“The universe is a pretty big place. If it’s just us, seems like an awful waste of space.”
This quote is attributed to Carl Sagan from his novel Contact (1985). It is often interpreted as reflecting Sagan’s optimism and belief in the possibility of extraterrestrial life. He strongly advocated for the search for extraterrestrial intelligence (SETI) and believed that the discovery of intelligent life beyond Earth would have profound implications for humanity.
In other words, Sagan suggested that if the Universe is so vast and we are the only intelligent life in it, it would be a shame to waste all that space on just one civilization.
A recent estimate (Conselice C.J. et al. 2016) says the observable Universe contains two trillion – or two million million – galaxies. Of course, this is a huge number, which math buffs can probably better appreciate if I translate it into scientific notation:
two trillion = two million million = one thousand billion = 2 x 1012
Even if we neglect 99.9999% of the Universe and consider only the Milky Way, we are left with a staggering number of about 100 to 400 billion stars.
Of course, these hundreds of billion stars vastly differ in age, mass, and chemical composition.
According to the stellar luminosity function:
A small percentage of stars are massive, young, and very bright (the so-called O, B, and A spectral types, with colors ranging from ultraviolet/white to blue);
A relatively large number of stars are medium-sized (the F and G spectral types, yellow to orange in color). Our “dull” Sun is one of them;
The majority of stars are small, old, low-mass stars (the K, M spectral types, a.k.a. red dwarfs);
Many stars are brown dwarfs (dark, spherical lumps of stellar material that never reached the star stage).
In the last few decades, roughly from the early nineties, it has become known that most, if not all, stars possess planets. Our Sun has eight major ones (excluding the KBOs or Kuiper Belt Objects). The former planet Pluto, now demoted to “dwarf planet,” is one).
Just like stars, planets also show a vast range of types.
I found a helpful classification in Imagined Life: A Speculative Scientific Journey among the Exoplanets in Search of Intelligent Aliens, Ice Creatures, and Supergravity Animals by James S. Trefil and Michael Summers. We can envisage the following kinds of exoplanetary environments as the most promising for alien hunters:
(1) Goldilocks Planets: planets like Earth, located at a distance from their star that allows them to have oceans of liquid water on their surface for extended periods;
(2) Subsurface Ocean Worlds: planets on which oceans of liquid water are bounded below by solid rock and above by ice. Examples in our solar system: the planet Pluto and several moons of Jupiter, Saturn, Uranus, and Neptune);
(3) Rogue Worlds: planets without a parent star. Such planets have been ejected from their solar system of origin and now wander through space. An example is OTS 44, a free-floating planetary-mass object located at 550 light-years, with approximately the mass of Jupiter;
(4) Water Worlds: planets with no dry land at all. That’s what a post-apocalyptic Earth would look like. (See, e.g., Kevin Reynolds’ 1995 movie Waterworld);
(5) Tidally Locked Worlds: planets that always present the same face to their star, much as the Moon does with Earth. Their peculiarity is that one side is perennially hot, while the other is an eternal Antarctica;
(6) Super-Earths: planets whose size falls between Earth and Neptune. Given their mass, the main characteristic of these planets is their intense gravity. Creatures must live in oceans or evolve a strategy to deal with this crushing force. A nice fictionalization of this is Edmond Hamilton‘s Starwolf series (1967-68), where Morgan Chane, the son of a human missionary family, grows up in a heavier-than-Earth world.
If these worlds exist, and there’s a tiny chance some might be inhabited, well… I want to see them. I’ll probably never do it in person (sadly, I’m not an astronaut). However, I can still dream about them, hoping someone will get there someday.
I wish someone to be able to say, just like the replicant Roy Batty in Ridley Scott’s 1982 movie Blade Runner:
“I’ve seen things you people wouldn’t believe.
Attack ships on fire off the shoulder of Orion.
I watched C-beams glitter in the dark near the Tannhauser Gate.
All those moments will be lost in time, like tears in the rain.
Time to die.”
Read more about this topic in this post and this other post.
