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Is it possible to find out how old is a planet, especially a rogue planet? I know that people measured the radioactive decays to determine Earth's age with some amazing accuracy, but what about interstellar planets be, they gas or terrestrial?

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  • $\begingroup$ If it were an ejected planet, then the same measures would likely apply as they do on Earth. If you're using some other model for the formation of the planet, you might have to first define when the clock starts ticking. $\endgroup$ – Mitch Goshorn Nov 30 '16 at 9:32
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    $\begingroup$ Are you talking by telescope or landing on the rogue planet and being able to take samples. Samples is much easier, as a variety of radio dating methods could work. Telescope observation dating, is harder but some modelling based on surface temperature could give a rough/ballpark age, with some possible quirks like a giant impact or the planet at one point being close to it's star. $\endgroup$ – userLTK Nov 30 '16 at 9:49
  • $\begingroup$ @userLTK: interstellar so I think it is beyond our landers. $\endgroup$ – user6760 Nov 30 '16 at 10:21
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    $\begingroup$ How do you determine the manner in which the planet was created? Accretion vs. Gas Giant vs. birth in a major collision between non-stellar bodies? For example, so far as I know there's no rule that a planet has to have formed w/ enough radioactive species to behave like Earth. $\endgroup$ – Carl Witthoft Nov 30 '16 at 12:52
  • $\begingroup$ @CarlWitthoft Technically you're right, but given the age of our universe, the chances of a planet forming without radioactive species is pretty small. $\endgroup$ – zephyr Dec 1 '16 at 1:47
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At the moment there is basically only one way. That is to associate the planetary-sized object with a cluster of stars or moving group of stars of known age.

That's basically it. If the planetary-sized object really can't be associated with another object, then only limits can be placed on its age by comparing it's luminosity to theoretical planet cooling models. But to use these models you need to know the mass!

So at the moment, the only "free-floating" planetary mass objects known (they could all be low-mass brown dwarfs actually) are those for which an age is estimated by association and therefore a (planetary) mass has been deduced from cooling models.

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