Opinion: thanks to the discovery in 2017 of four Earth-sized planets orbiting a star on our cosmic doorstep, the prospects of finding extra-terrestrial life within the next decade are much improved
We’ve all heard of Trappist beer and Trappist monks. But to a scientist, Trappist, or TRAPPIST, as it’s usually dubbed in this case, refers to two Belgian telescopes, one in Chile and one in Morocco. Their job is to search for exoplanets, that is planets orbiting any sun other than our own.
One of the TRAPPIST telescopes discovered a small star, which was named TRAPPIST-1, with three planets orbiting it in 2015. This year, further study brought the number of known planets in this system to seven – almost as high a number as our own eight (previously nine, before Pluto was demoted).
Three of these seven planets are in the star’s habitable zone, which is clearly encouraging for astrobiologists interested in the search for life in the Universe. Whether any of these particular three planets have life remains to be established.
But the question "do any such planets have life?" has been made redundant by this and many related discoveries over the last few years. Our question now should be: how many planets have life? And the answer is probably "trillions of them". That’s a big leap – from three planets to trillions of planets – so let’s examine why we’re justified in making it.
This involves doing a calculation, but luckily it’s a very simple one, especially if we use rounded, ball-park figures. Using several exoplanet-hunting telescopes including TRAPPIST, Spitzer, and Kepler (the king of them all in terms of number of planets discovered), our tally of exoplanets to date is about 3000. About 30 of these are Earth-like so that’s one in 100. All these figures refer to the small patch of our Milky Way galaxy that we’ve searched so far. To extrapolate from this patch to the whole galaxy we should multiply our figures by about a million, and to extrapolate from the galaxy to the Universe as a whole we need to multiply by the number of galaxies – about a trillion.
You can see where this calculation is going. The universe as a whole contains many trillions of Earth-like planets. Despite the use of a ball-park calculation, this conclusion is robust, unless our location in space is in some way special. But according to the Copernican principle, now universally acknowledged to be true (except perhaps by the Flat Earthers and the Intelligent Designers), our location is not special at all. Rather, it is very ordinary. We are not at the centre of things, as we were once arrogant enough and ignorant enough to believe.
Searching for extra-terrestrial life requires knowledge of biology as well as astronomy, so let’s now turn our attention to the science of life. We know an awful lot about the evolution of life on Earth over the past four billion years. But what does this tell us about its equivalents on other planets? I’d say quite a lot really.
The question "do any such planets have life?" has been made redundant; our question now should be: how many planets have life? The answer is probably "trillions of them"
Although there are millions of species on Earth at present, most of them represented by billions of individual organisms (seven billion in the case of the human species), every living being on our planet is carbon-based. There is not a single exception to this general rule, which is a rare thing in biology. And almost all of these organisms are built of one or more cells. As implied by "almost", there are exceptions, but they are very few in number. There are some creatures called slime moulds that have largish bodies (more than 10 cm across) that are not divided into cells. But these represent less than 0.01 percent of all living species on Earth.
To what extent can we expect these general rules to apply on planets with life throughout the universe? With regard to being carbon-based, "completely" may be a good answer, despite this view being criticised by some as "carbon chauvinism".
But it’s not chauvinism at all. That is, it’s not derived from a misplaced sense of the importance of our own key element. Large carbon-based molecules are sufficiently complex to give the specificity needed for life – such as the possession by humans of about 25,000 genes, each with its own specific role. Large silicon-based molecules are known, but their structure is very different and they simply can’t form the basis for the degree of specificity needed for life.
Claiming that all extra-terrestrial life is cellular in its construction is more dangerous, especially given that there are some exceptions to this rule on earth. But then again, there are no exceptions if we restrict our attention to complex life-forms – like animals and plants – with intricate organ systems. If our interest is in intelligent alien life, we should expect cellular construction to be the norm.
Can we go beyond this? Might some alien life be humanoid, as often depicted in sci-fi movies? "Maybe" is the only answer we can give at present. But let’s now ask a very different question: how long will it be until we discover evidence of extra-terrestrial life, and where will we find it? The discovery of TRAPPIST-1 and many other systems with planets orbiting in the habitable zones of their respective stars in our own local patch of the Milky Way suggests that the answers are as follows: very soon – perhaps within a decade – and right on our cosmic doorstep.
The views expressed here are those of the author and do not represent or reflect the views of RTÉ