Aliens

Are we alone in the Universe?

As far as the human race is aware Planet Earth is the only place in the universe with any form of life. But if you consider where we are, and what we are, you begin to realise that there is really no special reason why we should be the only life forms in the universe.

We live on an ordinary planet which we call Earth, which happens to be just 1 of 8 (sorry Pluto), which just happens to be the fifth largest and also the third from the Sun, which is classed as just an ordinary star (spectral class G2, of which there are many billion of throughout the galaxy). I have mentioned many justs & ordinarys, but that is because apart from it's number of planets, there really is nothing special about our solar system.

For many years people have been asking the same questions about what our position in the universe is. In 1961 a scientist by the name of Frank Drake put together all the factors believed required for life in the Universe, based on facts we already know, or can accurately predict, this is known as the Drake equation .

N = N* . f_p . n_e . f_l . f_i. f_c. f_L

N = No: of aliens capable of communication (intelligent life)

N* = No: of stars in our galaxy

f_p= fraction of these stars that have planets

n_e= No: of these planets capable of bearing life

f_l= fraction of planets on which life actually evolves

f_i= fraction of these life forms that are intelligent

f_c= fraction of intelligent life that want to communicate

f_L= fraction of the needed life time for the main star

Lets look at these individually.

N* = Through observations made in the last 20 years, this number is calculated at around 100 billion (although some people believe it could be as high a 400 billion) N* = 100 billion

f_p= As early as 1994 not 1 single extra solar planet had been found. This all changed in 1995 when two Swiss scientists, Mayor & Queloz accidentally found the first ever planetary system outside of our own. To this present date over 100 new systems have been found within 30 Pc. These systems may range from 1 planet up to 10 or more. f_p = 0.1

n_e= This depends on a number of factors.

Time it takes for a life form to evolve. Humans took around 4 billion years from the most basic of micro-organisms to the incredible beings that we are now.
Presence of water on the surface or in the atmosphere. This means that we would be looking for life that requires* water for survival. The planet would have to be at the correct distance from the main star for water to be in liquid form. *= they may not be Carbon based organisms.
The chemistry of the planet is vital if the right molecules are to get together and form life forms. As we know that this worked very well on Earth, then we should be looking for planets that are approximately the same temperature as Earth. n_e = 0.1

f_l = This one is more difficult simply because we don't know exactly what starts off life. We do know that things like amino acids are easily synthesized by geological processes and are therefore probably common throughout space. We may be a one off event never to be repeated in the universe. On the other hand, we know it has happened once, so chances are that it will happen again. Due to the fact we don't know which one of these is more accurate, all we can say is that f_l is somewhere between 10^-8 & 1

f_i = Although it took around 4 billion years for life to evolve from the most basic organisms to what you see around us today, it is known that around 100,000 years ago there were two intelligent speices living on Earth (Homo sapiens neanderthalensis & homo sapiens sapiens (us)). We luckily had the upper hand, and the other guys (& gals) became extinct. From what we can see, once life evolves it naturally becomes intelligent reasonably quickly. From this we can say that any life form with time will become intelligent. So f_i =1 Unfortunately we cannot leave here. We can clearly see by looking around us that of the 1 million species on Earth only 1 has evolved significant intelligence. So maybe intelligence is not as simple to evolve after all. Therefore f_i = 10^-6

f_c = Again this one is not easy to answer. What fraction of the above intelligence want to communicate?

First of all they may not want to communicate, then we will never know!
They may be more or less intelligent than ourselves and develop alternative technologies which do not require communication in the sense that we do. As we do not know whether they would want to communicate or even how they would do so, all we can say is that 0 < f_c >1 (more than 0, but less than or equal to 1).

f_L = I have read that this question is the same as asking "How long will a civilization survive?" As we know that the Sun is our cause of life (read Starstuff), we can therefore say that the Human race (however it may evolve) will live for however long the Sun is around. In our case 5.2 billion years. We are civilised in the sense that we have been capable of communicating into space for 70 years. Therefore the age of our civilisation is between 70 & 5.2 billion years.

Other events may shape the fate of a civilisation i.e.

Nuclear war (obviously high intelligence would be required)
Destruction of the atmosphere both natural and deliberate
Rogue asteroids
Killer viruses
Nearby supernovae
Stray black hole
Attack from other worlds (if they exist)
Things we do not yet want to know about.

Some of the above factors are avoidable, whereas some are completely beyond our control (that's life, wherever it may be). As the sun has a total life time of around 9.8 billion years we can say that 7.10^-9 < f_L < 0.5

Are we alone?

If we are optimistic about this, we can say by adding up all of the above that N = 5.10⁸. This statement says that about 1 in every 200 stars has a civilisation that is able to communicate. If this were the case, then we are less than 100 ly from our nearest galactic neighbours. Although that means that there are around 500 million civilisations out there somewhere in our galaxy (50 million trillion in the Universe) there are obviously going to be exceptions to the above factors, which may reduce or even increase the number of life forms in the universe. On a pessimistic scale though, it is just as easy to say that we are and will be the only forms of life ever to exist, & it is at present impossible to tell which statement it more accurate.

Which ever way you may look at it we are aliens ourselves. We fall into every possible criteria we have said extra-terrestrials should. As this is the case, do we need to know if there are other life forms in the Universe when we already know that we are surrounded by them every day?