There may be life forms made of plasma in the hearts of stars, and civilizations made of blocks of space-time at the tips of black holes. Maybe aliens are visiting us, we just haven’t recognized them yet.
How many habitable planets are there?
Let’s take the Milky Way as an example. It is generally believed that the Milky Way contains 200 billion stars. The radiation near the center of the Milky Way is so intense that it could break down any complex biochemicals, so we’re conservatively ruling out the closest quarter of the Milky Way. So there are only 150 billion possible stars left.
Next, since planets are by-products 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 another star other than the sun) were discovered in January 1992 and were named “Ghouls”, “Saoling” and “Dream Gods”. We have since discovered more than 3,500 exoplanets. I am reluctant to give more precise numbers, because humans are discovering exoplanets so fast, and by the time you read this article, the numbers I wrote may be out of date.
Of the 3,500 exoplanets that have been discovered, scientists suspect 21 are in the habitable zone, or 0.6% of the total, so if we use this to represent the Milky Way’s 135 billion planets, there are 810 million planets in the habitable zone Habitable zone. If we assume that only 10% of these planets have water (a conservative estimate, since we’ve already discovered an exoplanet K2-18b with liquid water), this suggests that 81 million planets could have life on them.
Unfortunately, our inferences can only go so far, as we don’t yet know how life on Earth originated. So we can’t assess whether this is a regular process or a rare one. But with 81 million habitable worlds (a conservative estimate), even if the odds of life were one in a million, there could still be about 80 planets with life on them.
Can aliens see us?
Suppose aliens are analyzing our solar system and assessing whether it is suitable for life, as we do for other star systems. Can they find us?
The most important clue they will notice is the oxygen in Earth’s atmosphere. Oxygen is a very active element and pure oxygen cannot exist for long because it usually finds its way into rocks and oceans. The only way a planet wants to maintain an oxygen atmosphere is if there is something constantly making oxygen on the planet.
Besides, Earth is not an obscure planet. We use a lot of radio communication technology, and these radios leak into space, and anyone can easily eavesdrop by placing probes in the corners of the Milky Way. We even deliberately sent some messages into space in case people outside couldn’t hear them.
In 1974, we beamed 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 “messages for teens” to 6 star systems, which included a Theremin concert (the earliest message will arrive in 2047, the latest in 2070). In 2008, the Earth’s Message project sent 501 greetings to planet Gliese 581c from British social networking site Bebo, expected to arrive in 2029.
We also installed 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 aluminum panels, these business cards are plated with gold (one of the most stable metals) and feature a map where the Earth can be found, as well as nude men and women.
On the two Voyager probes launched in 1977, we devised a more detailed message. Both ships were fitted with gold phonographs, which also contained 116 images of Earth, as well as videos of common Earth sounds, including music clips from Beethoven to Chuck Berry, greetings in 55 languages to Sample of whale song.
What if the aliens were fools?
The idea of reaching out to a fellow cosmic is exciting, but it often raises deep concerns. If alien species are hostile, sending them our details is basically telling the invading fleet how to attack Earth.
Has anyone seen the movie “Independence Day”? In this movie, an alien race called the “Reaper” (which is the first clue to watch) wiped out most of the Earth’s population within 24 hours, which is a reasonable amount of time.
A species only needs to be decades ahead of us to conquer us without any resistance. For example, imagine an army in the 1970s declaring war on an army in the early 20th century. It will be a war of nuclear weapons, planes, radars, submarines, guided missiles, satellites against rifles and horses.
Obviously, in Hollywood movies, invading aliens always have an Achilles’ heel that we can exploit. In the movie “Omens”, the alien is allergic to water (logically, it invaded a planet whose surface and atmosphere are full of water). In the movie “The Martians on Earth,” it was the yodel singing that helped us win. And in the aforementioned Independence Day, the alien’s weakness is Jeff Goldblum, who plays the computer engineer.
In fact, alien races cannot have a simple switch. So some might say that we should keep a low profile before we fall victim to space military. I know where this concern comes from, but if I may, I would like to offer a contrary view.
All living things face the same basic challenges: competing for limited resources, ensuring the survival of offspring, fending off competitors, and more. Aggressive tendencies do help some of these goals, but they get in the way of society as a whole. Creatures must fight when threatened, but must also refrain from attacking family members and tribal allies. A species that fails to develop character of restraint and patience will perish because of internal struggles. So there can be no super-aggressive species.
Also, I think alien species are likely to have sympathy for us. In 2016, the BBC’s landmark series Earth Pulse, which featured an episode about a baby turtle trapped in a sewer, sparked an outcry. Humans have no reason to sympathize with another species, but we do. We realize the helplessness of something and see it as pitiful rather than 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 own shoes. A species that learns and thinks is likely to develop some kind of empathy for more pathetic life forms as well.
And, from a pragmatic point of view, an alien invasion of Earth makes little 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, all kinds of minerals, a molten core and a sun, but these things are not scarce – you can find them in the rest of the galaxy effortlessly.
Alien species have declared war on us, just 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 at our disposal, penguins are not a threat to us, and we tend to find them cute. So that’s the way it is, humans, the penguins of the galaxy.
