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It is estimated there could be as many as 40 billion Earth like planets in the Milky Way galaxy. Of these about 11 billion are around stars which are similar to the Sun - let us only consider these particular stars for this calculation.

Stars like ours live for about 10 billion years at a relatively constant luminosity and size. After that they get very bright and very big and would destroy their habitable zones. So as a rough estimate, every billion years in the Milky Way, 1.1 billion stars destroy their potentially habitable planets. That’s roughly a star a year (in our galaxy).

Now there are likely more than 170 billion galaxies in the universe, of which about 60% of them are similar to our Milky Way (spiral or elliptical galaxies). We discard the others because, even though they’d also contain huge numbers of stars with habitable zones, the age/luminosity distribution and star populations can be quite different for other galaxy types (which would ruin the assumptions above). That leaves us with a minimum of 100 billion galaxies destroying about one habitable planet each a year.

The probability of biogenesis (the probability of life developing) on terrestrial planets was recently estimated to be at least 13%. That leaves us with 13 billion planets, that developed life, being destroyed each year across the universe.

Now, not all of those planets will have their life forms destroyed by their suns entering the ‘giant phase’. There are other possible apocalyptic scenarios (e.g, religious fanatics and nuclear warfare - should they really be that similar to Earth) - but all we are changing now is the method of destruction, rather than the rate. Remember, we have been generous and ignored 40% of the galaxies and about 75% of the stars from our estimate.

Finally, 13 billion planets per year is a rate equivalent to over 400 planets a second.

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