It seems that every few months newspaper and magazine headlines will declare something like “Scientists have discovered an Earth-like planet.” Such headlines probably lead the reader to imagine a planet with an environment much like we see on Star Trek or Star Wars where we could land our spaceship, take off our spacesuit, and frolic around the countryside. But what do scientists mean when they say that they have discovered an “Earth-like” planet? How likely is it to find a planet that can support higher life forms (defined as anything more complex than bacteria)? Are planets like the earth common or rare? Let’s explore the answers to these questions.
When scientists say that an Earth-like planet may have been discovered, they actually mean one of three things. Either (1) the planet is in such an orbit around its central star that allows the temperature on the planet to possibly harbor liquid water, or (2) the planet is about the same size as the earth, or (3) the planet is solid and rocky rather than gaseous. Of course any one of these criteria, or even all three, does not actually give us a true Earth-like planet. We know that our moon is in the correct location to contain liquid water, but it is not “Earth-like.” We know Venus is about the size of the earth, but it is not “Earth-like.” We know that Mercury is rocky and not gaseous, but it is not “Earth-like.” So none of these criteria really give an Earth-like planet. Headlines and sound-bites are not meant to be precise but to draw attention, and it is much more exciting to proclaim an “Earth-like” planet has been found rather than a “Venus-like” planet (if even that could be claimed).
Some scientists have cautioned their colleagues about making unsubstantiated claims regarding the habitability of planets, including an article by Leonard Davis at science.com on December 13, 2017 or an article by Mike Brown in the October 26, 2012 Wall Street Journal. Nevertheless, you can be fairly certain that such headlines will persist.
An enlightening book on this subject is Rare Earth: Why Complex Life Is Uncommon in the Universebby Peter D. Ward and Donald Brownlee, published in 2000. One of my favorite chapters in the book is titled “The Surprising Importance of Plate Tectonics” which documents why plate tectonics is required in order for complex life to survive. Having lived 23 years of my life in California, I am well acquainted with the consequences of plate tectonics, but had no idea that such activity was crucial for my survival. Ward and Brownlee document how plate tectonics not only forms and maintains continents, but promotes biological diversity, regulates global temperature, and helps maintain a planetary magnetic field. They write, “It may be that plate tectonics is the central requirement for life on a planet and that it is necessary for keeping a world supplied with water,” (p. 220).
The astrophysicist Hugh Ross has done a rough estimate of the probability of finding a single planet that could support even simple unicellular life for a sustained period of time. Including correlations and longevity factors, and assuming there are 10 billion trillion planets in the visible universe, he concludes that the probability of finding a single planet that could support unicellular life for a prolonged period of time is 1 in 10556. If this informed estimate is even close to being correct, then there are no other planets in the visible universe that can support life.
Of course, from a theological viewpoint, if God created one planet capable of supporting life he could have created many more. From a biblical viewpoint we are given no information about whether or not God made other life forms that live within our physical universe. (Although we are told there are other created beings like angels that can interact with our universe.) So, from a theological perspective, these kinds of probability estimations neither limit the possibilities or challenge any biblical doctrines, whether or not other complex life is ever discovered in the universe. But these informed probability estimates do make a strong case that the habitable Earth is not an accident and that there seems to be both a designer and creator behind our unusual planet.
A careful examination of the requirements for an Earth-like planet give more evidence that our universe was designed and created by an intelligent creator who has carefully crafted a home for humanity. Over and over again, the findings of modern science give credibility to the postulate that there is a God who has a plan for humans. Contrary to the rhetoric that claims that because there are a lot of planets many of them must be habitable, the known required characteristics of a habitable planet set stringent limits on the probability that any of the thousands of known extra-solar planets, or the trillions of planets out there, actually support any kind of complex life. So the next time you read that an “Earth-like” planet has been found, you can marvel at the amazing ability humans have to discover the wonders of the universe including the many rocky, earth-sized planets out there—planets that are almost certainly devoid of any life, but interesting places nevertheless—planets that emphasize the rarity and distinctiveness of our home the Earth.