How common is life in the universe?
The question is basically unanswerable. The well-known Drake equation feigns a certain degree of precision but suffers from the fact that it is nearly impossible to reach agreement on values for any of its seven factors. Right now, we have only one example for intelligent life, and for us to draw conclusions for the entire universe from just our own existence would, indeed, be very human, but would be scientifically problematic.
There is, however, an alternative. We could ask what the likelihood would be for life to develop on Earth if we turned back the clock and started over from the beginning. We know the conditions on Earth rather well, and astronomers are also fairly certain that there are planets somewhere out there that are similar enough to Earth that similar likelihoods would apply to them too.
In this context, researchers basically distinguish between four scenarios:
- Life is common, intelligence develops frequently
- Life is common, intelligence rarely develops
- Life is rare, intelligence develops frequently
- Life is rare, intelligence rarely develops
Which of these scenarios is the likeliest? That can be estimated, at least for our Earth, using statistical methods. To do that, researchers used two important pieces of evidence in a new paper:
- Life appeared on Earth relatively early, likely just 300 million years after it formed. That is a really fast start. If this is analyzed statistically, it is therefore 9 times more likely that life develops frequently. Even if the start of life is taken to be the appearance of the very first micro-fossils, the scenario “life is common” is still 2.8 times more likely than the scenario “life is rare.”
- The development of intelligence, however, took a significantly longer period of time. The Earth was already 4.5 billion years old when that first occurred. Whether that was the best thing for the planet or not remains to be seen. Viewed statistically, however, this means that the development of intelligence is a rather rare process with a likelihood of 3:2.
Therefore, habitable, Earth-like planets should likely have had life for a long time – but for us to find other thinking beings, we’re probably going to have to explore a large number of these planets.