Are we alone?

Are We Alone in the Universe?

Are we alone in the universe? So far, the only life we know of is right here on Earth. NASA is exploring the solar system and beyond to help us answer fundamental questions about life beyond our home planet. From studying the habitability of Mars, probing promising “ocean worlds,” such as Titan and Europa, to identifying Earth-size planets around distant stars, our science missions are working together with a goal to find unmistakable signs of life beyond Earth (a field of science called astrobiology).

How Likely is it That We're Alone?

There is a common belief among astrophysicists and other scientists that studying the universe has revealed our own planet as something less than special. The reasoning is as follows: Earth, long assumed to be stationary and unmoving, is just one of many planets orbiting our sun. Our sun is nothing more than a regular, nondescript star, one of hundreds of billions found within the Milky Way. The Milky Way itself is just one of an estimated 2 trillion galaxies strewn across the expanse of our observable universe. As our own insignificant home, Earth, is teeming with life, including intelligent and technologically innovative human beings, wouldn’t it be reasonable to infer that whatever is common here is plentiful throughout the universe?

According to this default assumption, the same ingredients found here—elements, molecules, and various favorable conditions—can be found practically everywhere we look. The same physical rules that apply here are no different elsewhere in the universe. Given all the stars, planets, and chances for life that surely exist within our galaxy and beyond, we’ve mostly stopped asking whether life exists beyond Earth. Instead, we now ask how common it may be.

But for all this impressive theorizing, the best evidence hasn’t matched expectations. Despite decades of searching, we haven’t detected even a single robust signal that indicates the presence of intelligent aliens. This conundrum is commonly known as the Fermi paradox, after the famed physicist Enrico Fermi. It goes like this: If the ingredients for life are everywhere, and there are astronomically large numbers of stars and planets where it’s possible for life to have arisen, then we’d expect many instances in which intelligent aliens rose to prominence well before the advent of human life on Earth. Such beings should have had plenty of time either to have colonized the galaxy or designed a broadcasting system that would be unmistakable as a sign of intelligent life. Yet we haven’t discovered a shred of credible evidence favoring the existence of intelligent extraterrestrials. 

 Where is everybody If the universe is teeming with life?

While we certainly owe it to ourselves to look for their presence with all the resources we can muster, we must confront the possibility that perhaps we’ve got it all wrong about just how common life in the universe is. Perhaps the ingredients and conditions on Earth don’t inevitably lead to life arising on a potentially habitable world beyond our planet. And even if life does arise elsewhere, it may be the case that it frequently fails to thrive. Maybe it’s the case that even successful life only rarely becomes complex, differentiated, or intelligent as we understand those terms. Or, quite possibly, it’s exceedingly rare that even intelligent life becomes technologically advanced. In all of space, as far as intelligent life goes, perhaps humanity is truly alone.

The first scientific estimate concerning the number of intelligent, spacefaring, communicative extraterrestrials came from the American astronomer Frank Drake. His method of constructing estimates for the number of intelligent extraterrestrial civilizations—developed in 1961—gave rise to what’s now known as the Drake equation.

Although his estimates—and even his framing of the problem—are outdated today, we no longer rely on the degree of guesswork we once did. In the decades since Drake first set about his task, we’ve surveyed the vast abyss of the distant universe and discovered many important things. We’ve learned the size of the observable universe and the duration of time since the hot Big Bang. We know that there are at least 2 trillion visible galaxies. We now understand star formation, stellar populations, and how stars burn through their fuel and die. We know that over the entire cosmic history of the observable universe, there have been approximately 1024 unique stars.

That’s our starting point for estimating the number of chances that the universe must have produced Earth-like life. 

So are We Really Alone?

In the absence of evidence, all we have is speculation.

 For nearly 60 years, humanity has earnestly searched for life beyond Earth. We’ve attempted to quantify the odds of there being life elsewhere in the universe, and, more specifically, of intelligent, spacefaring extraterrestrials. Yet, for all of our efforts, we have yet to produce a meaningful estimate that’s anything more than guesswork. We do not know if there are millions of extraterrestrial civilizations thriving throughout the galaxy, or whether in all the visible universe, there’s only us.

When we ask the big question—“Where is everybody?” —it’s worth keeping a great many possibilities in mind. Aliens might be plentiful, but perhaps we’re not listening properly. Aliens might be plentiful, but they might self-destruct too quickly to maintain a technologically advanced state. Aliens might be plentiful, but they may choose to remain isolated. Aliens might be plentiful, but they might purposely choose to exclude Earth and its inhabitants from their communications. Aliens might be plentiful, but the problems of interstellar transmission or travel might be too difficult to overcome. 

But there’s another valid possibility that we must keep in mind, as well: Aliens may not be there at all. The probability of the three vital leaps, as described above, is enormously uncertain. If even one of these three steps is too cosmically improbable, it may well be that in all the universe, there’s only us.