Aug 18 2015

How Common is Life in the Universe?

We don’t know, but we can do some interesting thought experiments.

The only confirmed life in the Universe is here on Earth. That gives us one data point, so we know it is at least one planet per universe. The universe is an extremely large place. There are hundreds of billions of galaxies in the visible universe, each with hundreds of billions of stars on average, most of which seem to have planets. According to Ethan from Starts with a Bang:

With at least 200 billion galaxies out there (and possibly even more), we’re very likely talking about a Universe filled with around 1024 planets, or, for those of you who like it written out, around 1,000,000,000,000,000,000,000,000 planets in our observable Universe.

This number may be much larger. There are also many rogue planets not orbiting stars, and there may be as many stars between galaxies as there are within galaxies. So maybe, rough estimate, tack another zero on there.

It seems intellectually profane to argue that out of all of those planets, ours is the only one with life. Sure, we may have won the cosmic lottery and our own existence is profoundly unlikely, but there is no reason to think that’s the case.

Finding Life in the Universe

We can add a tremendous amount of information to the question if we could find life elsewhere. Empirical evidence would be best. In the short term (unless life comes to us) our best bet is to either find life in our own solar system, or to find the signatures of life in exoplanets.

Sending a probe to Europa is perhaps one of the best scientific investments we could make, in terms of understanding our place in the universe. If the liquid ocean under the icy surface of Europa (a large moon of Jupiter) contains life, and that life is not directly related to life on Earth, then our evidence-based estimate of the density of life in the universe goes from “we have no idea” to “life is likely to be everywhere.”

If life on Europa is related to life on Earth, then it’s possible that our solar system is just contaminated with life that arose once. It may have even arisen on Mars, then spread throughout the solar system through meteors.

Unique life, however, means life arose twice, in the same solar system, on two vastly different worlds. If that is the case the most parsimonious interpretation is that life is ubiquitous in the universe.

We can also look for the signatures of life in the atmospheres of exoplanets. When planets around other stars pass in front of them (transit) from our perspective on Earth, that is one way of finding them. It also gives us the opportunity to look as the light from the star as it passes through that planet’s atmosphere. We can then do spectral analysis to see what molecules are in the atmosphere. If we see lots of oxygen, it’s a good bet that there is life on that planet. This is indirect evidence, but it’s pretty compelling. Oxygen is very reactive, it won’t survive in an atmosphere unless it is being constantly replenished. Life is one process that creates oxygen.

Finding life on Europa or oxygen in the atmosphere of an exoplanet are things that can happen in our lifetime.

Theoretical Considerations

Short of evidence for life, can we figure out from theoretical principles how common life is likely to be? One question we can ask is – how common are the building blocks of life in the universe? It turns out, they are very common.

We find organic molecules on meteors, and recently we found them on a comet. “Organic” can mean that it just contains carbon, and not necessary biochemistry, but it’s a start. We have also, however, discovered amino acids in space. Now we’re getting closer. Amino acids are the basic components of proteins, which are the fundamental building blocks of life.

Scientists have also found that if you combine lightning with the likely constituents of the Earth’s early atmosphere, you make a goo of organic molecules (the classic Miller-Urey experiment). And just yesterday scientists published a study in which they show in their simulations that meteors striking the early ocean could have also created amino acids and nucleic acids. The latter are the building blocks of DNA – so now you have DNA and proteins and you’re off to the races.

The building blocks of life are still a long way off from life, but it shows that the raw material is likely common, so at least that is not an impediment to life being common.

What we really need to know, however, is how probable it is for life to evolve from the raw material. You can think of probability as how long an event will take on average to occur. The oldest evidence for life on Earth is from 3.65-3.85 billion years ago. This time period ranges from the end of the later heavy bombardment (when the Earth was pelted with meteors) to shortly after the bombardment.

So either life was forming on earth even while the surface was still being bombarded with meteors, or it arose very shortly after that. In the former case this could support the panspermia idea that life formed elsewhere and was seeded on the Earth.

Either way it seems that life arose very quickly. Again, it is very difficult to extrapolate from one data point, but statistically speaking it is therefore likely that the mean time for life to evolve when the raw materials are present and the conditions are suitable is very short, perhaps only hundreds of millions of years.  This argues for life being very common in the universe (unless we are deceived because Earth is a statistical outlier and therefore a quirky data point). This is also why a second data point would be so profound.

What about the evolution of complex multicellular life? The Earth was home solely to single-celled creatures and colony creatures for about 2.5 billion years. Multicellular life arose about 1 billion years ago, but we are not exactly sure. The Cambrian Explosion, where multicellular life really takes off and evolved hard parts that fossilize began between 550 and 600 million years ago.

If this timeline is typical (big if) then the mean time to evolve complex life may be several billion years. Of the millions of species of multicellular life, only one evolved advanced technology.

So if we assume that Earth is statistically typical we would infer that life is common in the universe, and that most of that life is likely to be simple, even single-celled. On worlds with billions of years of relative stability, we will sometimes (perhaps often) find complex life. On some of those, although it really is hard to say what percentage, there may be a technologically advanced species.


We only know of one instance of life in the universe, but if we are typical, or at least not a profoundly unlikely fluke, then it is probable that life is common in the universe. It is progressively more difficult to estimate the probability of complex life and technological civilizations.

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