Much of the story of life on planet Earth has been due simply to luck. This is particularly true of the physical environment which has allowed evolution to produce such a diversity of species.
One day in 1950 over lunch with his scientific colleagues, physicist Enrico Fermi heard someone speculate about how many advanced civilizations there must be out in space. Fermi quipped, “So, where are they?” He meant that if there were many advanced civilizations, some of them must be more advanced than we are, and must have invented space travel—and at least some of them should have contacted us by now. This has come to be known as “Fermi’s Paradox.” One answer to this paradox is that there are so few advanced civilizations in the universe that they have not found us yet and probably never will. According to this view, Earth-like planets might be very rare. Very few planets have been as lucky as Earth. According to the “Rare Earth Hypothesis” of planetary scientists Peter Ward and Donald Brownlee, we can begin by thanking our lucky star, the Sun.
First, the Sun is a calm and stable star. Many stars fluctuate wildly in their energy output. Such pulsations in energy may prevent life from ever getting started on any planets that revolve around variable stars. In contrast, the Sun has been stable for billions of years. Not perfectly stable, of course. The Sun has had occasional “coronal mass ejections,” in which it propels energy and particles from its outer layer out into the Solar System. One of these mass ejections, on September 1, 1859, was strong enough that it shut down the telegraph systems in the United States and Europe and caused auroras to occur in the skies of even tropical regions. The Sun also has an 11-year sunspot cycle. These variations, however, have not had much effect on Earth. Coronal ejections have not been known to have ever harmed life on Earth (no organisms were harmed when the telegraphs shut down), and solar intensity shifts by only 0.1 percent during the sunspot cycle. The Sun has, in fact, changed its energy output over the billions of years of its existence. It has increased the intensity of its radiation by about 30 percent during that time—but it has done so very gradually.
Second, the Sun is an isolated star. Many stars have partners, forming multiple-star systems—most commonly, binary systems in which two stars swing around each other like dancers. If the Sun were part of such a close family of stars, the other stars would prevent planets from having stable orbits, which might prevent the evolution of life. The Sun is also far away from stars that emit so much energy that they would disrupt or destroy life. For example, a supernova anywhere within a few dozen light years of Earth would wipe out all of life—but there have been no supernovae in the Sun’s neighborhood for at least several billion years. Moreover, stars in the centers of galaxies may be so close together that they would disrupt the revolution of one another’s planets, even if they are not part of multiple-star systems. But the Sun is on a swirling arm far from the center of the galaxy. If we were near the center of the galaxy, many stars would be so close to us that night would not be very dark, and those stars would yank and tug us around and disrupt the stability of our planet’s conditions.
Therefore, it appears that complex life would not have had time to evolve on planets that revolve around most stars. Earth is not just a special planet in our solar system, but in the universe. The story of evolution, as presented in this blog, might be something quite rare in the universe.
The foregoing was adapted from a portion of my upcoming book, Life of Earth: Portrait of a Beautiful, Middle-aged, Stressed-out World, soon to be released by Prometheus Books.