By Josef Eisinger©
In Part One of this article, I posed the question: Is our story of life on Earth unique or did life also develop elsewhere? I also discussed some of the observations and laws of physics that affect the likelihood of life emerging and evolving on another planet. In this segment, I explore the habitability of Earth’s sister planets and of planets orbiting stars in our immediate neighborhood of the Milky Way.
We might 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, its orbital radius must be in a fairly narrow 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 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, however, by no means the only requirement for life to evolve. Thus, a habitable planet must also be large enough to maintain an atmosphere that retains heat (but not too much, we Earthlings now say!) and it should also be surrounded by a magnetic field that protects it from space radiation. That implies, in turn, that a habitable planet should be young enough to have retained a liquid core, whose slow motion generates the shielding magnetic field—as is the case for 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. Our sun, aged about 5 billion years, is fortunately only in her middle-age. The age of Earth is estimated to be 4.5 billion years—and because that is a time span too vast to be easily grasped, it is useful to convert it to one in which the Earth’s age is equivalent to just one year.
According to that make-believe time-scale, primitive life appeared on Earth only a month after the planet was formed and then evolved slowly in water. It took indeed 10 more months—i.e. until a month ago—before land animals began to inhabit Earth. About two weeks ago, dinosaurs roamed the Earth, and they were, in time, succeeded by mammals among other species, and eventually by that optimistically-named species, Homo sapiens, which appeared on Earth about four hours ago. 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 which is aptly named Proxima Centauri—might be accompanied by one or more planets. Although Proxima Centauri is only about four light years from us, its luminosity is so low, that it is not visible to the naked eye. 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. We learn from spectroscopic observations that Proxima b has a mass very similar to that of Earth, that its orbital period is just 11.2 days, and that it is much closer to its star than Earth is to the sun. Because it is so close to its dim star, the planet receives almost as much radiant energy as we do from the sun. It is, nonetheless, unlikely to be hospitable to life because the frequent flares emitted by its uncomfortably close star probably blew the planet’s atmosphere away a long time ago.
Fortunately, the number of stars with potentially habitable planets is so huge that astronomers ought not be discouraged by failures to find extraterrestrial life. On the basis of a recent survey of stars conducted with 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!
Josef Eisinger, Professor Emeritus at the Mount Sinai School of Medicine in New York, is the recipient of two Guggenheim Fellowships, and the author of more than 150 scholarly articles that range from nuclear physics and molecular biology to the history of science. While at Bell Laboratories, Eisinger’s research base for 30 years, he and colleagues developed the Hematofluorometer—a device for diagnosing lead poisoning. Eisinger is also the author of Einstein on the Road; Einstein at Home (Prometheus Books 2011, 2016); and his memoir, Flight and Refuge: Reminiscences of a Motley Youth (Amazon 2016).