Are we alone in the universe? Humanity has been asking this question for millenia. Our creation of thousands of gods, spirits, and intelligent creatures is fueled, in part, by our intense desire for cosmic company. As long as we have been asking this question, there have been a plethora of answers. This quest for company continues today—embodied predominantly in the study of extraterrestrial life. Today our big questions take the form: “Is ET out there? If so, is it intelligent?”
With our present telescopes comes the ability to peer deeper into space than we ever have before, and the findings in recent years have been immense. This has dramatically changed the potential for extraterrestrial life. In the observable universe there are about 100 billion to 1 trillion galaxies. Spiral galaxies like our own have about 100 billion to 1 trillion stars. Elliptical galaxies like Maffei 1 have about 1 trillion to 10 trillion stars. Since elliptical galaxies make up about a third of all galaxies, the number of stars in the universe is about 100 sextillion to 300 sextillion: “or three times as many as scientists previously calculated. That is a 3 followed by 23 zeros. Or 3 trillion times 100 billion” (Borenstein, “Number of Stars”).
What does all this mean for extraterrestrial life? There are 50 billion or more planets in our galaxy. “At least 500 million of those planets are in the not-too-hot, not-too-cold zone where life could exist” (Borenstein, “Cosmic census”). This means that one of every two stars has a planet and one of every 200 stars has a planet in the habitable zone (Borenstein, “Cosmic census”). If you do the math, that's around 50 sextillion planets in the universe, conservatively. Around 500 quintillion of those are habitable. That's 100 billion times 5 billion planets that could harbor life like us.
Professor Andrew Watson of the University of East Anglia estimates that the chances of life like us occurring is “less than 0.01 per cent over four billion years” (Science 2.0) The oldest known planet is around 13 billion years old (Savage, Neal, and Villard). This means that a very rough estimate of the number of Earth-like planets with life like us on it is 16.25 quintillion, or 16,250,000 in our galaxy. Now, when I say “life like us” I don't mean that they will look a lot like humans. I mean life that shares four similar formational attributes. However, even this estimate may be far too narrow: “if life originated independently, even within our own solar system, it might have nonterran characteristics and, thus, not be detectable by NASA’s in situ or remote-sensing missions designed explicitly to detect terran biomolecules or their products” (National Academies: Space Studies Board). In other words, life could be drastically different from us and could exist in greatly varied environments.
This is all well and good, say the skeptics, but what is the likelihood of us actually coming in contact with anything intelligent if there are such life forms out there? That question is answered by the Drake Equation. “The Drake equation was developed as a means of predicting the likelihood of detecting other intelligent civilizations in our galaxy” (Astrobiology Magazine). The Drake equation takes the rate of star formation per year, the rate of production of new planetary systems per year, the number of planets in the continuously habitable zone, the fraction of potentially habitable planets that actually give rise to life, the fraction of living systems that give rise to an intelligent species, the fraction of intelligent civilizations which give rise to a technology which we might detect, and the longevity of such a civilization and returns the number of communicative civilizations (Astrobiology Magazine). Frank Drake's current solution to his equation is 10,000 such civilizations (Ford).
The final issue with extraterrestrial intelligence is if there are really that many civilizations out there that we could have come in contact with, why haven't we come in contact with them yet? This question is known as the Fermi Paradox. The problem with the Fermi Paradox is that it assumes that extraterrestrials haven't already attempted to contact us. There are a number of sufficient explanations for why they may already have been (or are) in our solar system and we simply cannot observe them yet. Freitas says, “The current observational status of the Solar System is insufficient to support the assumption that ETI are not here. Most advanced civilisations also would be either invisible or unrecognisable using current human observational methods, so millions of advanced societies may exist and still not be directly detectable by us. Thus the Fermi Paradox cannot logically be raised as an objection to the existence of ETI until these major observational deficiencies have been corrected.”
Ultimately, there is still a lot that we don't know about space, life, and intelligence, but what we do know so far is overwhelming support for the existence of extraterrestrial life. Not only are they out there, but some of them may be intelligent, some may be contactable, some may have already attempted to contact us, and some...we may yet be on the verge of discovering.