What is the chance that extra-terrestrial life exists? It is a question that we astronomers often get asked, and I always respond by saying that we have no idea. To predict the probability of something occurring, you need to start with some relevant, prior knowledge. For instance, meteorologists can figure out how likely it is to rain tomorrow because they already have lots of data about weather patterns. But in the case of predicting how common it is for life to arise in the universe, we currently only have one datum: the existence of life on earth. That’s just not enough.
However, we are getting closer to having the extra data we need. On June 19, NASA announced that its Kepler space telescope team had identified 219 new planet candidates. This followed on the widely-covered February report by another team of scientists that they had discovered seven earth-sized planets, all orbiting a single star named “TRAPPIST-1.”
We assumed that planets orbiting other stars would look like the planets in the solar system. What we have found is that reality is much richer than we imagined.
What makes these exciting news stories is not the discovery of planets per se—thousands of extra-solar planets have been cataloged over the past 25 years—but that a few of them belong to a much shorter list of planets that are potentially habitable. They are similar in size to earth and, crucially, orbit their stars at a distance that could result in just the right temperature to host liquid water, a prerequisite for the only kind of life we know. The Kepler team has 10 new candidates of this kind (which still need to be followed up), while TRAPPIST-1 can boast three confirmed planets in the sweet spot.
According to Vatican Observatory astronomer Paul Gabor, S.J., an expert in planetary detection, the TRAPPIST-1 discovery is particularly tantalizing because so many Earth-sized planets have been found around such a small star: TRAPPIST-1 is only one 12th of the sun’s mass, roughly the size of Jupiter. “Thirty years ago,” Father Gabor comments, “we assumed that planets orbiting other stars would look like the planets in the solar system. What we have found is that reality is much richer than we imagined.” This richness has its benefits. Because TRAPPIST-1 is so small and dim, its habitable zone is much closer in, and its planets consequently complete their orbits in a matter of days. Detecting planets with such short orbital periods is more practicable because you do not have to observe their stars for long periods of time. Other small stars could thus provide rich terrain for Earth-like planet-hunting.
Still, to return to my initial point, we have no idea whether there might be life on these planets. They could be barren rocks with empty oceans. But scientists are already forecasting that if any of them have atmospheres, the James Webb Space Telescope, due to be launched next year, will be sensitive enough to detect atmospheric chemicals typically produced by biological processes, like oxygen and methane. And that could finally give some empirical grounds for inferring the existence of extraterrestrial life. Father Gabor says that such results would probably be “tenuous”—it is a really hard measurement to make—but that it could encourage funding for more advanced space telescope instrumentation to investigate further.
Right now, all we can do is speculate about what alien life might be like. And we cannot merely extrapolate from our knowledge of life on Earth. The bodies of alien species could be wildly different. Further, we cannot say whether evolution on other planets would result in higher forms of life analogous to those we have on Earth. It could be that life is common but intelligence is rare. And even if there are other rational creatures, their patterns of thought could be totally different from ours. Last year’s film “Arrival,” for instance, imaginatively explored the difficulties that might be involved in communicating with intelligent aliens.
These are only some of the questions to ponder about alien life, and that is before you even get to the religious issues—before you can ask the titular question of the recent book by Father Gabor’s colleagues, Guy Consolmagno, S.J., and Paul Mueller, S.J., Would You Baptize an Extraterrestrial? (Their answer in brief: It depends.)
So, it is too early for the Holy See to erect a Dicastery for Extraterrestrial Life. But at least in the meantime the Catholic faith has already made a concrete contribution to the search for alien life, albeit indirectly: The Belgian leaders of the recent study say that the mission acronym TRAPPIST was inspired by the beer brewed by the monks of the same name.