Recent observational results show that the outskirts of spiral galaxies contain
a huge amount of stars in the peripheries of the discs. In
these regions the gas density is so low (≤ 10 Mo pc-2) that no efficient
star formation could be expected that could form these stars. There
is a growing consensus that these stars were not formed in-situ but
rather migrated from the inner parts of the disc, effectively resulting
in an age gradient beyond the star-formation cut-off radius. To test
whether the predictions of migration or alternative theories are indeed
correct we need to probe the stellar populations in the disc. In this
talk I am going to present the different paths our group has followed
in recent years using stellar population techniques. I will show recent
results suggesting that migration can play a signicant role in the
evolution of disc galaxies, but that we are still left with many open
questions. Proving whether this theory is correct or not will be essential
to understand the goals of many large surveys (e.g. Gaia, RAVE)
and revise ideas about the Galactic habitable zone and astrobiology.
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