To begin, those are two significantly different questions: The first one asks whether there are aliens out there, presumably intelligent ones; the second one only asks whether life, even of the most basic type (bacteria, algae, or such), exists elsewhere.
We know the answer to the first one at least inside the solar system: There are certainly no aliens on any of our neighbor planets, moons or asteroids. In our Milky Way galaxy, many scientists, myself included, are convinced that there are no aliens, though we don’t know for sure. We are convinced because, considering the age and size of the Milky Way, any intelligent species would by now have made such huge technological advances as to populate many places, most probably with robots, thus making itself manifest.
The answer to the second question, whether there is any life elsewhere, is totally unknown, even inside our solar system. Spacecraft and probes have landed on the Moon, Mars, Venus, Titan (a moon of Saturn), and Comet Chury; they have dug in search of any signs of life, and we have yet to find a single cell.
We now follow several approaches for the search of life (advanced or primitive). For aliens, we continue to scan the sky for any radio or laser signal with artificial features. We also keep an eye on objects that enter the solar system from interstellar space, like the “cigar” asteroid that was recently discovered and named Oumuamua, and which might possibly (but very improbably) be an alien spacecraft.
For primitive life, we continue to send spacecraft with instruments that scan planets, moons, asteroids, and comets within our solar system, searching for gases that would indicate biological processes. And for the 3,700 exoplanets that we have discovered so far, we are now readying big telescopes with instruments that can detect biosignatures, which are gases in the atmospheres of those planets that could only be explained by the presence of life underneath.
We have come a long way in the exploration of both our solar system and our galaxy in the last few decades. The first exoplanet was discovered 25 years ago, and now we have thousands. To understand the feat, remember that a planet like Earth looks 10,000 times smaller than the star it orbits, and faraway stars appear like points in the sky even when seen through large telescopes. But we have developed ingenious techniques of observation and detection, and so can now attempt what has long been a hugely challenging search.
Indeed, we are at an extraordinary moment in history: In the next few years (five to 10 at the most), we shall be able to determine whether there is at least basic life elsewhere and how widespread it is. Should we discover biosignatures somewhere, we won’t know what kind of life that will be — is it built on the same biochemistry as life on Earth, is it based on DNA, or is it fundamentally different?
If we turn out to be the sole custodians of life, intelligence and consciousness, then our responsibility toward the entire universe becomes huge.
Whatever the outcome of the discoveries, even if they turn out to be negative, it will be a very important development. As my favorite science-fiction writer, the late, great Arthur C. Clarke, famously put it: “Two possibilities exist: Either we are alone in the Universe, or we are not. Both are equally terrifying.”
Indeed, even if we find the most basic life somewhere, and since we will have explored only the closest regions of our galaxy (a few thousand light-years, compared to a diameter of 100,000 light-years), it will mean that life is probably widespread. And if life is ubiquitous, then it will probably have evolved somewhere to more complex organisms, animals, and perhaps aliens. And if life exists in basic form but has not evolved in our galaxy, then it will surely have done so in some other galaxy, since there are at least a trillion galaxies, each with 100 billion stars and planets, in the Universe. And, if aliens do exist somewhere, then they most likely are much more advanced than us, since our planet was a latecomer (4.5 billion years old) to the Universe (13.8 billion years old). I’ll leave that prospect for the philosophers and the theologians to discuss.
If we don’t find any trace of life among the tens of thousands of planets that we will have discovered and analyzed over the next 10 years, then it won’t prove that life exists only on Earth, but it will make life a very rare phenomenon. That too would have momentous implications, both for science and for our view of the cosmos and of our existence in it. If we turn out to be the sole custodians of life and of (relatively) high intelligence and consciousness, then our responsibility toward the entire universe becomes huge.
The search for life is one of the greatest scientific quests. We are getting closer to at least some tentative answers, which would raise a number of fascinating questions for science, for religion, and for humans of all persuasions.
• Nidhal Guessoum is a professor of physics and astronomy at the American University of Sharjah, UAE. Twitter: @NidhalGuessoum.