms, it will be organic and fleshy as in carbon based rather than hard and metallic as in silicon or metal based. Perhaps this may give us a little comfort that we will not meet a race of strange, unfeeling robots and be eliminated.

Earth is not the Centre of the Universe

To say the universe is vast is an understatement, it’s beyond vast, its size is almost unimaginable for our mortal minds to grasp; a rough approximation would be “practically infinite.”

Let’s see, stars are organized into galaxies and our sun is part of the Milky Way galaxy which contains at least 200 billion stars. As for the number of galaxies, no definite figure is available but estimates range from 100 billion to 500 billion and estimate numbers are rising as detection methods improve. It has been said that the total number of stars in the universe is greater than all the grains of sand on all the beaches of our Earth!

Early astronomers in their self-importance thought that the Earth was the centre of the universe as all the stars seemed to recede away from us as the universe expands. But this is fallacious as can be shown by the analogy of an expanding balloon that all points on the balloon seem to be receding away from each other; in other words any point looks like the centre of expansion.

Within our own galaxy, our sun is just a speck in a far flung corner so Earth’s position is merely a random distribution of cosmic dust rather than occupying any special position in space. This observation is important because it puts paid to the notion that we are in any way special.

By logical extension, if Earth is nothing special in the universe, why should life on Earth be any special? By passing an electric spark through a mixture of primal gases and irradiating it with ultraviolet light to simulate early conditions on earth, scientists are able to create many complex hydrocarbon molecules that form the building blocks of life so it appears that life is a natural consequence of habitable planets.

The Tyranny of Distance

Using our miles and kilometers to measure distances in space is of course nonsensical. In space, distances are measured in light years – the distance light travels in a year bearing in mind that light takes a mere 1.5 to travel from Earth to moon. The nearest star is 4.3 light years away from us. Our nearest galactic neighbour, the Andromeda Galaxy is 2.5 million light years away.

Light presents a natural speed limit in the universe. Physicist Stephen Hawkings has shown that it is not possible to travel faster than light or this would mean being able to travel back in time which is forbidden in our universe. Furthermore, postulations of shortcuts to travel the vast distances like worm holes and cosmic tunnels do not exist because they would effectively mean being able to travel faster than light.

As the nearest galaxy is millions of light years away and we can’t travel faster then light nor find some comic shortcuts, we shall confine our discussion on the existence of extraterrestrial life to our own galaxy.

The Abundance of Planets

Planets were once thought to be rare but astronomers have established that planets are indeed common as they are the by-product of the formation of stars.

But not all stars will give rise to solid planets. The early generation stars comprise mainly of hydrogen and helium and do not give rise to solid planets while later generation stars contain heavy elements may have solid planets. Scientists estimate that as many as 60% of the 200 billion stars in our galaxy may form rocky planets. That gives is 120 billion possible solar systems.

To be habitable, a planet must have a nearly circular orbit around its star so as to receive a constant source of energy without wild fluctuations in temperature. This would exclude binary star systems as there will be no stable orbit for planets. About 50% of the stars are binary which leaves us 60 billion suitable stars.

As we’ve discussed previously, liquid water is essential to carbon based life which is the only type we expect to find in the universe. This means that a possible life bearing planet must be located at just the right distance from its star in order to fall within the narrow temperature range for liquid water to exist.

In each solar system, we can assume that only one planet can be located at the right distance to support liquid water. Of course there may not be a planet in the habitable band at all, it may just be empty space. The planet must also be solid, not gaseous. It must also be large enough to possess the gravitational force to hold an atmosphere or the water will evaporate away into space. There is also a problem with planets which are too large as gravity will be too crushing.

The question is, “How many of the 60 billion solar systems have planets which are suitable for life to develop?”

The Key of Life

Liquid water holds the key to life for without it there is no life. The problem is that water can only exist as a liquid within an extremely narrow range of 100 degrees C. To appreciate how thin this range is, the temperature from the surface of the sun to the cold of deep space range over 6500 degrees C.

Even on Earth, this narrow range of temperature cannot be maintained throughout the whole planet. Near the poles and at high altitudes, water freezes into ice. The Earth has also gone through many ice ages due to varying output from the sun. The slight tilt of the Earth’s rotational axis causes the seasons as it revolves around the sun.

This mean that a planet’s orbit must be exactly spot on in distance from its sun to receive just the correct amount of heat to have liquid water and hence life. To top it off, the planet must be rocky and of the correct size. If this is the case, life would be rare indeed. So are we are alone in our galaxy after all, a mere quirk of nature?

But closer examination shows that this need not be the case. The age of stars is 10-15 billion years and its energy output varies throughout its life as it burns its thermonuclear fuel. Going by Earth standard, life takes a few hundred million years to develop given the right conditions. So as stars age, planets which were once uninhabitable may become habitable and civilizations may grow and then die.

Given this scenario, most of the planets out there should have life at some point in time. However, we shall arbitrarily discount 50% of them as unsuitable for life to develop for some reasons or other. This leaves us 30 billion life bearing planets.

The Abundance of Life

The problem now is to estimate how many planets out there have life at the same blip of time as us. The age of the Milky Way is estimated to be 13.7 billion years, almost as old as the universe. The era of plants and animals on Earth will reign for 1 billion years before the expanding sun wipes out life.

If we assume that on average life on a planet lasts for 1 billion years and its occurrence is distributed evenly throughout the age of our galaxy, (imagine the galaxy’s age chopped into 1 billion year time slices, each slice containing an equal number of life bearing planets) then a simple proportion of time (1b/13.7b) yields 7.3% of the life bearing planets sharing the same slice of cosmic time. This means there are about 2.2 billion life-bearing planets in our galaxy right now.

But if life is so abundant, why aren’t we inundated with visitors from outer space and why have none contacted us?

Our Nearest Neighbours in Space

We are not concerned with primitive life forms but with intelligent life which is sufficiently advanced to reach out into space and advertise their presence.

This would certainly slash down the numbers. Human civilization reach back about 5000 years but the space age began a bare half century ago. How long will we last before we wipe out each other in thermonuclear wars or we kick ourselves into extinction for unsustainable use of the Earth’s resources?

It’s an open question but let’s be very generous and say a modern civilization much wiser than our own can last 100,000 years and their occurrence is evenly distributed over time. This means over the 1 billion years time slice, just 0.01% of the planets have intelligent civilizations simultaneously by taking a simple time proportion. So out of the 2.2 billion life-bearing planets, just 220,000 of them have intelligent civilizations at any one time.

The extent of the Milky Way can be approximated by a disc of 100,000 light years diameter and 1000 light years thick. If we distribute this 220,000 planets evenly throughout this volume, we end up with a spacing of 330 light years between habitable planets with intelligent life.

Reaching Out to Each Other

It’s actually worse than it looks. Our spaceships can’t travel at light speed. To escape our solar system, a speed of 0.3% of the speed of light is required. It could carry more fuel to accelerate to higher speed but cost and weight are limiting factors. If we assume our spaceship with high technology can eventually reach 1% of the speed of light, it would take 33,000 years to reach our nearest neighbour (provided we know where to go) and 66,000 years to make a round trip!

So it looks like there is little chance of contact with an alien civilization but this doesn’t mean we’ll be forever islands in space, cut off from each other. Radio waves travel at the speed of light and we could possibly pick up some evidence of intelligent life if we are lucky. Of course the civilization which originated it may have already died out by the time their signals reach us. We’ve been actively looking for alien transmission for the past 40 years with no luck so far, but the places to look are vast.

Perhaps one day in the not too distant future, out of the cold blackness of the cosmos, a signal in a pattern which can only be generated by intelligent life will come beeping into our radio telescope receivers and we realize in one shocking burst of euphoria that we’re not alone and the world will never be the same again.

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