Last month, astronomers from the Kepler astronomical satellite team announced the discovery of 1,284 new planets. They all orbit stars outside our solar system. The total number of such "exoplanets", whose existence was confirmed thanks to the Kepler telescope and other means of search, today exceeds three thousand.
This is a real revolution in our knowledge of the planets. About ten years ago, the discovery of a single exoplanet immediately became a scientific sensation. Much has changed today. Thanks to the improvement of techniques and technologies for astronomical observations, we have moved from retail to wholesale planetary discovery. For example, we now know that every star in the sky in orbit has at least one planet.
But the planets are just the beginning of history. Everyone wants to know if these worlds are inhabited by aliens. Does our newfound knowledge of the planets help us get closer to the answer to this question?
In fact, yes, albeit a little. In the May issue of Astrobiology, I, together with astronomer Woodruff Sullivan, published a paper in which we show that, although today we do not know anything about the existence of advanced extraterrestrial civilizations in our galaxy, we have enough information to conclude that that they probably existed at some point in cosmic history.
In the scientific community, there is the so-called Drake equation. This is a formula used to determine the number of extraterrestrial civilizations in the galaxy with which we have a chance to come into contact. In 1961, astronomer Frank Drake was asked by the National Academy of Sciences to hold a scientific meeting on the possibilities of "interstellar communication." Since the chances of establishing contact with alien life are determined by the number of advanced extraterrestrial civilizations that exist in the galaxy, Drake deduced seven factors that this number depends on and included them in his equation.
The first factor is the number of stars born per year. The second is the proportion of stars with planets. Then comes the number of planets per star that orbit in places where life can form (assuming that life requires liquid water). The next factor is the proportion of planets where life actually originated. And there is also such a factor as the proportion of planets where life has developed to a reasonable one, and advanced civilizations have appeared (capable of emitting radio signals). And the last factor is the average lifespan of a technically advanced civilization.
Drake's equation is completely different from Einstein's formula E = mc2. This is not a statement of a universal law. It is a mechanism for facilitating an organized discussion, a way to understand what we need to know to answer the question about alien civilizations. In 1961, only the first factor was known - the number of stars forming annually. We remained in such ignorance until recently.
This is why discussions about extraterrestrial civilizations, no matter how scientists they are, have long been reduced to the usual expressions of hope or pessimism. For example, what is the fraction of planets where life is formed? Optimists may come up with complex molecular biological models, claiming to be great. Pessimists, on the other hand, cite their own scientific data, arguing that this proportion is closer to 0. But since we have only one example of a planet with life (our own), it is rather difficult to understand which of them is right.
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Or let's think about the average lifespan of a civilization. Humans have only been using radio technology for about 100 years. How long will our civilization last? A thousand years? One hundred thousand? Ten millions? If the average lifespan of a civilization is short, the galaxy is most likely unpopulated most of the time. But again, we can only use one example, and that again brings us back to the battle between pessimists and optimists.
But our new knowledge of planets has removed some of the uncertainty from this debate. Three of the seven factors in Drake's equation are known today. We know the number of stars that are born each year. We know that the proportion of stars with planets is about 100%. And we also know that 20-25% of these planets are in a place where life can form. Thus, for the first time, we can say something definite about extraterrestrial civilizations - if we ask the right questions.
In our most recent work, Professor Sullivan and I shifted the focus of the Drake equation. Instead of asking how many civilizations there are at the moment, we decided to find out what the likelihood is that our civilization is the only technically advanced one that ever appeared. By asking this question, we were able to bypass the factor of the average life span of a civilization. Thus, we have only three unknown factors, which we have reduced into one "biotechnical" probability: the chances of the emergence of life, the chances of the emergence of intelligent life and the chances of technical development.
It can be assumed that such a probability is small, and therefore the chances for the emergence of another technically advanced civilization are small. But our calculations showed that even if this probability is extremely low, the chances that we are not the first technically advanced civilization are quite high. Specifically, if the probability of a civilization on a habitable planet is less than one in ten billion trillion, then we are not the first.
Let's introduce some context to better understand the numbers. In previous discussions on the Drake equation, the probability of civilizations forming one in ten billion was considered very pessimistic. According to our calculations, even under such conditions, a trillion civilizations should have appeared in the entire history of space.
In other words, given what we know today about the number and orbital position of planets in the galaxy, the degree of pessimism necessary to doubt the existence of an advanced extraterrestrial civilization at one time or another is contrary to common sense.
An important step forward in science can be the search for a question that can be answered with data in hand. This is exactly what we did in our work. As for the most important question - are there other civilizations today - here we will have to wait a bit until the relevant data appears. But we must not underestimate the successes we have achieved in such a short time.
Adam Frank is a professor of astrophysics at the University of Rochester, co-creator of the 13.7 Cosmos and Culture blog and author of About Time: Cosmology and Culture at the Twilight of the Big Bang.