Beyond Our Sun
The possibility of life of any sort appears more promising when we venture beyond our solar system. But consider: our galaxy is 100,000 light years across.
A light year connotes distance: how far light travels in a year. As far as is known, nothing in the natural world is faster than light. And as we learn while young, light travels 186,000 miles per second, specifically, 186,282 mps. One light year, then, connotes a distance of 5,878,500,000,000 miles (5.9 trillion miles). Travel from one end of our galaxy to the other involves a distance of nearly six trillion miles multiplied by 100,000. The distance and time are unfathomable to human beings, frail creatures whose individual life spans run about eighty years.
Occasionally, Earthlike planets within our galaxy present themselves to us. In 2014, a 97.5-inch ground telescope at Spain’s La Palma Island detected 55 Cancri e, an exoplanet with nearly eight times the mass of Earth, and twice its diameter. The continually sun-kissed side of 55 Cancri e develops temperatures as high as 3,000 degrees Fahrenheit—hot enough to melt iron. That’s an unpromising environment for life, but if 55 Cancri e has hydrocarbons and organic chemicals, and if moderated temperatures outside the hot zone allow the planet to retain water, life there is not an impossibility. Future European and American study of 55 Cancri e will attempt to address those points.
NASA
NASA’s Kepler spacecraft, as well as ceaseless study from the ground, suggests other Earthlike planets within our galaxy. But our Milky Way is not the only galaxy, of course. There are many others, and they are far away—so far away that the 100,000-light-year distance from one end of our galaxy to the other seems paltry by contrast. Andromeda, the galaxy closest to our own, is 2.2 million light years away. It is comprised of some 600 billion stars, but we have little hope of physically reaching any of them within a useful time span. If the expanse of our own Milky Way is, for now, physically closed to us, crewed journeys to other galaxies seem inconceivable.
The Milky Way, Andromeda, a pair of Magellanic Clouds (dwarf, satellite galaxies of the Milky Way), and about twenty other systems comprise the so- called Local Group portion of the universe. The Local Group is home to about 1.5 trillion stars. Many galaxies exist beyond the Local Group. In the observable universe, there may be a billion trillion stars. Not every star is suited to have attracted planets, of course. And on stars that have, many factors come to bear on the possibility of life. A very small planet cannot sustain or hold an atmosphere. The distance separating a planet’s sun from other suns is important, because close proximity invites annihilating collisions, chain reactions, and black holes. The biosphere (life-supportive) region around a star is relatively shallow, so a planet’s distance from its sun is critical. The age and configuration of those suns matters, too, for an elderly sun will have begun to collapse upon itself, putting planetary life at risk. A sun that gives off continual blasts of ultraviolet light and X rays militates against planetary life. The particular gaseous makeup of stars is also important, given that helium and hydrogen are conducive to life (though those and other elements may be found in interstellar dust clouds).
Yet another factor to bear in mind is that conditions ideal for life do not guarantee the creation of life. On Earth, congenial combinations of proteins and amino acids came together to form life by creating reactions that built on one another, growing more sophisticated at every stage. The same or similar ingredients and combinations may not have appeared elsewhere. Despite Miller- Urey, the “somehow” aspect of the germination of life on Earth continues to intrigue and challenge laypersons, scientists, and theologians. What is it, exactly, that provides the spark that sets life’s building blocks into motion? One theory cites “space spores” that drift to Earth (and other planets) from parts unknown.
Once deposited on Earth, the spores generate life. The notion is captivating, but a nearly infinite collection of spores would have to make this journey in order for a probability of just one reaching Earth, or elsewhere. Equally discouraging is that while moving through space, such spores die from exposure to cosmic rays. Even assuming limitless spores that escaped radiation, solar gravity would repel them from our Sun (and any sun) long before they reached Earth or an Earthlike planet.
Another theory, spontaneous generation of life, is more mystical than empiric, and has no basis in known science.
Perhaps life’s spark arrives in meteorites. Some examples that reach Earth’s surface, called carbonaceous chondrites, carry amino acids—building blocks necessary to life as we know it. However, meteorites of this type are not robust; they do not travel well through space and fare even worse after hitting an atmosphere. The chemical properties of those few, over eons, that reached the Earth’s surface quickly degraded, leaving them inert, and ill-suited to be spark plugs for life. Further, the amino acids carried by carbonaceous chondrites are evenly divided between those with links to simple life as we understand it and those that have no links at all. The life-in-a-meteorite theory has merit, but must be approached cautiously, at least if we remain wedded to the idea of amino acids being necessary for the generation of any kind of life at all.
The Drake Equation
The likelihood of intelligent life becomes a little more comprehensible because of the Drake Equation, an invention of leading American astronomer Frank Drake. The equation estimates the number of communicating technological civilizations in the Milky Way by taking seven factors into account: the formation rate of life-friendly stars; the fraction of those stars with planets; the average number of life-friendly planets per solar system; the fraction of those planets where life evolves; the fraction of those planets where intelligence evolves; the fraction of those planets where interstellar communications capability evolves; and the length of time those civilizations remain detectable from Earth. Although the equation seems to paint an unpromising picture (one can discern the numbers of planets growing fewer and fewer at each step), the outcome is realistic rather than hopeless, as it estimates the existence of between two civilizations and 280 million civilizations in the Milky Way with the ability to communicate. Even two is a wonder, and the possibility of some 280 million others is downright heartening.