William Reville: Copernicus might have been wrong about Earth
A computer model has found Earth to be uniquely suited to life among 700 million trillion terrestrial planets
Erik Zackrisson’s study indicates there are far fewer places than we thought where life such as ours could evolve. Photograph: iStock
Nicolaus Copernicus (1473-1543) initiated the demotion of the importance of the Earth and its inhabitants in 1543 with his proposal that the sun, not the Earth, sits at the centre of the universe. Subsequently science discovered that we live in a universe teeming with stars and planets and popularly adopted a “Copernican principle” stating that Earth is not a special planet circling a special star and neither are we humans special creatures.
I have, more than once, saluted the Copernican principle in this column. Following Copernicus, in the continuing saga of the humbling of humankind, Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) proposed a theory of evolution explaining how humans arose naturally on Earth as just another animal in the animal kingdom. The theory also provided a mechanism capable of populating many other planets with intelligent life. However, the latest study by Erik Zackrisson (Uppsala University) and others, submitted to the Astrophysical Journal and published on arxiv.org, indicates that Earth may be a very special planet after all, thereby challenging the Copernican principle.
Zackrisson and colleagues created a computer model of the early universe, incorporating all the data on planets (exoplanets) discovered beyond our solar system by the Hubble space telescope and programmes such as the planet-hunting Kepler space observatory.
They then modelled the evolution of the universe up to the present day under the known laws of physics. It is estimated that the universe contains 700 million trillion (quintillion) terrestrial planets, although only about 2,000 exoplanets have been discovered to date. Zackrisson’s study found that none of these 700 quintillion possible planets looks like Earth.
Most planets in the model exist in galaxies bigger than the Milky Way and orbit stars of different composition to our sun. The composition of a star affects the properties of its orbiting planets. Most of the planets in galaxies such as the Milky Way are much older than Earth and very unlikely to support life. Earth’s age and position within the Milky Way galaxy may make it unique – “a mild violation of the Copernican principle”.
Because Earth formed relatively early in the expanding universe’s history, we can look backwards in time using powerful telescopes such as the Hubble and trace the development of galaxies back to the universe’s creation in the Big Bang. If the universe continues to expand at its current rate, which is most likely, such observational evidence will be erased billions of years from now and civilisations arising then will have no astronomical evidence as to how the universe began and evolved. We humans are very fortunate to be living at this special time.
What are the implications for extraterrestrial life? Well, there is certainly plenty of time left for extraterrestrial life to arise if it doesn’t exist already. When the Earth formed about 4.6 billion years ago, only about 8 per cent of the potentially habitable planets that will ever form in the universe had been born.
When our sun burns out about five billion years from now, the great majority of planets destined to form in the universe will still not be born (Peter Behroozi and Molly Peeples, Monthly Notices of Royal Astronomical Society, October 2015). Neither is there any shortage of planets (700 quintillion), and the Copernican Principle predicts that life will surely start up on some of these planets.
The prospects of extraterrestrial life being biologically similar to life on Earth may well be very small, however. Such life would need food, water, sunlight, a rocky planet and so on, and Zackrisson’s study indicates – contradicting the Copernican principle – that there are far fewer places in the universe than we thought where life such as ours could potentially evolve. We may very well not only be alone, but remain alone, in the universe – a very lonely thought.
However, much is uncertain in this area of research. As one wag put it: “Few statements can be made in this area with the certainty I feel when I say: ‘I think I will have another beer.’ ”
William Reville is an emeritus professor of biochemistry at UCC, http://understandingscience.ucc.ie