Earth is the only planet known to harbor life. A few other places in our solar system—e.g. oceans beneath the icy surfaces of Jupiter’s moon Europa and Saturn’s moon Enceladus—could potentially house aquatic life,See this NASA press release for more information about the potential habitability of Europa and Enceladus. and some scientists speculate that microbial organisms (microscopic organisms like bacteria) may have lived on Mars in the distant past.See this NASA page for more information about the search for evidence of microbial life on Mars. So far, however, there is no direct evidence to support these conjectures.
On the other hand, the sun is not the only star with planets. More than 5,000 planets—or exoplanets, as they are called when located outside our solar system—have been discovered orbiting other stars, and the number is increasing rapidly as new discoveries are added to the list.As of November 10, 2022, a total of 5,265 confirmed exoplanets are listed in The Extrasolar Planets Encyclopaedia Catalog. Many of these exoplanets are classified as earth-like, although that term is somewhat misleading: it simply refers to any exoplanet that is similar in size and density to Earth, and which is located in the habitable zone—that is, it orbits its star at a distance that could potentially allow liquid water to exist on the planet’s surface. We do not yet have telescopes powerful enough to determine whether there actually is water on any of these so-called “earth-like” exoplanets, much less whether they harbor any life.For further discussion of why the term “earth-like” is misleading, check out this video by PBS Spacetime. I also recommend this Scientific American article by astrophysicist Caleb A. Scharf, Director of Astrobiology at Columbia University. According to recent estimates, however, there may be tens of billions of earth-like planets in our own Milky Way Galaxy, and surely there are countless more in other galaxies throughout the universe.See this New York Times article for further discussion of recent discoveries and estimates. For an example and the mathematical details of one such estimate, check out this article in the Proceedings of the National Academy of Sciences. It seems overwhelmingly likely that at least some of these planets have liquid water and the numerous other features necessary to sustain biological life.
For an explanation of the methods used to detect exoplanets, check out this Crash Course Astronomy episode. Want to help find exoplanets? You can join hundreds of thousands of other volunteers who are analyzing data from NASA’s Kepler space telescope and Transiting Exoplanet Survey Satellite (TESS) to search for undiscovered exoplanets using the transit method. See this tutorial for instructions.
Could there be life outside our solar system, on some distant exoplanet? Might there be other intelligent civilizations, perhaps even more technologically advanced than we are? Many astronomers think so. In 1961, American astronomer Frank Drake and a dozen other scientists met to discuss a newly emerging field of scientific inquiry: the search for extraterrestrial intelligence (SETI). Making an educated guess about the number of habitable planets in our galaxy, and some assumptions about the likelihood that life would spontaneously arise (via evolution) on any planet that could support it, Drake and his colleagues tried to estimate how many alien civilizations might be out there. They predicted that the number of extraterrestrial civilizations in our galaxy should be in the thousands at least, possibly in the millions.
Drake obtained his famous estimate by multiplying together a number of relevant factors, like the number of sun-like stars and the proportion of those stars thought to have earth-like planets. Most factors in his equation, which came to be known as the Drake equation, are unknown, so the calculation was based mostly on speculative guesses. Drake recounts the story of that historic meeting and explains the reasoning behind the prediction here.
Drake’s prediction inspired a frenzy of research aimed at detecting evidence of extraterrestrial civilizations, including two NASA-sponsored SETI initiatives and numerous privately funded SETI programs.In 1971, NASA developed plans to build a huge array of radio telescopes to look for alien radio signals. This project, codenamed Project Cyclops, was cancelled due to the high cost: the projected budget was $10 billion. A second NASA project, the Microwave Observing Program (MOP), used existing radio telescopes to look for alien signals in the microwave spectrum. For more on the history and significance of SETI research, check out this National Geographic article written by Frank Drake’s daughter, Nadia Drake. The United States Congress later cancelled funding for SETI research, but a number of radio telescope facilities in other countries around the world are still listening for extraterrestrial signals.See here for more information about recent SETI research. So far, however, no sign of alien life has been detected. The fact that no extraterrestrial civilizations have been detected despite the apparent likelihood of their existence is called the Fermi paradox.
There are many possible explanations for the Fermi paradox. Perhaps we’ve been looking for the wrong sorts of signals, or maybe our technology is just too weak to detect them. Perhaps alien civilizations usually conceal themselves to avoid being attacked by other aliens. Or, perhaps intelligent life doesn’t arise spontaneously on its own as Drake and his colleagues assumed.
In any case, the possibility of life on other planets raises interesting questions for Christians to consider. Might God have created life elsewhere in the universe? If extraterrestrial life were discovered, what would be the theological implications of that discovery? If there are intelligent civilizations living on other planets, is God’s relationship with them similar to his relationship with us? Are they made in God’s image, as we are, and did Jesus die for them too? I don’t know the answers to any of these questions, of course, but it’s fun to speculate about what other things God may have created in this vast universe.
Christian theologians have debated these questions for centuries. During the medieval period, the Catholic Church officially recognized belief in extraterrestrial life as compatible with Christian orthodoxy. Likewise, some of the 16th-century Protestant Reformers (especially Lutheran theologian Philip Melanchthon) accepted the possibility of creatures living elsewhere in the cosmos, and by the 18th century a consensus had emerged among Christian theologians that extraterrestrial life was not only possible but likely to exist. Nevertheless, there has never been a consensus on the theological questions raised by that possibility.For a brief but fascinating summary of the history of this conversation, I recommend Lecture 34 of Frederick Gregory’s course on “The History of Science: 1700–1900” in The Great Courses lecture series. For a more detailed discussion, see Michael Crowe, Extraterrestrial Life Debate, 1750–1900 (New York: Dover
Publications, 1999) and Michael Crowe (ed.), Extraterrestrial Life Debate, Antiquity to 1915: A Source Book (Notre Dame: University of Notre Dame Press, 2008).
Here’s my opinion, for what it’s worth. Our planet may or may not be the only place where God said “let the earth bring forth living creatures” (Genesis 1:24), but I find it hard to believe that He would leave all but this one little speck of the universe barren and sterile. Given what the Bible reveals about God’s creativity and His love for living creatures, I can easily imagine that He created unfathomably diverse creatures on countless planets throughout the cosmos. We already know that God created a majestic variety of living organisms on this planet, and we also know that He created other intelligent creatures besides human beings: angels, cherubim, seraphim, and other mysterious and wonderful creatures described in scripture. Angels are spiritual beings rather than physical creatures, presumably; but given that we’re not the only intelligent creatures God made, surely it’s not out of the question that He created other forms of intelligent biological life as well. As to the theological implications of this possibility, I am less inclined to speculate. Maybe God’s relationship with them is the same as His relationship with us, or maybe His ways of dealing with other creatures are as diverse as the creatures themselves. That’s really none of our business. In any case, the discovery of extraterrestrial creatures would only increase my appreciation for the mystery, beauty, and grandeur of creation.
Speaking of mystery, beauty, and grandeur, let’s turn now to some of the most exotic—though non-living—things in the universe: black holes, dark matter, and dark energy. The discoveries of these strange entities have given rise to some of the greatest unsolved mysteries in astronomy and astrophysics, as we’ll see in what follows.