If aliens ever arrive on Earth maybe they would first contact astronomers. Here is an imaginary scene of aliens visiting Sydney Observatory. Digital mischief by Nick Lomb with help from Microsoft clipart
This is the second of two posts contemplating the possibilities of life, especially intelligent life, in space. In the first post we established that that there is a plentiful supply of organic molecules, essential for life as we know it, in the clouds of gas and dust from which stars and planets form. Also that in our galaxy there are an estimated two million Sun-like stars circled by Earth-size planets in their habitable zone, the zone in which water can exist on the surface as a liquid. Finally, we considered the seven-factor Drake equation that contains the ominous term on the lifetime of civilisations.
It is difficult to use the Drake equation to draw any conclusions on the possibility of intelligent life elsewhere as that involves making guesses about the values of the, as yet, unknown terms. A completely different approach is to use the Fermi Paradox that uses only one fact and yet gives a definitive answer.
Before considering the Fermi Paradox though let’s have a numerical interlude and consider the likely size or height (h) of any animal-like creatures on another world. A creature to lift itself above the ground must have legs of sufficient strength to support its weight. The weight of the creature will scale with its volume, that is, with h^3 and with the local acceleration due to gravity (g), while strength scales with the cross-sectional area of its legs, that is, with h^2. This gives the ratio of the weight to strength as proportional to gh and, assuming biological muscles are the same strength everywhere per unit area, we find that h ∝ 1/g. So on a rocky body the size of Earth creatures should be roughly Earth-size, on a smaller body with half the gravity they would be about twice the size and on a larger rocky body with twice the gravity they would be half the size.
Returning to the Fermi Paradox that says that let us assume that there are other civilisations in our galaxy. Some are likely to have developed much faster than on Earth, while some maybe less advanced. Those that are more advanced by, say, a million years would have technological capabilities that would be completely unimaginable to us. One capability they would have, and possibly a civilisation only a little more advanced than ours could have as well, is to be able to colonise the galaxy. This could be done, for example, by sending to a few stars autonomous machines that could find and land on suitable planets, reproduce themselves a number of times utilising available resources and then launch themselves and the copies to new stars. It does not matter if it would take, say, a 100 years to build copies, the galaxy could still be colonised within a relatively short period of a few multiples of 10,000 years.
This analysis suggests that if there are other civilisations in our galaxy at least one should have made its presence felt long ago. Since there is no evidence of any significance to suggest that we have been visited or colonised by an extra-terrestrial civilisation in the past, reluctantly we need to conclude that we on Earth are likely to have the one and only civilisation in our galaxy. Other explanations are canvassed in a recent and excellent article in the Huffington Post, but still we have yet another reason to look after our own planet.