Is there intelligent life in our universe?

ET, the friendly alien who wanted to phone home.
ET, the friendly alien who wanted to phone home.

Think about it like this. There are so many stars in the observable universe that there are roughly 10,000 for every grain of sand on Earth. At least 5% of those are Sun-like stars, which means that there are 500 quintillion (500 billion billion) stars like our Sun out there.

 
Of these, many have habitable planets orbiting them (habitable in this case meaning those with warm temperatures and liquid water); conservative estimates suggest that there’s an Earth-like planet orbiting at least 1% of the stars in the universe. That’s 100 for every one of those grains of sand.
 
Perhaps 1% of those planets evolves life, meaning that each grain of sand now represents 1 planet with life. And maybe 1% of those planets evolves intelligent life. That would mean that in the observable universe there are 10 quadrillion (10 million billion) planets with intelligent life. 

That’s a lot. Assuming that these civilisations are spread out evenly, there are probably 100,000 in our galaxy alone. It’s pretty certain that we’re not alone in this universe.

The Clangers, a children's programme about cute aliens who live on the same moon as the Soup Dragon.
The Clangers, a children’s programme about aliens who live on the same moon as the Soup Dragon.

But if that’s the case, why haven’t we encountered any aliens? This is known as the Fermi Paradox, named after physicist Enrico Fermi; he asked why, since our solar system is pretty young compared to the rest of the universe, haven’t we been visited by intelligent life that evolved ahead of us? There are almost as many answers to this as there are people to think about it. Some of the most popular suggestions are:

Intelligent life doesn’t exist anywhere apart from Earth.

It does exist but aliens have no interest in making contact.

Aliens visited Earth before we evolved; after all, in the lifetime of our planet humans are a very recent phenomenon.

Since the universe is more than 13 billion years old, millions of civilisations could have risen, fallen and become extinct before our solar system even existed.

We assume that aliens would be carbon-based and similar to us, but there’s no reason why they couldn’t exist in a form that we’re unable to observe.

They just haven’t arrived yet.

We’re searching for alien life by sending out radio waves and scanning different frequencies of waves. But as the recent announcement about the discovery of gravitational waves demonstrates, there’s probably a lot of the universe that we can’t currently detect.

(Credit: xkcd)
(Credit: xkcd)

What do you think?

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