It sounds like a fun thought experiment, but it’s not really a question, but rather a whole nested set of questions about how to find extraterrestrial life, how to determine the presence of alien intelligence, how to determine the nature of that intelligence, and how we will study or even study it. We will try to make contact. There’s a big moral issue at the end, and plenty of juicy scientists along the way.
First, how are we going to find a truly Earth-like planet? Despite all the incredible recent discoveries of planets around other stars, astronomers have yet to find a true twin of Earth—that is, an Earth-sized planet orbiting a sun-like star at a distance about the same as Earth. Land.
The closest they have come is to a planet called Kepler 452b. It is about five times the mass of Earth and probably belongs to the class of planets called super-Earths. Habitable, perhaps; Like Earth, no.
The same goes, even more so, for the recently discovered Earth-size planets around the star Trappist-1. These are extremely exciting worlds to study, and some of them may be compatible with life.
But the Trappist-1 planets orbit a dim red dwarf star, meaning they’re bombarded with energetic radiation and likely tidally locked, with one hemisphere always facing the star.
The closest analogs are surely there, but they are hard to find. NASA’s Keplerspace telescope sees planets by the way they blot out some of their star’s light when they transit, that is, pass between the star and us.
But for a planet in a one-year orbit, like ours, that means you see a shadow only once a year, and very briefly. To confirm that the planet is real – and not, say, a random flicker of the star – we must observe at least three identical events.
Therefore, you need to observe many stars and observe them for several years. Even then, the only planets we’ll find are the ones aligned exactly between us and their star.
Second, how would we know if a superficially Earth-like planet is actually habitable? The main method at the moment is to observe the light that streams through the planet’s atmosphere as it passes between us and the star.
That is an extremely difficult task even for giant planets. Right now, we don’t have the technology to do it for a true Earth twin, if we have one to study at all.
Another approach is to directly search for other Earths next to their stars, and then analyze their light. To do that, we need a very powerful telescope and a highly effective way to block the star’s glare, since an Earth-like planet would be next to it in the sky, but on the order of a trillion times fainter!
Astronomers have some clever ideas about how to do that, either by using an external light blocker (called a starshadow) flown in front of a space telescope, or an internal device (coronograph) built into the telescope to create a kind of artificial eclipse. .
The upcoming WFIRST telescope will test the coronagraph concept, but it won’t be sensitive enough to observe other Earths. That will require improved telescopes that are unlikely to be built until at least the 2030s.
Third, how would we know if there is intelligent life on the planet? Proving the presence of any kind of life will be quite a challenge, even after we get past steps one and two above.
Astronomers will analyze the planets’ atmospheres for “biosignatures” — unusual out-of-balance chemical compositions, such as those associated with life on Earth (for example, free oxygen plus methane).
SETI searches focus on possible signals sent by extraterrestrial civilizations, but aliens with 16th-century technology won’t send us any radio messages. We would have to find them in more subtle ways.
For example, we might be able to detect heavy metal signatures associated with smelting and other types of simple industry, though that still wouldn’t give unequivocal proof.
Some types of extreme speculative optical telescopes might be powerful enough to see evidence of cities or the geometric clearing of forests; since such observations would certainly be much more convincing.
Let us note, by the way, that it would be very unlikely to find an alien civilization that is so close to us in technological development, even if intelligent alien life is quite common.
Stars and planets have been forming in our galaxy for more than 10 billion years. Perhaps it is normal that it takes 4 billion years for intelligent life to emerge; maybe not.
In any case, life on other worlds could have started billions of years before or after life on Earth.
Outside of that wide range, the odds of an alien civilization being within 500 years of our stage of development are millions to one, and that assumes other planets follow the same path as ours, which is a big jump of faith. But let’s go deeper into the question.
Fourth, what would we do if we did find strong evidence of a pre-industrial civilization on a planet around another star?
We were unable to communicate with them by any currently known method. Unless physicists make some kind of wildly unforeseen new discovery, there’s also no practical way for humans to travel there.
We could potentially send miniature interstellar probes to survey the planet and learn more about its inhabitants.
A project called Breakthrough Starshot is exploring the kind of technology needed to do something like that. Such probes would be so small and fast that the aliens would have no idea they were being watched.
Let’s assume the best case. Sometime in the next decade we will find a potentially Earth-like planet around Alpha Centauri A, the closest star to the sun, or perhaps find encouraging information about the Earth-size planet orbiting its dwarf companion star. red Proxima Centauri.
In 2030 we find plausible biosignals on one of these nearby exoplanets. In the 2050s, we envision the planet and see plausible indications of an intelligent civilization.
What a moment of discovery that would be! We launch interstellar probes there, which arrive in the 2090s. By 2100, we are convinced that the planet is not only inhabited, but is home to an advanced, semi-industrial civilization. And now that?
That brings us to the fifth and final part. Should we try to make contact?
We could potentially use interstellar probes to send coded messages all over the planet. Perhaps the inhabitants could respond by setting huge geometric fires that we could observe from space.
We would have plenty of time to think about whether making contact is a good idea and, if so, what is the best way to go about it. Each round trip message would take about 9 years, and remember, this is to the nearest star.
Suppose we find a civilization on Kepler 452b. It is 1,400 light years away. Each round-trip message would take at least 2,800 years. A Starshot-style probe would take at least 7,000 years to get there.
But let’s go back to all the “what ifs” all along the way. What if we found this quasi-tech alien civilization, and what if we humans developed some kind of faster-than-light technology so we could jump on lightning and go visit, Star Trek-style?
How would we behave?
Right now, NASA has a detailed set of planetary protection rules to make sure humans don’t contaminate Mars or other possibly habitable worlds.
That’s part of the reason the Cassini probe crashed into Saturn: to make sure it doesn’t contaminate the moons Enceladus or Titan.
We like to think that when we’re smart enough to find life in other planetary systems and advanced enough to travel there, we’ll have advanced versions of those planetary protection rules to make sure we don’t contaminate other civilizations either.
On Earth, meetings between technological and non-technological cultures have generally not gone very well. If we ever get to the point where we find a non-technological culture on another planet, I hope we have enough common sense to watch from afar and not interfere, even though human history has always shown us how violent we are.