Is there any other life in the universe?
"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
M15 globular star cluster
Whether or not extraterrestrial life exists is of course open to speculation, for until we actually find it the answer has to be that we do not know, for not finding it does not preclude the possibility of its existence.
When considering the vastness of the universe, containing in all probability many millions of planets, it is difficult to imagine that our planet is the only one that harbours life. However, If that unlikely possibility should turn out to be the case, then it would perhaps be only reasonable to assume that it must be impossible for life to form by the process of natural development, but rather that it must have been uniquely created. To take the opposite view, that life formed spontaneously on only one planet in the entire universe, is pushing the laws of probability to virtually unlimited bounds. I would suggest therefore we have only two realistic options to consider. 1) Life was created exclusively on this planet by God. 2) Life has formed on many planets spontaneously.
Taking the first option, that life was created exclusively on this planet by God, raises a number of problems. One problem is the story of how God created human life as recorded in the bible, by forming Adam from the 'dust of the ground' (Genesis 2.7). Suffice it to say that this version of the formation of the species 'Homo Sapiens' does not accord with the findings of anthropologists, and I do not feel it necessary to embark on a lengthy discourse explaining why - we all know why. But in fairness, it has to be said that the bible was written in the style of the people the best part of 2000 years ago and should not perhaps be taken too literally today. We now live in age where man has walked on the Moon, has sent probes to all the planets in the solar system, and has even sent probes to investigate comets and land on an asteroid. Armed with all this knowledge that we have gathered about other worlds and distant galaxies, the vast majority of us no longer believe that the world was made in six days, so I will not pursue the pointless exercise of arguing against the bible. Those of you that do believe in the literal truth of the bible have your own reasons for doing so, but I do not believe that God created life on Earth. I do not argue that God did not create the universe which resulted in life on Earth, but that is not the same thing at all.
If then we discard the idea of God creating life on Earth we are left with the second option that life was created here by chance and therefore it is most likely that it will also form elsewhere. You may ask how I reach this conclusion, and my reply is that it is based on probabilities. Let's look at how many planets there may be in the universe, starting with the number of known galaxies.
1) The number of galaxies. An estimated 50 billion galaxies are visible with modern telescopes and the total number in the universe must surely exceed this number by a huge factor, but we will be conservative and simply double it. That's 100,000,000,000 galaxies in the universe.
2) The number of stars in an average galaxy. As many as hundreds of billions in each galaxy.
Lets call it just 100 billion.
That's 100,000,000,000 stars per galaxy.
3)The number of stars in the universe.
So the total number of stars in the universe is roughly 100 billion x 100 billion.
That's 10,000,000,000,000,000,000,000 stars, 10 thousand, billion, billion. Properly known as 10 sextillion. And that's a very conservative estimate.
4) The number of stars that have planetary systems. The original extra-solar system planet hunting technology dictated that a star needed to be to close to us for a planet to be detected, usually by the stars 'wobble'. Better technology that allows us to measure the dimming of a stars brightness when a planet crosses its disk has now revolutionised planet hunting and new planets are being discovered at an ever increasing rate. So far (August 2003) around 100 have been discovered so we have very little data to work on for this estimate. Even so, most cosmologists believe that planetary formation around a star is quite common place. For the sake of argument let us say it's not and rate it at only one in a million and only one planet in each system, as we want a conservative estimate, not an exaggerated one. That calculation results in:
10,000,000,000,000,000 planets in the universe. Ten million, billion, as a conservative estimate.
5) The number planets capable of supporting life. Let's assume that this is very rare among planets and rate it at only one in a million. Simple division results in:
10,000,000,000 planets in the universe capable of producing life. Ten billion!
For another approach I recommend The Drake Equation. This states that the number of communicating civilisations in our galaxy (note, our Galaxy only, not the universe) likely depends on a number of factors which must combine to yield a habitable planet where life has the chance to develop to a certain level of technological know-how. These factors include the rate of formation of stars like the Sun, the fraction of those with planets, the fraction of Earth- like planets, the fraction of such planets where life develops, the fraction of those where life becomes intelligent, the fraction of intelligent species who can communicate in a way we could detect, and the lifetime of the communicating civilisations. As you may imagine, There is a lot of debate about reasonable values for most of these factors.
Frank Drake's own estimate puts the number of communicating civilisations in just our Galaxy alone at 10,000.
Even though the figures I have used cannot of course be considered to be accurate, at least the figure of 10 sextillion stars in the universe is most definitely an underestimate. The number of life supporting planets that may be orbiting those stars is impossible to say, but by any reasonable estimate must surely run into the millions, if not billions. This is easy to justify on the basis that following the Big Bang the most abundant material in the universe was hydrogen and helium, being the most simple atoms, and this material forms the bulk of the raw ingredients for star formation. All stars begin life in the same manner, by the gravitational drawing together of these basic elements that then gravitationally collapse to form a star. Apart from size, all stars begin pretty much the same, with the remnants of the hydrogen and helium clouds that are not absorbed into the stars forming an orbiting disc that goes on to form the protoplanets. With this same process repeated many billions of times it would be only statistically reasonable to expect that many planets would have similar characteristics, and would be capable of supporting life of one form or another, just as our planet does.
In order to answer the question of the existence of extraterrestrial life, it need exist on only ONE other planet. Given those odds, how can it not exist?
In view of the incredibly high probability of extraterrestrial life existing the question remains why we haven't yet detected any signs of it. Maybe there has not yet been enough time, or maybe we are using the wrong technology. I appreciate that there are a number of people that claim that aliens are already visiting us in the form of UFO's, but as that is very speculative I have dealt with the subject separately. See UFO's : Fact or Fiction?
The SETI team (Search for Extra Terrestrial Intelligence) has been scanning the heavens for many years searching for a radio signal from an alien source, but so far without success. Having said that, they do have many very good candidate signals that they will follow up on. Radio signals do seem to be the logical way to send messages over astronomical distances, but an alien society with a mere hundred years advantage in technology could be using something very different. With a thousand years advantage, and a thousand years in evolutionary terms is the blink of an eye, they may have technology beyond our comprehension. It could be that their radio signals - if they ever used radio at all - were transmitted only briefly and may have reached us years before we had radio receivers, so we will never find a radio signal. Alternately, they may have started using radio more or less the same time as we did, so if they are thousands of light years away we will not receive their message for thousands of years to come, by which time we may no longer be using it. The window of opportunity for two distant civilisations to have compatible communication systems may be very small indeed.
We need to take into consideration the length of time it would take for a signal to reach us. Our own 'Milky Way' galaxy measures roughly 100,000 light years across, so a message from the opposite side of the galaxy would take 100,000 years to reach us. Our nearest spiral galaxy, the Andromeda Spiral Galaxy, is over 2,000,000 light years away. For a message to reach us today it would need have been transmitted towards the end of the Pliocene age, when Homo Erectus first appeared and began to diversify. To put it another way, the race that transmitted such a message would be 2,000,000 years more advanced then we are and we can only imagine how primitive we would appear to them. We need to consider also that if we did receive a message from the Andromeda Spiral galaxy, by the time we did they may no longer exist.
Even if there are many civilisations in our own Milky Way galaxy, and even if we do hear from them, do we really want them to know where we are? I for one would rather hide in the long grass, my mother taught me never to speak to strangers!
What do I think?
There simply HAS to be other life in the universe. It happened here, it must have happened elsewhere. Whether or not we will ever communicate is another matter entirely.
Update: August 2003
Astronomers have published a new estimate of the total number of stars in the universe. The international group of astronomers presented their findings at the General Assembly of the International Astronomical Union in Sydney, Australia. The figure they have arrived at is 70 sextillion, seven times higher than my estimate (hey, at least I got the right number of zeros!). This figure does not represent the actual total number of stars in the universe, just those that are in range of our telescopes. The actual number could be very much larger.
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