How old could alien civilizations be?

Guest post by Sarah Pearson, Columbia Astronomy Graduate student and creator of the Space with Sarah YouTube channel ( Today Sarah is describing her latest YouTube episode.

Within the last couple of decades, humans have detected thousands of planets around stars other than our own Sun (exoplanets). The enormous number of galaxies each with billions of stars which statistically all have a planet orbiting them, makes it weird to think that life here on Earth should be the only life that exists in the entire Universe.

A question which hasn’t received that much attention yet is: how old could the oldest planetary system be?

We know that our own solar system is roughly 4.6 billion years old, which is actually quite young compared to the whole Universe which is ~13.8 billion years old. The Big Bang, mostly produced Hydrogen and Helium, while Earth’s crust consists mostly of oxygen, silicon and iron. This means Earth couldn’t have formed right after the Big Bang. But for how long would we need to wait?

To create the elements that rocky planets like Earth consist of, stars in the Universe actually need to first be created and then die to spread elements heavier than Hydrogen and Helium into space. Heavier elements are mostly produced in stellar interiors through fusion and when the stars eventually explode and shed their layers to their surroundings. It takes hundreds of thousands of years for the stars’ material to fully mix into nearby space, and subsequently this material needs to collapse and form new stars and planets.

While there’s definitely an observed correlation between the amount of time passed since the Big Bang and the amount of heavier elements in the Universe, astronomers are still having a hard time creating a precise timeline for the amount of heavy elements created at what time. But we do know that something like Earth could not have formed until enough stars in the Universe had exploded, and we also know that this could have happened a lot earlier than when our own solar system formed.

One of the most interesting planetary systems astronomers have found in our own Galaxy is Kepler-444 (Campante et al. 2015, ApJ) which consists of five rocky planets orbiting a star which is 6.6 billions years older than our own solar system, meaning that it formed only 2.6 billion years after the Big Bang! While this system probably doesn’t harbor life (the planets are too close to their star to have liquid water), its existence demonstrates that planetary systems could have formed a lot earlier in the history of the Universe than our solar system. This begs the question: how intelligent would alien civilization be if they have evolved for billions of years longer than life here on earth?

Find out more about this topic in the newest Space with Sarah’s YouTube episode:

On the Space with Sarah YouTube channel (, astrophysicist Sarah Pearson answers frequently asked space related questions in 3-6 minute videos.


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