Relative to the average age of other known planets, is the earth young, old or in the middle? Does it make a difference if you compare the earth against nearby planets (say < 500 LY), or other planets in our galaxy, or other galaxies?

  • $\begingroup$ We don't (yet) have the technology to observe planets in other galaxies. We are just starting to be able to resolve individual stars in other galaxies from the Local Group. $\endgroup$
    – usernumber
    Jan 8 '20 at 7:47

We do not have direct data on the age of any exoplanets, but we can estimate the age of their host stars, around which they presumably formed at the same time. These methods are uncertain and may have systematic biases but gives at least some information. If we use the Wikipedia page and plot the ages (again, both old data and now extra selection biases!) the resulting distribution looks like this: Estimated age histogram based on Wikipedia data The mean age is 4.9161 Gyr and the median age is 4.5260 Gyr, comparable to the age of the Earth. The skew distribution means Earth is younger than the mean, but fairly typical.

Another way of estimating whether Earth is young or old is to compare to star formation rate estimates across the history of the universe. This is of course also presently uncertain and may be different in different galaxies (elliptic galaxies have low star formation, so their planets would tend to be older than the ones in spiral and irregular galaxies). Overall, it appears to have peaked 3.5 Gyr after the big bang and has declined since. That would perhaps make the average planet that ever formed much older ($\approx 10$ Gyr old) than Earth.

However, many of these have also been "lost" since they formed around bright stars that died long ago and are not measured in the exoplanet surveys. There is also the issue of whether early low-metallicity stars form many planets: it might be that planet formation got going much later (Lineweaver has a -- somewhat old -- paper arguing that Earth formed after 75% of the other planets). Again, this is a very active research field and future answers are likely to be better.


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