By Fabo Feng
Humans have always wondered about their place in the universe—specifically, whether we are alone. And in today’s scientific age, we have added more details to that initial question. We wonder, why is there life on Earth rather than on Mars or planets around other stars? Is life common or rare? Are there intelligent beings living on other planets? If so, then those planets likely require Earth’s marbled appearance—including 71% of its surface comprised of water—in order to be habitable, according to a recent study.1 We’ll explore that discovery after some foundational work on planetary habitability.
Current Research Shows Life Is Rare
Some of these questions have become scientifically addressable since the detection of the first planet around main sequence stars in 1995.2 Since then, thousands of planets have been discovered to orbit around other stars. Thus, many people hope that future space-based telescopes such as the James Webb Space Telescope (JWST) or large ground-based telescopes such as the European Extremely Large Telescope (ELT) will usher in an era of Earth-sized planet discoveries and their potential signatures of life.
However, a survey of 1,327 stars seemed to dampen hope when scientists could not detect radio technosignatures (evidence of past or present technology).3 This radio interference detection seems to suggest that the rate of extraterrestrial intelligence (ET) is rare. Nevertheless, astronomers continue to survey other technosignatures to provide conclusive evidence of the occurrence rate of ET.
The Fermi Paradox
Famous physicist Enrico Fermi first proposed the rare ET hypothesis decades ago in a lunch talk with colleagues. He said, “Where are they?” in response to the apparent contradiction between the lack of evidence for, and high probability estimates of, many civilizations in the Milky Way Galaxy. Why don’t we find any evidence of their existence—particularly, if they are advanced enough to travel to Earth? Astrophysicist Michael Hart further exacerbated this concern in his 1975 thesis.4 There Hart examined and criticized the physical, social, psychological, and timing arguments for the high occurrence rate of ET. He concluded that humans are probably the only intelligent beings in our galaxy.
Not all astronomers have adopted that conclusion. Many continue to work toward a scientific resolution of the Fermi paradox. And while researchers uncover evidence for habitability, a just-right planetary land-ocean ratio seems to be another constraint for life.
Just-Right Amounts of Water and Land
A recent study of the amount of water on temperate planets in the habitable zone (where liquid surface water can exist on a planet) shows that about 95% of them would have more than 90% ocean coverage.5 By contrast, Earth’s waters cover about 71% of its surface, close to the water world limit corresponding to the basin saturation on the surface of a planet. In other words, Earth exhibits a balance between the volume of water it retains and the capacity of its oceanic basins. The author concludes that this rareness is due to an anthropic selection effect (humans observe it because we happen to be here) or just because we are very lucky.
This research indicates that human beings can originate and survive only on a planet with a large landmass (29% is large compared to other planets in the liquid water habitable zone). From an evolutionary point of view, if the landmass is small, few species would survive and there would not be enough genetic diversity to allow intelligent beings to emerge. Another reason for a large landmass is species’ resilience to catastrophic events. For example, during a glaciation period, animals and plants need to migrate from cold habitats to warm ones to survive. However, without sufficient land to be migrated to, these species would become extinct. Thus, landmass is crucial for the origin and survival of species.
Since most temperate planets would be uninhabitable water worlds, humans find themselves living on a planet which is close to the water world limit. Too much water limits habitability, but Earth’s water-to-landmass ration seems just-right. This so-called Goldilocks principle can be illustrated by the example of filling an empty cup with an arbitrary amount of water. Since the capacity of a cup is limited and fixed, it is easy to fill the cup and overflow it. If it is essential for a person to hold the cup without spilling, then a successful filling would be close to a 75% limit. (This Goldilocks principle can also be observed in the curvature of the universe, in the size of the earth, in the architecture of the solar system, and in the physical properties of the Sun.)
Fortune or Foresight?
Instead of viewing this Goldilocks principle as a selection effect—or sheer luck—Christians would assert that it is a scientifically affirmed creation principle, or a design rule. Consider the tragedy of an air disaster in which only one person survives. Even though the passenger would say he is lucky, he would still be surprised to learn how so many small-probability factors led to his survival.
Rather than take humanity’s survival and flourishing on Earth for granted, we might ask a deeper question: Why are we so “lucky”? As a Christian, I find scientific discoveries to be deeply consistent with a Christian worldview. In that worldview, humans have been specially created in a state of innocence on a special planet. However, humans have fallen from their original state and can be reconciled to their holy, loving Creator by receiving the atoning work of a special Savior, the incarnate God, Jesus Christ.