How’s this for an astronomical estimate? There are at least 50 billion exoplanets in our galaxy. What’s more, astronomers estimate that 500 million of these alien worlds are probably sitting inside the habitable zones of their parent stars.

So how many of these exoplanets have life? Unfortunately, there’s no estimate for that question.

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This announcement was made on Saturday by Kepler science chief William Borucki at the American Association for the Advancement of Science in Washington D.C. However, Kepler didn’t actually count 50 billion exoplanets, this number comes from extrapolations of the data taken so far by the exoplanet-hunting space telescope.

For example, as Kepler has spotted 1,235 exoplanet candidates so far — 53 of which orbit stars in their habitable zones — knowing approximately how many stars there are in our galaxy (there are thought to be around 300 billion stars in the Milky Way), an estimate can be made of how many worlds are orbiting these stars.

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Kepler has only studied 1/400th of the sky, and it can only detect exoplanets that pass in front of (or “transit”) their parent stars. Also, it needs more time to detect exoplanets that orbit further away from their stars.

Taking all these factors into account means that a lower estimate can be made. There’s likely to be more than the 50 billion exoplanets Borucki describes.

Making this estimate is a relatively simple task, not so simple is estimating how many of these worlds might play host to life. As we know that only one planet in the Milky Way has life on it (Earth, in case you were wondering), no amount of statistical guesswork can arrive at an estimation for the number of alien beings that are out there.

Making estimates may sound trivial, but it does put the search for ET into perspective. There’s at least 50 billion worlds, which have fostered the development of basic lifeforms? How many have allowed advanced civilizations to evolve?

If there are any space-faring alien races out there, “the next question is why haven’t they visited us?” Borucki asked. He responded with: “I don’t know.”

I wonder if we’ll ever know.

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Image: An infrared observation of the core of the Milky Way as imaged by the Spitzer Space Telescope (NASA/JPL-Caltech/S. Stolovy SSC/Caltech)