The Plurality of Worlds

The following post was written by Professor Stephen Kane, who has been researching planets around other stars for almost 20 years and has discovered and characterized hundreds of exoplanets, including Kepler-186f - which is the smallest planet yet to have been found in the Habitable Zone of a star. After spending many years working at the NASA Exoplanet Science Institute, Kane is now a Professor of Astrophysics at San Francisco State University.

“The Earth is dying. As the caretakers of this once beautiful planet, we have made many errors along the way. Some of these have been correctable, but the Great Mistake has left a scar across this land that will never heal. We must leave, but where shall we go to begin again? Science is the source of our redemption. Over the past 200 years our scientists have discovered many possible places of refuge. So we begin again, amongst the stars.”

The world of Civilization: Beyond Earth poses the dramatic scenario in which the human race travels to various planets on which to settle. But how realistic is this scenario and how many places will we have to choose from 200 years from now? Predicting the future of science and technology is always a perilous task. After all, the science fiction of mid-twentieth century depicted humans already living on other planets by now, at least within our own Solar System. In 1996 I attended one of the first conferences held to discuss the recent planet discoveries. At that conference, NASA released a handbook called a “A Road Map for the Exploration of Neighboring Planetary Systems”. That handbook attempted to predict the next 10 years of exoplanet discoveries in the below table.

Almost everything in this table turned out to be incorrect. The reason these predictions, along with most predictions involving the future of science and technology, are wrong is because they extrapolate the current technology and don't take into account new ideas, new instrumentation, and difficult hurdles that need to be traversed. For example, the planet detection method of “microlensing” turned out to be far more difficult than originally imagined, whereas the transit method (successfully used by the NASA's Kepler telescope) is nowhere to be seen. Even as I write this, new ideas are being proposed to both detect and characterize planets as small as the Earth outside of our Solar System. Like all active areas of scientific research, this is a topic that is in a constant state of flux.

One of the ways we can see this is to look at the rate of exoplanet discoveries over the past couple of decades. At this point in human history we are aware of more than 1,700 planets outside of our solar system. However, the number of known planets has not increased uniformly with time. The below histogram shows the number of exoplanet discoveries per year and color-coded by the different methods which are used to discover them. This single histogram shows how chaotic the brief history of exoplanet discoveries has been, with detection rates of various methods rising and falling and then an enormous spike in discoveries this year due to results from the Kepler telescope being released. Clearly we can never predict what will happen from one year to the next.

Having said all that, I will attempt to project forward 200 years with an estimate of how many potential new homes humanity will have to choose from. The reason I will do that is because of some important pieces of the exoplanet puzzle that have come to light. Firstly, we know that almost all, if not all, stars have planets and that they are a natural consequence of star formation. Secondly, we know that smaller terrestrial planets like the Earth are far more common than giant planets like Jupiter. Armed with these two facts, I will stare into the crystal ball of possible future outcomes.

Knowing that there are many Earth-like planets out there provides an enormous drive towards building better telescopes and developing more sensitive techniques to find them. Pending government spending on this science and technology, it isn't something I see going away anytime soon and there are already several new telescopes planned which will contribute a great deal towards further discoveries. Over the next decade alone, NASA will launch a further exoplanet discovery telescope called TESS and the successor to the Hubble Space Telescope called the James Webb Space Telescope (JWST). Meanwhile, the European Space Agency (ESA) will be launching two exoplanet telescopes (CHEOPS and PLATO). On the ground, the race is on to finish construction of the next generation of giant telescopes, including the European Extremely Large Telescope (E-ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT). These facilities will be capable of both discovering and characterizing a multitude of planets that are not accessible with current telescopes. With these tools in place over the next decade, I expect that the number of known exoplanets will rise above 10,000.

Beyond 10 years I see that the discovery of Earth-size planets will become commonplace. The decades after that will focus on space-based telescopes designed specifically to take detailed pictures of these planets and attempt to confirm which out of these might actually be habitable. I predict that 50 years from now, the number of known exoplanets will be around 100,000 with most of those being terrestrial. If only a relatively small fraction of those have the right temperatures for liquid water oceans, then that is already a large number of possible abodes. After that I predict that the discovery of new habitable worlds will go through a period of “stamp collecting”, where there are so many known that the focus on finding new ones decreases and the detection rate slows down. Instead the human race will focus on the issue on traversing the large distance between worlds and explore the notion of expanding the human race to new lands. Discoveries will continue though and 150 years from now we will know of more than 1 million planets and thousands of habitable planets within 100 parsecs of the Earth. It's hard to say how the Great Mistake will affect the progress of scientific research, but I will assume that it will lead to a general decline of active research on exoplanets as well as many other science disciplines.

Based on these projections, the world of Civilization: Beyond Earth will be one in which the people take for granted that there are more places to move to than can easily be counted and that, if needed, many more could be discovered and explored. Thousands of years ago, Greek philosophers such as Epicurus popularized the idea of the “Plurality of Words”, hypothesizing that there are an infinite number of other planets that are habitable and, in some cases, inhabited. Our journey on this path is just beginning and I have pointed out the difficulty in predicting the future on topics such as this. However, I can say that our knowledge of exoplanets will continue to expand and our thirst for finding other Earths will not expire anytime soon. I fully expect that the next hundred years will likely show that the plurality of worlds is not only real, but essential for the survival of the human race.