There may be organisms made of plasma in the heart of a star, or civilizations made of chunks of spacetime at the tip of a black hole. Maybe aliens are visiting us and we just haven’t recognized them yet.
How many habitable planets are there?
Let’s take the Milky Way as an example. It is now generally believed that the Milky Way contains 200 billion stars. The radiation near the center of the galaxy is very intense and would probably break down any complex biochemistry, so we’re going to conservatively exclude the closest quarter of the galaxy to the center. So there are only 150 billion possible stars left.
Next, since planets are byproducts of stars, we assume that most stars have planets. If we take a pessimistic approach, assume that only 90% of stars have planets around them, and that there is only one planet around each star. That’s 135 billion worlds.
The first exoplanets (planets orbiting a star other than the sun) were discovered in January 1992, and were named “Ghoul”, “Soul” and “Oneiroi”. Since then we have discovered over 5,000 exoplanets. I am reluctant to give a more precise number, because the speed at which exoplanets are discovered by humans is too fast, and by the time you read this article, the numbers I wrote may be out of date.
Of the 5,000 exoplanets that have been discovered, scientists suspect that 50 are in the habitable zone, accounting for 1% of the total, so if we use this to represent the 135 billion planets in the Milky Way, there are 1.35 billion planets in the habitable zone. If we assume that only 10% of these planets have water (this estimate is conservative, since we have already discovered an exoplanet K2-18b with liquid water), this suggests that 135 million planets may host life.
Unfortunately, we can only go so far because we don’t yet know how life on Earth originated. So we cannot comment on whether this is a regular process or a rare one. But with 135 million habitable worlds (a conservative estimate), even if the chances of life were one in a million, there could still be about 135 planets where life could exist.
Can aliens see us?
Suppose aliens are analyzing our solar system and assessing whether it is suitable for life, just like we have done with other star systems. Can they spot us?
The most important clue they’ll notice is oxygen in Earth’s atmosphere. Oxygen is a very reactive element and pure oxygen cannot last long because it usually finds its way into rocks and oceans. The only way a planet can maintain an oxygen atmosphere is if there is something on the planet that is constantly making oxygen.
Besides, Earth is not an obscure planet. We use a great deal of radio communication that leaks into space and can be tapped by anyone with no trouble putting probes in the corners of the galaxy. We even sent some messages into space on purpose, in case people outside couldn’t hear them.
In 1974, we sent a 3-minute message to the M13 star cluster through the Arecibo radio telescope in Puerto Rico, which contained information on the chemical composition of human genetic material DNA. In 2001 we sent a “teenager message” to 6 star systems, which contained a theremin concert (the earliest message will arrive in 2047, the latest message will arrive in 2070). In 2008, the “Earth’s Message” project sent 501 greetings to the planet Gliese 581c. These greetings came from the British social networking site Bebo and are expected to arrive in 2029.
We also have portable business cards on the side of space probes like Pioneer 10 and Pioneer 11, which flew out of the solar system in 1983 and 1990, respectively. Decorated with plates of aluminum plated with gold (one of the most stable metals), the cards feature a map where Earth can be found, as well as nude men and women.
On board the two Voyager probes launched in 1977, we designed a more detailed message. Both spacecraft are equipped with gold gramophones, which also contain 116 images of the earth, as well as videos of common earth sounds, including music clips from Beethoven to Chuck Berry, from greetings in 55 languages to A sample of a whale song.
What if the aliens were fools?
The idea of reaching out to fellow cosmic beings is exciting, but it often raises deep concerns. If alien species are hostile, then sending them our details is basically telling an invasion fleet how to attack Earth.
Has anyone seen the movie “Independence Day”? In this movie, an alien race called the “Reapers” wiped out most of Earth’s population within 24 hours, which makes sense.
A species only needs to be decades ahead of us to conquer us without any resistance. For example, imagine an army of the 1970s declaring war on an army of the early 20th century. It will be a war of nukes, planes, radars, submarines, guided missiles, satellites against rifles and horses.
Obviously, in Hollywood movies, the invading aliens always have an Achilles heel that we can exploit. In the movie Signs, the aliens are allergic to water (logically, they wouldn’t have invaded a planet with a watery surface and atmosphere). In the movie “The Martian Plays Earth,” it was yodeling that helped us win. And in the aforementioned Independence Day, the alien’s weakness is Jeff Goldblum, who plays a computer engineer.
In fact, there’s no way an alien race could have an easy switch. So some people might say that we should keep a low profile while we are not victims of the space military. I know where this concern comes from, but I’d like to offer a counter-view if I may.
All organisms face the same basic challenges: competing for limited resources, ensuring the survival of offspring, fending off competitors, and so on. Aggressive tendencies do serve some of these goals, but they hinder the functioning of society as a whole. Creatures must fight when threatened, but must also refrain from attacking family members and clan allies. A species that fails to develop the qualities of restraint and patience will perish through internal strife. So there can’t be a super-aggressive species.
Also, I think it’s very likely that alien species will feel sympathy for us. In 2016, the BBC aired the landmark series Planet Earth, including an episode about a baby sea turtle trapped in a sewer, which sparked an outcry. Humans have no reason to sympathize with another species, yet we actually do. We are aware of something’s helplessness and feel sorry for it rather than seeing it as a threat.
My hunch is that empathy arises because the ability to learn triggers the ability to imitate, allowing us to think in our shoes. A species that learns and thinks will likely also develop some kind of empathy for more pathetic forms of life.
And, from a pragmatic standpoint, an alien invasion of Earth doesn’t make much sense. The technology of a species capable of interstellar travel must be so advanced that our tiny planet has nothing to offer. We have water, various minerals, a molten core and a sun, but these things are not scarce – you can find them in other parts of the galaxy without any trouble.
Alien species have declared war on us as we have declared war on penguins. Penguins are too far from our comfortable homes, they live in places we don’t want to live, it takes effort for us to get there, there are no resources available to us, they don’t pose a threat to us, and we tend to find them cute. So this is the case, humans, are the penguins of the galaxy.
