The author of this guest posting – ruminations about extraterrestrial life – is the physicist Josef Eisinger, author, among many other publications, of Einstein on the Road.
In these troubled times it is useful and instructive to ponder our place in the larger scheme of things. We are today privileged to have gained a realistic perspective of our place in the universe thanks to the progress scientists have made in observing our universe and discovering the laws of nature that explain the physical and biological evolution that brought humankind to the state it is in. I find this insight to be most gratifying and it, in turn, raises the question posed in the title of this essay: Is our story of life on Earth unique or did life emerge on other planets scattered by the billions in our own galaxy, the Milky Way, and in our universe at large? And beyond that, did life elsewhere evolve to a technological state, only recently attained on Earth, that allows us to contemplate interstellar communications by means of electromagnetic waves in the near future.
The raw materials that make up a living organism, its constituent atoms and molecules, are at the base of all such speculation. We know from observations and the well-tested laws of physics that the same ninety-two stable nuclei that exist on Earth are also found in all reaches of our universe. We also know that under suitable conditions these positively charged nuclei are surrounded by electrons to form atoms and molecules that interact with each other according to the laws of chemistry – or, more precisely, according to the rules of quantum mechanics, which determine their affinities to each other. Many different experiments give us confidence that the same physical laws are indeed valid everywhere in our universe.
The raw materials of extra-terrestrial life – the elements and their chemistry – are therefore very familiar to us, whatever planet they find themselves on. But for a living organism to evolve from them, the prevailing conditions must be “just right,” and must remain so for a very long time. We know that under suitable conditions the sun’s radiation can generate the basic building blocks of life (e.g., carbohydrates, amino acids) in water, although the crucial steps of assembling the components into a self-reproducing organism, remain hidden. The same building blocks are, incidentally, also found in meteorites, those messengers from outer space that land on Earth.
All forms of life on Earth, from the lowly ant to the majestic whale, employ the same biological master plan to create a living organism: a genetic memory, coded with four letters (the bases of DNA), which is expressed in proteins that consist of some twenty different amino acids, strung together in the particular order that determines each protein’s form and function. Other schemes for generating living organisms may have existed on Earth in the distant past but could not compete with the scheme outlined above; its remarkable stability and flexibility made Darwinian evolution possible.
While somewhat different biological schemes may well have evolved under different conditions, it seems likely that they would employ the same matchless solvent, water, and would also employ carbon-based molecules, because the rules of quantum mechanics (Pauli Principle) endow the carbon atom with uncommon chemical versatility. In searching for extra-terrestrial life it is, in any case, reasonable to look where the prevailing conditions resemble those on Earth – home of the only example of a successful biological evolution we know. We might therefore begin our search for extraterrestrial life by looking for planets with a stony surface that can support lakes or oceans and receive sufficient radiation from their star to keep water in the liquid state, at least some of the time. For a planet to be habitable for life, its orbital radius must be in a fairly narrow habitable range which depends on the luminosity of its star. With regard to Earth’s nearest neighbors in the solar system, for example, Venus is too close to the sun and therefore too hot for life to take hold, while Mars is much too cold – now, although it may have supported life in its more temperate past.
A balmy climate is by no means the only requirement for life to evolve. Thus, a habitable planet must be large enough to retain an atmosphere that retains its heat (but not too much, many Earthlings say!) and it should be surrounded by a magnetic field that protects it from space radiation. In other words the planet should be young enough to have retained a liquid core whose motion generates the protective magnetic field – as on Earth.
As stars age, they become brighter and their habitable planets become inhospitable to life. Stars that are much brighter than our sun may therefore be too old to allow biological evolution to reach a high level. Fortunately, our sun, aged about 5 billion years, is only in her middle-age. The age of Earth is estimated to be 4.5 billion years – and because that time span is too vast to be readily grasped, it is useful to convert it to a time scale in which Earth is just one year old. According to that time-scale, primitive life appeared on Earth about a month after the planet was formed, and it was only about a month ago that land animals began to inhabit Earth. About 2 weeks ago, dinosaurs roamed on Earth, and in time, they were succeeded by that optimistically named species, Homo sapiens, which made its appearance on Earth only 4 hours ago, while all of recorded human history took place in just the last half hour.
Having eliminated our sister planets as potential habitats of life, it is natural to search for planetary life in our immediate neighborhood. Thus, astronomers looked to see if the sun’s nearest neighbor, a star in the constellation Centaurus, aptly named Proxima Centauri, was accompanied by planets. Proxima Centauri is only about four light years from us, but because of its low luminosity, it is not visible to the naked eye, in spite of its proximity. It belongs to a class of long-lived stars known as “red dwarfs,” which are very abundant in our galaxy. Quite recently it was discovered that Proxima Centauri is indeed orbited by a planet, prosaically named Proxima b. Spectroscopic observations have made it possible to determine that Proxima b has a mass 1.2 times that of Earth, that it is much closer to its star than Earth is to the sun, and that its orbital period of just 11.2 days. Due to its proximity to its dim star, Proxima b receives almost as much radiant energy as the Earth receives from the sun. It may, nonetheless, not be hospitable to life because its atmosphere may have been blown away long ago by the frequent flares emitted by its uncomfortably close star.
Fortunately, the number of stars with potentially habitable planets is so huge that astronomers ought not be discouraged even by repeated failures to find extraterrestrial life: On the basis of a recent survey of stars (SWEEPS) conducted by NASA’s Hubble space telescope, it is estimated that the number of stars likely to possess “habitable” planets is approximately one billion – in our galaxy alone.
Artist’s impression of the planet Proxima b, which orbits our nearest star Proxima Centauri on the horizon, with its companion stars Centauri A & B above it and to the right.)