The past half-century of solar system exploration has reinforced the lesson that no arbitrary division should be placed between life on Earth and astrobiology. In this chapter, I will discuss the probability that life exists elsewhere in the universe. I will also consider some possible explanations for its absence.
In the last few years, astronomers have discovered thousands of exoplanets orbiting stars outside our own galaxy. These worlds are thought to be the most promising candidates for finding intelligent life beyond Earth because they orbit their parent star at distances from it where liquid water could potentially exist on the planet’s surface as well as being conducive to supporting complex biological systems such as photosynthesis or life support. Of course, we cannot know for certain if any of these planets harbor life. But there is reason to believe that many do.
One problem with studying exoplanets is that they are so far away from us that we can’t directly observe them. We need to look at how their atmospheres change over time; what kinds of chemical reactions take place in those atmospheres; and what sorts of physical processes might affect the atmosphere itself. All of these observations would require large telescopes capable of gathering light from distant galaxies. Such instruments are not yet available, but NASA recently announced plans to build one, which will allow us to study exoplanets even further away than those currently explored.
A second problem is that we don’t know much about the habitability of exoplanets. For example, do they have oceans? Do they have continents? Are there any biotic communities on them? What is their atmosphere and what kinds of life-supporting cycles do they have, such as the carbon cycle between plants and animals? All these factors affect habitability.
A third problem is that we don’t know how frequently life arises on planets. Why is Earth the only planet that developed life? Did it just happen to have the right conditions for it to develop?
I will attempt to answer these questions below.
The Drake Equation
The Drake equation is a formula for computing the number of civilizations in our galaxy with which we might be able to communicate and is used as a working model for making these calculations. The information needed for this working model comes from four main sources: the number of stars of various types in the galaxy, the number of these stars that have planets, the number of these planets that can potentially support life, and the proportion that actually do.
The number of stars in the galaxy is estimated to be between 100 billion and 400 billion, depending on the size of the galaxy. The amount of stars of a given type (i.e. size and age) is also taken into consideration, since this affects how many planets will be able to orbit them.
The number of planets in the galaxy is harder to estimate, but it is known to be high. Earth is a very special planet: not just another rock orbiting a random star. It has liquid water, an oxygen/nitrogen atmosphere, and life. However, it is suggested that other planets must exist that have similar properties conducive to life support, which implies that there are other worlds protected against harmful radiation, gamma ray bursts, and meteor strikes. These are worlds in which life could flourish.
The ‘potential for life’ is very important when considering the possibility of communicating with lifeforms from other planets. This is extremely difficult to estimate, and indeed it is unknown if other life exists at all. But it seems unlikely that our planet was created in all its complex beauty just so that we could exist. I strongly suspect there is life elsewhere– it’s just a question of identifying it among the untold number of potential habitable systems in the universe.
The final variable is the probability that a planet produces intelligent life. This, too, is difficult to decide, but it is assumed that it is non-zero (because Earth itself has produced intelligent life). The Drake equation takes this into account by multiplying the value by a very large number.
The final probability of finding other life in the galaxy is so small that it is usually just taken to be zero, but with the large number multiplying it, it becomes a considerable (though still very small) possibility.
There are many factors that affect whether a planet produces intelligent life, such as axial tilt, atmosphere, and even the number of moons it has.
All these factors interact in complex ways to produce an overall likelihood of life, or indeed a given kind of life.
The Drake equation shows us how to find this probability.
Given this, it seems likely to me that life exists elsewhere. Statistically, it seems likely that at least one other planet would be able to support life, given how many planets are out there. That’s not to say that they would necessarily communicate with us, but life probably exists.
It is more probable that we will find microbial life on other planets before we find intelligent life. Like I discussed above, it requires a very specific set of conditions for intelligent life to develop and certain conditions must be met for it to survive. For one thing, an atmosphere is required. Life can obviously exist without an atmosphere as it does in the deep ocean or underground, but an atmosphere is needed for it to be at the surface. Other factors affecting whether life can exist may include how much energy a planet receives from its parent star, how much matter it has, how warm the planet’s surface is, and whether the planet is at the right distance from its parent star to have liquid water. Life may even exist below the surface of a planet. However, it would be difficult to study this without digging deep beneath the surface and building very complex machines in order to see what lies beneath.
What would we do if we found intelligent life elsewhere in the universe? I think we’d have three choices: flee from them and hope they’re peaceful; try to make contact with them; or destroy them. If we make contact with another civilization, it’s very likely that they would be technologically superior to us, so perhaps we should fear them. But to fear something just because you don’t understand it is a childish mindset. And if they were at the same level of technological advancement as us, then we could always trade with them. This would be very profitable to us and help human civilization reach heights far beyond what we can imagine.
We may even find non-organic life elsewhere in the universe. Non-organic life is life that does not require carbon and uses different elements, such as silicon, instead. It may very well be that the fundamental basis of non-organic life is so easy to create that it spontaneously arises in a process very similar to evolution on Earth.
It is clear that space exploration will play a large role in expanding human civilization, but there are still many unknowns about exoplanets and the lives that may exist around them. And those lives may already be at a level far beyond what we can imagine.
Consider what would happen if they had a level of technology that was so far beyond us that it could be used to travel to other parts of the universe in an instant. Perhaps they could even manipulate space and time itself to achieve such feats. If we ever do make contact with such a species (or even a more advanced version of our own), we should hope that they are benevolent; because if not, then it may very well mean the end of humanity.
It is also possible that perhaps humanity was not meant to be a spacefaring race. Perhaps Earth was meant to be the only place in the universe with life. It could be that we are destined to live and die here, without ever knowing what lies beyond our own planet. Maybe we have a purpose, maybe we don’t. Only time will tell.