Mario Livio welcomes a lucid description of attempts to evaluate how special humans are.
NASA/ESA/Z. Levay (STSCI/AURA)
An artist's impression of the Milky Way — one of hundreds of billions of galaxies in the Universe.
All the astronomical discoveries made since Nicolaus Copernicus demoted Earth from its position at the centre of the Universe have continued to erode humanity's perceived physical significance in the grand scheme of things. Consider this sequence of events: in 1920, US astronomer Harlow Shapley showed that the Solar System does not occupy the centre of the Milky Way, but is about two-thirds of the way out. Then, Edwin Hubble discovered that there are many other galaxies — a few hundred billion in the observable Universe, according to the latest observations (made, fittingly, by the Hubble Space Telescope). Next, it was found that even the stuff we are made of — ordinary baryonic matter — constitutes less than 5% of the Universe's energy budget. To top it all, speculative models based on cosmic inflation and string theory suggest that our entire Universe may be but one member of a 'multiverse', a huge ensemble of some 10500 universes.
On the planetary scale, there has also been an explosion of discoveries. Until 1992, there had been no confirmed discoveries of any planets outside the Solar System. However, observations since then (especially by the Kepler satellite) suggest that about 20% of all Sun-like stars in our Galaxy harbour approximately Earth-sized planets orbiting in the stars' 'habitable zones' — the regions of space that are neither too hot nor too cold, allowing liquid water to exist on a planet's solid surface. Given that liquid water is considered potentially a necessary ingredient for life, these statistics are (at the very least) promising for those who believe that there could be life elsewhere.
That Earth hosts life remains its last qualification for being special. How reasonable is it to think that we are alone in the vast expanses of space? And how significant is life on Earth on the Universal (or multiversal) scale? These are the questions that astrobiologist Caleb Scharf addresses intelligently and comprehensively in his beautifully written The Copernicus Complex. The book offers a grand tour of important findings from astronomy to biology that are relevant to the cosmic and microscopic search for life.
What sets this book apart from those that simply describe the hunt for exoplanets is Scharf's emphasis on the significance, or lack thereof, of our own existence. For instance, the realization that the human body contains ten times as many microbial cells as human cells — as well as impressive advances in the understanding of the chemical origin of life — has forced us to rethink how we classify the 'importance' of life forms on Earth, and perhaps even to consider placing microbes at the top of the hierarchy, rather than at the bottom.
Scharf tackles in some detail the question of whether we can conclude anything about the expected frequency of extraterrestrial life from the known facts about the emergence and evolution of life on Earth, particularly two important clues. The first is that some form of life arose very early in Earth's history, within only a few hundred million years of the planet's formation; the second, that the appearance of 'intelligent' beings took a few billion years. After sketching the basics of Bayesian probability theory, Scharf describes the interesting results of astrophysicists David Spiegel and Edwin Turner. They have shown that in the absence (so far) of any evidence of life arising independently of our lineage, one cannot reach any conclusions about the rarity (or not) of life in the Universe. This highlights the importance of the search for that evidence.
“Our place in the universe is special but not significant, unique but not exceptional.”
Scharf ends his book with the reflection that life inhabits the border between order and chaos. For instance, the dynamics of the planetary orbits in our Solar System are so complicated that they may become unstable within a few billion years. Similarly, Earth's climate and geophysics occupy that interface between order and disorder. From that, Scharf concludes that “our place in the universe is special but not significant, unique but not exceptional”. Note, however, that from the perspective of thermodynamics (entropy), life itself is an extremely ordered system.
I see two other important messages, with which I am sure Scharf would agree. One is that given the number of space telescopes that either are being built (such as the James Webb Space Telescope, to be launched in 2018) or have been proposed (including the Advanced Technology Large-Aperture Space Telescope), we may, for the first time in human history, be close to determining whether extraterrestrial life exists. Some optimistic estimates predict such a discovery in the next two decades. The second is that, notwithstanding our physical insignificance, the human mind is significant. Why? Because all the discoveries described in this book, from the subatomic realm to the multiverse, were made by us.