Your existence is unbelievably unlikely. Think of everything that happened for you to be born: your parents met, a particular sperm fertilised a particular egg, ultimately giving rise to the specific sequence of genes that is you.
But if it hadn’t happened that way, someone else would be reading this in your place. We’re unique, but that doesn’t make us special: there are 7 billion other humans on the planet. Now, thanks to a glut of data on planets in other star systems, astronomers are starting to realise the same is true of Earth itself.
Researchers have discovered nearly 2000 exoplanets so far, the majority found by NASA’s Kepler space telescope. Getting a better picture of our galactic neighbours helps put our solar system into context, says Peter Behroozi of the Space Telescope Science Institute in Baltimore, Maryland. “Kepler has been fantastic for setting some of the limits on how many planets are likely to be found around stars, especially for Earth-like planets.”
Behroozi and his colleague Molly Peeples have combined the latest exoplanet statistics with our understanding of how galaxies form stars. The result is a formula that tracks the growth in the number of planets in the universe over time (arxiv.org/abs/1508.01202).
It suggests there are currently 1020, or 100 billion billion, Earth-like planets in the universe, with an equivalent number of gas giants. “Earth-like” doesn’t mean an exact replica of our planet, but rather a rocky world that, if blanketed by a suitable atmosphere, would hold liquid water on its surface. Applied to the solar system, this definition would include Mars and Venus but not Mercury or the moon.
And that’s just the start. Only a fraction of the gas within all the galaxies in the cosmos has cooled enough to start collapsing, so stars and planets will continue forming for billions of years. That means 92 per cent of the universe’s Earth-like planets won’t exist until long after the sun has died and taken the Earth with it.
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